It's not terminal -- think I can buff it out -- but my new piston and rebore (modified Wombat cylinder with CW Victory Port) temporarily seized today.
There are two seizure points 180 degrees apart with the ring gap lining up with one of them. I think this is called ring-gap seizure which is usually cause by the ring ends butting.
I filed the ring to .010 before assembling the top end, following the .004 time bore diameter inches.
Also, the piston to cylinder clearance is tighter than I specified (.0025) to the machine shop. It's more like a tight .002 or a correct .0015. Even so, I'm not seeing the seizure or score pattern typical of a too tight piston-cylinder. Is there a cause-effect dynamic between the tight cylinder and the ring-gap seizure?
Thanks for your wise feedback.
Seized new Combat piston
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Bullfrog
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Re: Seized new Combat piston
Modern Wiseco piston? If so, .0015" is simply a bit too tight.
So many additional questions:
- Thumb push fit of piston pin into piston (ie, not too tight?)
- Jetting? (what were operating conditions at time of seizure? Throttle position? Steady state or accelerating?)
- Gasoline? (straight or ethanol?)
- Oil? (brand and mix ratio)
- Compression, timing?
All of the above suggests that I'm not leaning toward "ring end gap seizure" as you describe it. I don't think the gap closed up and the ring deformed and caused the problem. And I don't think there is any correlation between tight piston/cyl clearance and your description of "ring end gap seizure", since proper ring end gap addresses what is needed for the ring to do it's job in the bore. My 2 cents.
Ed
So many additional questions:
- Thumb push fit of piston pin into piston (ie, not too tight?)
- Jetting? (what were operating conditions at time of seizure? Throttle position? Steady state or accelerating?)
- Gasoline? (straight or ethanol?)
- Oil? (brand and mix ratio)
- Compression, timing?
All of the above suggests that I'm not leaning toward "ring end gap seizure" as you describe it. I don't think the gap closed up and the ring deformed and caused the problem. And I don't think there is any correlation between tight piston/cyl clearance and your description of "ring end gap seizure", since proper ring end gap addresses what is needed for the ring to do it's job in the bore. My 2 cents.
Ed
Keep the rubber side down!
Re: Seized new Combat piston
Thanks Ed, your two cents USD is worth a bit more than Canadian after factoring rate exchange.
Here's the whole back story:
- NOS Art piston and rebored cylinder to suit.
- Piston and bore had already gone through several thermal cycles in about 25 miles of riding.
-Wrist pin needed moderately strong thumb pressure to fit.
-Jetting is stock Wombat specs. Half to 3/4 throttle on a steady uphill climb in fourth gear.
-Gasoline ethanol free 94 octane
-Yamalube 2R at 35:1
-Compression right after rebuild was 180 psi. Post-seizure compression was 175 psi. The Wombat head I'm using had been shaved by a PO to a degree where the squish band/or lip is about gone.
- Set timing and points not that long ago.
I also added Torque (or Singh) Grooves to the combustion chamber and was pretty well overjoyed at how well the bike performed pre-seizure. The grooves should have brought down the compression ratio on the shaved head, I believe.
There's another piston ring here - it will also need end filing to squeeze into the bore, as well as some 400 and 600 wet-dry, and muriatic acid to buff out the seize marks.
I'm wondering if I should get a different machine shop to check if the bore is tapered. I can insert a feeler gauge from the top of the piston and bore, but not the piston skirt at the bottom of the bore.
Thanks,
Darrell
Here's the whole back story:
- NOS Art piston and rebored cylinder to suit.
- Piston and bore had already gone through several thermal cycles in about 25 miles of riding.
-Wrist pin needed moderately strong thumb pressure to fit.
-Jetting is stock Wombat specs. Half to 3/4 throttle on a steady uphill climb in fourth gear.
-Gasoline ethanol free 94 octane
-Yamalube 2R at 35:1
-Compression right after rebuild was 180 psi. Post-seizure compression was 175 psi. The Wombat head I'm using had been shaved by a PO to a degree where the squish band/or lip is about gone.
- Set timing and points not that long ago.
I also added Torque (or Singh) Grooves to the combustion chamber and was pretty well overjoyed at how well the bike performed pre-seizure. The grooves should have brought down the compression ratio on the shaved head, I believe.
There's another piston ring here - it will also need end filing to squeeze into the bore, as well as some 400 and 600 wet-dry, and muriatic acid to buff out the seize marks.
I'm wondering if I should get a different machine shop to check if the bore is tapered. I can insert a feeler gauge from the top of the piston and bore, but not the piston skirt at the bottom of the bore.
Thanks,
Darrell
Re: Seized new Combat piston
Generally, skirt clearance is verified at least three times, top, middle, and bottom. If you can't get relatively similar readings top and bottom, something is out of whack. If the bore is tapered so soon after a rebuild, I would question the term rebuild. Was it rebored? If not, was it checked for taper before honing?. And, are ya' sure the clearance at the bottom is so tight you can't get a feeler in there? If you really can't get any thickness feeler in the bottom reading, it needs rebore. Most shops won't attempt to remove taper without rebore to the next size over.
Personally, I think 170-180 is high for a non-race bike. Just my perspective. It puts real demands on the fuel.
On the other hand, you said the scuffs were at the top of the piston at or near the ring gap, and directly opposite. With high pressures, combustion gas can pass the ring gap and degrade oil under the gap. On the other, other hand, those who claim to know say you can run really large ring gaps without problems. So I would be back to the skirt clearance.
Personally, I think 170-180 is high for a non-race bike. Just my perspective. It puts real demands on the fuel.
On the other hand, you said the scuffs were at the top of the piston at or near the ring gap, and directly opposite. With high pressures, combustion gas can pass the ring gap and degrade oil under the gap. On the other, other hand, those who claim to know say you can run really large ring gaps without problems. So I would be back to the skirt clearance.
GMc
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Bullfrog
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Re: Seized new Combat piston
Darrell:
Do you remember the strange sounds coming from the engine just before it seized? They were almost certainly there . . . it was almost certainly detonating like crazy. No squish band AND excessively high compression? It just had to be making strange sounds immediately before it got wayeee too quiet! 180 psi compression may be too high to survive even on 100% race fuel pre-mix. High compression will really grab your attention with the way it has "punch" when you grab some throttle, right? Well, um, it is impressive right up until all goes quiet. If you want to get to the finish line (or back to the van, or back home) without pushing (or calling for lift), you'll have to take action to reduce the compression to a liveable level.
With an original ART piston, .0015" piston/cylinder clearance would be fine (with proper break in) in a Combat Wombat cylinder - in fact that is what I request for my Combat Wombat cylinders with ART pistons when re-boring. That same clearance in an iron Wombat cylinder makes me a bit nervous. However, with your seizure "removing" the "high spots" it might run just fine IF (and only if) you get the compression down to a liveable level. (If it were me, I'd try it . . . but only after reducing compression to 160 or less.)
Ed
PS: I prefer an easy thumb push fit on the piston pin. A too tight pin can lead to seizures too. Still you did confirm that it IS a thumb push fit . . . so that is probably OK.
PSS Did I mention that you must reduce the compression?
Do you remember the strange sounds coming from the engine just before it seized? They were almost certainly there . . . it was almost certainly detonating like crazy. No squish band AND excessively high compression? It just had to be making strange sounds immediately before it got wayeee too quiet! 180 psi compression may be too high to survive even on 100% race fuel pre-mix. High compression will really grab your attention with the way it has "punch" when you grab some throttle, right? Well, um, it is impressive right up until all goes quiet. If you want to get to the finish line (or back to the van, or back home) without pushing (or calling for lift), you'll have to take action to reduce the compression to a liveable level.
With an original ART piston, .0015" piston/cylinder clearance would be fine (with proper break in) in a Combat Wombat cylinder - in fact that is what I request for my Combat Wombat cylinders with ART pistons when re-boring. That same clearance in an iron Wombat cylinder makes me a bit nervous. However, with your seizure "removing" the "high spots" it might run just fine IF (and only if) you get the compression down to a liveable level. (If it were me, I'd try it . . . but only after reducing compression to 160 or less.)
Ed
PS: I prefer an easy thumb push fit on the piston pin. A too tight pin can lead to seizures too. Still you did confirm that it IS a thumb push fit . . . so that is probably OK.
PSS Did I mention that you must reduce the compression?
Keep the rubber side down!
Re: Seized new Combat piston
Thanks Ed:
It's an .030 re-bore straight from the machine shop. I'm going to retract my concerns about the tapering because I sat down in a quiet well-lit corner and removed the feelers from the bunch so that I got a straight pull-push on the feelers. Now I'm satisfied that I have a tight .0025 piston-cylinder -- at least at the top and bottom. The feelers tend to bind when you're manipulating the leafs while they're attached a fat stack of them.
There was some prejudice on my part because most rebuilders ship their machining to the mainland because they don't have confidence in the local shops. But they lost my confidence when they returned my new piston with a mildly pinched ring groove -- it looks like they dropped or knocked the crown somehow. The Combat piston is more delicate than something like a CAT diesel piston which they're more used to. I fixed it with one of my wife's emery boards for her fingernails. I was careful to file just enough without widening the groove.
Anyhow, back to the event. Yes there was some pinging just starting. Similar to the noises my 250SL made before it's piston holed a couple of months back.
You're theory about high-compression induced blow-by sounds like that might be the smoking gun. It looks like the ring was weakened, or fractured, 180 degrees from the gap (the other point of seizure) because it later snapped without undue twisting or bending. Could this also be a symptom of butting ring ends?
I have my original unmodified head to put on this time to bring the compression back down to a normal level. I still want to cut in a few torque/Singh grooves because I think they really made a difference. The bike idled really evenly, even when cold.
Darrell
It's an .030 re-bore straight from the machine shop. I'm going to retract my concerns about the tapering because I sat down in a quiet well-lit corner and removed the feelers from the bunch so that I got a straight pull-push on the feelers. Now I'm satisfied that I have a tight .0025 piston-cylinder -- at least at the top and bottom. The feelers tend to bind when you're manipulating the leafs while they're attached a fat stack of them.
There was some prejudice on my part because most rebuilders ship their machining to the mainland because they don't have confidence in the local shops. But they lost my confidence when they returned my new piston with a mildly pinched ring groove -- it looks like they dropped or knocked the crown somehow. The Combat piston is more delicate than something like a CAT diesel piston which they're more used to. I fixed it with one of my wife's emery boards for her fingernails. I was careful to file just enough without widening the groove.
Anyhow, back to the event. Yes there was some pinging just starting. Similar to the noises my 250SL made before it's piston holed a couple of months back.
You're theory about high-compression induced blow-by sounds like that might be the smoking gun. It looks like the ring was weakened, or fractured, 180 degrees from the gap (the other point of seizure) because it later snapped without undue twisting or bending. Could this also be a symptom of butting ring ends?
I have my original unmodified head to put on this time to bring the compression back down to a normal level. I still want to cut in a few torque/Singh grooves because I think they really made a difference. The bike idled really evenly, even when cold.
Darrell
Re: Seized new Combat piston
Hard to say about the ring breaking 180 away from the gap. Rings are fragile anyway. If the area of the break shows damage not associated with the break itself, then possibly yes, it was compromised by the seizure. It is easy, though, to break rings by removing them with too much gusto.
Double check the ring groove. I would rather take a ding out beyond just level with the rest of the groove than leave it a bit short because it can seize the ring at that spot. Just by chance, the ding wasn't 180 away from the gap was it?
If your ring end gap was 0.010" it should not have gone shut. Now that it is broken it will be pretty hard to verify what the gap was. Just be sure the new one has a proper gap.
Two strokes make so much noise it is hard to hear detonation itself. Soft seizures such as yours was will make noise associated with the impending seizure. The screech, rattle, or whatever is hard to distinguish especially since you weren't listening all that closely. Unless you were expecting a seizure at that moment. I never hear them coming, but then haven't seized all that many two strokes. Just lucky.
But with compression as high as you report, detonation is not unexpected. If it was detonating, there should be trace evidence around the outer surface of the piston dome, like little sand grain strikes. Detonation will initially take carbon residue off the dome, then start to erode the surface. If an area of the piston dome is noticeably cleaner than the rest, or if it is stippled, it was detonating. Because it also causes a temperature spike in the area of detonation, detonation can push a piston to the point of seizure by causing oil failure. In the extreme, it will lead to a holed piston, but mild detonation will show signs even if it doesn't cause failure. Take a look. Check the head surface also for signs.
It made me shudder to think the shop that did your rebore is most familiar with Cat diesels.......
Double check the ring groove. I would rather take a ding out beyond just level with the rest of the groove than leave it a bit short because it can seize the ring at that spot. Just by chance, the ding wasn't 180 away from the gap was it?
If your ring end gap was 0.010" it should not have gone shut. Now that it is broken it will be pretty hard to verify what the gap was. Just be sure the new one has a proper gap.
Two strokes make so much noise it is hard to hear detonation itself. Soft seizures such as yours was will make noise associated with the impending seizure. The screech, rattle, or whatever is hard to distinguish especially since you weren't listening all that closely. Unless you were expecting a seizure at that moment. I never hear them coming, but then haven't seized all that many two strokes. Just lucky.
But with compression as high as you report, detonation is not unexpected. If it was detonating, there should be trace evidence around the outer surface of the piston dome, like little sand grain strikes. Detonation will initially take carbon residue off the dome, then start to erode the surface. If an area of the piston dome is noticeably cleaner than the rest, or if it is stippled, it was detonating. Because it also causes a temperature spike in the area of detonation, detonation can push a piston to the point of seizure by causing oil failure. In the extreme, it will lead to a holed piston, but mild detonation will show signs even if it doesn't cause failure. Take a look. Check the head surface also for signs.
It made me shudder to think the shop that did your rebore is most familiar with Cat diesels.......
GMc
Re: Seized new Combat piston
Hi Arizona,
There's no detonation damage to the piston. No carbon pattern either - it has gone only about 30 miles since new.
For a split second there was a quick screech when it seized and the wheel locked, but I pulled the clutch in right away.
The ding on the edge of the piston is about 30-35 degrees from the ring gap. Also the site of the ding is outside of the seized zone.
I'm leaning towards Ed's theory that the big-time compression might have been squeezing through and burning out the oil under the rings. Perhaps the new ring still had to seat itself in the bore. Maybe I should still have been giving the bike an easier time for longer to break in.
Thanks,
Darrell
There's no detonation damage to the piston. No carbon pattern either - it has gone only about 30 miles since new.
For a split second there was a quick screech when it seized and the wheel locked, but I pulled the clutch in right away.
The ding on the edge of the piston is about 30-35 degrees from the ring gap. Also the site of the ding is outside of the seized zone.
I'm leaning towards Ed's theory that the big-time compression might have been squeezing through and burning out the oil under the rings. Perhaps the new ring still had to seat itself in the bore. Maybe I should still have been giving the bike an easier time for longer to break in.
Thanks,
Darrell
Re: Seized new Combat piston
Just a couple more observations:
Piston is new-in-box clean, except for the seize marks. Underside of piston crown is clean and shiny too.
Spark plug showed a medium to dark brown insulator tip right after the seizure.
Darrell again
Piston is new-in-box clean, except for the seize marks. Underside of piston crown is clean and shiny too.
Spark plug showed a medium to dark brown insulator tip right after the seizure.
Darrell again
Re: Seized new Combat piston
Um, that was me, not Ed. And I agree that it was probably not broken in. At least I would hope that 30 miles would not break in a piston and ring, otherwise it won't last too long. I also agree that if the ding was not co-located with the scuffing, it wasn't the issue.Darrell wrote:I'm leaning towards Ed's theory that the big-time compression might have been squeezing through and burning out the oil under the rings. Perhaps the new ring still had to seat itself in the bore. Maybe I should still have been giving the bike an easier time for longer to break in.
Thanks,
Darrell
An observation only, your results may vary-- 35:1 is a bit oil lean for a break in. You might want to do your break in over a longer period of time, as you suggested, but also with more oil in the mix. I don't know how lean you can go with Yamalube. I have never run leaner than 32:1. I broke in at 28:1 or thereabouts. You could even do 20:1. All it is likely to do is kill mosquitos. Technically, 20:1 would require a rejet, but I wouldn't worry too much about jetting until it is broken in, unless you can't get more than 5 minutes out of a spark plug.
GMc
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Bullfrog
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Re: Seized new Combat piston
Um, my theory is not nearly as detailed as you described Darrell. I've stopped with short of that. I'm suggesting that at 180 psi compression and premium pump gas you WILL experience top end seizures (and I'm not really worried about exactly, precisely, what the final detail failure was a micro-second before it seized). I'm suggesting that when you left the drive way with 180psi compression and premium pump gas . . . the writing was already on the wall.
I'm not sweating the details because it is clear that ART pistons at .0015" (or greater) clearance with a properly gapped ring and with 160psi compression and premium pump gas WILL run and survive just fine. (subject to all of the normal caveats - no leaks, proper timing, proper heat range of plug, proper jetting, good oil, etc., etc.)
Ed
It's kind of like the story - "Hey, I heard Zeke died. Did you hear how it happened?" -- "Oh, yeah. Sad thing. He drowned." -- "Whew, that's hard! Another guy told me that an alligator got him." -- "Well, yeah . . . the gator was holding him under water. He probably really shouldn't have climbed the fence at the gator exhibit."
I'm not sweating the details because it is clear that ART pistons at .0015" (or greater) clearance with a properly gapped ring and with 160psi compression and premium pump gas WILL run and survive just fine. (subject to all of the normal caveats - no leaks, proper timing, proper heat range of plug, proper jetting, good oil, etc., etc.)
Ed
It's kind of like the story - "Hey, I heard Zeke died. Did you hear how it happened?" -- "Oh, yeah. Sad thing. He drowned." -- "Whew, that's hard! Another guy told me that an alligator got him." -- "Well, yeah . . . the gator was holding him under water. He probably really shouldn't have climbed the fence at the gator exhibit."
Keep the rubber side down!
Re: Seized new Combat piston
http://www.2strokeheads.com/tech2.htm
I am sorry but the cranking compression does NOT tell you what octane your engine requires.. The article above hopefully can shed some light on this..
I have had engines that crank 140PSI that require 110 octane fuel and CURRENTLY have engines that crank 190 and run WOT on 88 octane pump gas.. If you don't believe me, AS, feel free to make a trip up here with your comp tester and you can take them (yes have more than 1 engine) for a spin.
Not trying to be flip... but these ideas of cranking comp dictating octane requirement and plug color dictating the tune of the engine are simply just not reality anymore.. KNOWLEDGE has progressed, we have NEW and BETTER information...
Just like Jennings telling everybody in his book (from the 70's) the the Reed Valve would never be good... How did that turn out? and Jennings was a genius.. a man well ahead of his time IMO.. But he simply did not have the data and the equipment that we have now.. so his predictions fell short.
There have been advancements.. Some of the old "recipes" and theories still hold true today.. Others not so much.. You would be shocked how much better an old engine can run when it is tuned using "alternate" methods
I am sorry but the cranking compression does NOT tell you what octane your engine requires.. The article above hopefully can shed some light on this..
I have had engines that crank 140PSI that require 110 octane fuel and CURRENTLY have engines that crank 190 and run WOT on 88 octane pump gas.. If you don't believe me, AS, feel free to make a trip up here with your comp tester and you can take them (yes have more than 1 engine) for a spin.
Not trying to be flip... but these ideas of cranking comp dictating octane requirement and plug color dictating the tune of the engine are simply just not reality anymore.. KNOWLEDGE has progressed, we have NEW and BETTER information...
Just like Jennings telling everybody in his book (from the 70's) the the Reed Valve would never be good... How did that turn out? and Jennings was a genius.. a man well ahead of his time IMO.. But he simply did not have the data and the equipment that we have now.. so his predictions fell short.
There have been advancements.. Some of the old "recipes" and theories still hold true today.. Others not so much.. You would be shocked how much better an old engine can run when it is tuned using "alternate" methods
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Bullfrog
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Re: Seized new Combat piston
I take your point Kels - there are a LOT of modern developments which I don't know anything about. So my comment about the "writing was on the wall" was a bit grandiose.
On the other hand, Darrell's engine is made up of components which were designed and produced in the 1970's . . . and experience does show that in it's current state of tune with its antique suite of components (and antique combustion chamber shape) . . . it isn't going to live with 180psi compression. Could the engine be modified to live with that? Maybe so - but I think Darrell just wants it to stay together with the more-or-less stock components he's got. And maybe start studying what mods would let him run higher compression . . .
Ed
On the other hand, Darrell's engine is made up of components which were designed and produced in the 1970's . . . and experience does show that in it's current state of tune with its antique suite of components (and antique combustion chamber shape) . . . it isn't going to live with 180psi compression. Could the engine be modified to live with that? Maybe so - but I think Darrell just wants it to stay together with the more-or-less stock components he's got. And maybe start studying what mods would let him run higher compression . . .
Ed
Keep the rubber side down!
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taber hodaka
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Re: Seized new Combat piston
Kels
I have always tuned my hodaka using jet's, needle, slides, pilot jets ect. can you explain the new alternate modern way that would make it run so much better that I would be shocked. Besides that jets are harder for me to find. Always looking for the simple better way------------Clarence
I have always tuned my hodaka using jet's, needle, slides, pilot jets ect. can you explain the new alternate modern way that would make it run so much better that I would be shocked. Besides that jets are harder for me to find. Always looking for the simple better way------------Clarence
Re: Seized new Combat piston
Whoops! Sorry to Ed and Arizona about the misattribution of statements! I'm often trying to read the computer screen in short glimpses while the cat is walking back and forth in front of it.
Ennyway, as I posted earlier, I'm going to sand off the piston and muriatic clean the cylinder before reverting back to the unmodified head and shelving the hi-compression head, for now. But it was fun while it lasted! I was climbing hills in fourth and fifth gear that the old tired bore needed second for.
Point taken regarding the richer premix and more patience with the break-in. I've read lots of differing opinions about break-in procedure -- as well as other facets of two-stroke tuning. Does anybody else find that it seems like a mix of engineering science with a few dashes of legend, wizardry, and superstition? It's fascinating how the simplest engine design begets so much complexity.
Darrell
Ennyway, as I posted earlier, I'm going to sand off the piston and muriatic clean the cylinder before reverting back to the unmodified head and shelving the hi-compression head, for now. But it was fun while it lasted! I was climbing hills in fourth and fifth gear that the old tired bore needed second for.
Point taken regarding the richer premix and more patience with the break-in. I've read lots of differing opinions about break-in procedure -- as well as other facets of two-stroke tuning. Does anybody else find that it seems like a mix of engineering science with a few dashes of legend, wizardry, and superstition? It's fascinating how the simplest engine design begets so much complexity.
Darrell
Re: Seized new Combat piston
Ed, yes , I fully agree with you.. If the history of this engine in this state has shown that 180 PSI is too much for pump gas (especially today's gas) then the data is valid..Bullfrog wrote:I take your point Kels - there are a LOT of modern developments which I don't know anything about. So my comment about the "writing was on the wall" was a bit grandiose.
On the other hand, Darrell's engine is made up of components which were designed and produced in the 1970's . . . and experience does show that in it's current state of tune with its antique suite of components (and antique combustion chamber shape) . . . it isn't going to live with 180psi compression. Could the engine be modified to live with that? Maybe so - but I think Darrell just wants it to stay together with the more-or-less stock components he's got. And maybe start studying what mods would let him run higher compression . . .
Ed
My point was that just because it is valid in this particular case (known variables) does not mean that everytime you see 180PSI on an engine that you can apply the same logic.
Re: Seized new Combat piston
Me again,
Here's the final installment (I hope) to this piston saga...
The aluminum transfer on the cylinder melted away with the muriatic acid. The seize marks on the piston sanded smooth with 400 wet-dry.
This time I generously gapped the ring to .014 thousands.
I used the original head this time and the compression is 160 (down from 180) psi now.
Timing may have been a bit retarded but that's reset now.
I haven't ridden it but the engine runs really well, I had to turn down the idle a bit from the setting used for the shaved/high-compression head. The exhaust note has become a lot snappier and crackly with using the Combat piston and Victory Port vs the tired Wombat top end. It sounds like some real combustion is taking place in there.
Thanks to all that keep the Hodaka ThinkTank moving forward!
Darrell
Here's the final installment (I hope) to this piston saga...
The aluminum transfer on the cylinder melted away with the muriatic acid. The seize marks on the piston sanded smooth with 400 wet-dry.
This time I generously gapped the ring to .014 thousands.
I used the original head this time and the compression is 160 (down from 180) psi now.
Timing may have been a bit retarded but that's reset now.
I haven't ridden it but the engine runs really well, I had to turn down the idle a bit from the setting used for the shaved/high-compression head. The exhaust note has become a lot snappier and crackly with using the Combat piston and Victory Port vs the tired Wombat top end. It sounds like some real combustion is taking place in there.
Thanks to all that keep the Hodaka ThinkTank moving forward!
Darrell
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Bullfrog
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Re: Seized new Combat piston
Dang, I wish I had sent this earlier! The ring end gap specification listed in the Official Hodaka Workshop Manual is .004" to .012". While I'm having trouble reconciling the .004" with my aging memory that .006" is the proper gap for a CW OEM ring . . . I'm having no trouble with the concept that your .014" gap setting when new . . . is um, "worn out" before you start. The manual specification of .012" maximum is an indication that a new ring is called for when normal wear enlarges the ring end gap to .012". Sorry for not mentioning this sooner.
Ed
Ed
Keep the rubber side down!
Re: Seized new Combat piston
No sweat Ed,
I was aiming for .012 but overshot a bit. The .004 rule is per inch of bore, that would make the 2.25 inch bore diameter ring gap would be .009. That's also a Wiseco spec, and if the engine is subject to running hot Wiseco suggests .005 per bore inch. It's been pointed out that the all-iron Wombat won't shed heat as well as the alumiferric CW cylinder I leaned that way.
First time I was sure I had set the gap at .010, but the pattern of the seizure had me wondering if the gaps had butted... But next time I'm gapping the ring(s) I'll put on more powerful glasses and better lighting to make sure I've indeed got the feeler inside the gap, and not riding on top. I often second guess myself after buttoning up an engine. I guess that's normal.
Regardless, I have a reasonable compression reading now, and when it gets too low a new ring is pretty cheap.
Darrell
I was aiming for .012 but overshot a bit. The .004 rule is per inch of bore, that would make the 2.25 inch bore diameter ring gap would be .009. That's also a Wiseco spec, and if the engine is subject to running hot Wiseco suggests .005 per bore inch. It's been pointed out that the all-iron Wombat won't shed heat as well as the alumiferric CW cylinder I leaned that way.
First time I was sure I had set the gap at .010, but the pattern of the seizure had me wondering if the gaps had butted... But next time I'm gapping the ring(s) I'll put on more powerful glasses and better lighting to make sure I've indeed got the feeler inside the gap, and not riding on top. I often second guess myself after buttoning up an engine. I guess that's normal.
Regardless, I have a reasonable compression reading now, and when it gets too low a new ring is pretty cheap.
Darrell
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Bullfrog
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Re: Seized new Combat piston
The information from the manual lists the actual allowable gap range for the OEM dykes type ring on the OEM ART piston in the Combat Wombat cylinder - with ring replacement indicated at .012" ring end gap. As for the rule of thumb for gap per inch of bore, the NOS Poppy rings I have laying around list .003" to .004" per inch of bore. For the (nominal) 56mm bore, that calculates out to .0066" - round up to .007" on the snug end (which is what I would use). At least that calculates out to very close to what I remember doing on my engines -- .006" ring end gap. And I think it is safe to apply that rule to OEM rings for the Combat Wombat ART piston.
While the iron Wombat cylinder may require a bit more ring end gap, .008" has got to be plenty. Sooooo . . . I'm still of the opinion that your new ring with .014" end gap is "worn out" before you start the engine. Note that the Workshop Manual lists exactly the same ring end gap for both rings in the iron Wombat engine. .004" - .012"
Again, I'm sorry I didn't comment on this topic sooner. You have the compression down where it ought to be . . . but now you have to worry about excessive ring end gap "blow by" and spot lubrication failure (seizure).
Ed
While the iron Wombat cylinder may require a bit more ring end gap, .008" has got to be plenty. Sooooo . . . I'm still of the opinion that your new ring with .014" end gap is "worn out" before you start the engine. Note that the Workshop Manual lists exactly the same ring end gap for both rings in the iron Wombat engine. .004" - .012"
Again, I'm sorry I didn't comment on this topic sooner. You have the compression down where it ought to be . . . but now you have to worry about excessive ring end gap "blow by" and spot lubrication failure (seizure).
Ed
Keep the rubber side down!
Re: Seized new Combat piston
Here's what Wiseco has to say:
" As a general rule, most engines will require .004" per inch of bore diameter for intended ring end gap minimum. More extreme engine applications, where the piston and ring will experience excessive heat, it is recommended to use .005" per inch of bore diameter ring end gap minimum. Generally, when measuring ring end gap, it is most always better to have too much than too little; a tight ring end gap can potentially cause the ring ends to touch during operation, which may cause a failure." (my emphasis)
A 56mm bore is 2.204" not corrected for overbore. Following Wiseco recommendation would result in a ring end gap of minimum 0.01102" end gap at 0.005" per inch of bore. For a 0.40" overbore, the end gap would be 0.005 x 2.244 = 0.01122". Allowing some leeway for "too much is better than too little" , setting the gap at 0.012" is simply well within acceptable range for a hand gapped ring, and not worn out at all.
Check with any piston and ring manufacturer, or any two stroke high performance tuner and the answer is the same: too much is better than too little. Anyway, 0.01122" is arguably the same as a hand measured 0.012". I don't have a feeler calibrated ten thousandths or more of accuracy, and it certainly doesn't matter. As long as the ring ends don't touch in operation, it is likely to work just fine. After all, when hot the gap shortens. If 0.009" is a minimum, arguably a hot ring might expand, say, 0.008", or just a bit less than the minimum clearance, leaving one or two thousandths clearance. Add that 0.008" expansion to a ring gapped at 0.012" cold, and you get a hot gap of 0.004" at operating temperatures. To me that seems acceptable and certainly not worn out.
Consider also that regardless of the piston style, cast or forged aluminum, ring expansion bears no relation to piston material type. Different styles of ring expand differently, and we have little to no background information from manufacturers on ring expansion attributable to ring style or material. Original Wombat top ring end gap clearances are stated at 0.006" to 0.014", and Super Combat, using a Dykes ring, is stated also at 0.006" to 0.014". A gap of 0.012" falls within this range. Further, rings made in the 70's when stock gaps were published were cast iron, didn't have a lot of spring to them, and wore in fairly quickly. Wiseco rings these days are really hard, have more spring, and wear in slowly, according to Klemm Racing. Klemm also points out that Wiseco rings are factory gapped too closely, suggesting to me that using their wider factor isn't that bad an idea. Bottom line here is that there is little to go on in setting end gaps other than manufacturers suggestions and long standing generalities, but most agree that a little too wide beats a ring that closes to zero in use.
The downside to wider ring gap has already been mentioned, in that combustion gases can bypass the gap and degrade oil film in the immediate area. However, all rings operating correctly have an open gap and pass these combustion gases to some extent. A piston that has never seized will still show discoloration under the ring gap due to gas leakage. The answer to this is to use a two stroke oil with a high film strength and high heat tolerance, mixed at an adequate ratio. If you use high quality two stroke oil known to work in air cooled bikes, and mix at conservative ratios, the ring end gap is not a problem even when the gap widens in use.
The reality is that the ring has already been gapped to 0.012". The choices are to use it or throw it away. My choice would be to use it, use good air cooled suitable two stroke oil, mix at not more than 32:1 for long term, use 20:1 or 24:1 for breakin, and give the engine an extended breakin before putting the engine under stress.
" As a general rule, most engines will require .004" per inch of bore diameter for intended ring end gap minimum. More extreme engine applications, where the piston and ring will experience excessive heat, it is recommended to use .005" per inch of bore diameter ring end gap minimum. Generally, when measuring ring end gap, it is most always better to have too much than too little; a tight ring end gap can potentially cause the ring ends to touch during operation, which may cause a failure." (my emphasis)
A 56mm bore is 2.204" not corrected for overbore. Following Wiseco recommendation would result in a ring end gap of minimum 0.01102" end gap at 0.005" per inch of bore. For a 0.40" overbore, the end gap would be 0.005 x 2.244 = 0.01122". Allowing some leeway for "too much is better than too little" , setting the gap at 0.012" is simply well within acceptable range for a hand gapped ring, and not worn out at all.
Check with any piston and ring manufacturer, or any two stroke high performance tuner and the answer is the same: too much is better than too little. Anyway, 0.01122" is arguably the same as a hand measured 0.012". I don't have a feeler calibrated ten thousandths or more of accuracy, and it certainly doesn't matter. As long as the ring ends don't touch in operation, it is likely to work just fine. After all, when hot the gap shortens. If 0.009" is a minimum, arguably a hot ring might expand, say, 0.008", or just a bit less than the minimum clearance, leaving one or two thousandths clearance. Add that 0.008" expansion to a ring gapped at 0.012" cold, and you get a hot gap of 0.004" at operating temperatures. To me that seems acceptable and certainly not worn out.
Consider also that regardless of the piston style, cast or forged aluminum, ring expansion bears no relation to piston material type. Different styles of ring expand differently, and we have little to no background information from manufacturers on ring expansion attributable to ring style or material. Original Wombat top ring end gap clearances are stated at 0.006" to 0.014", and Super Combat, using a Dykes ring, is stated also at 0.006" to 0.014". A gap of 0.012" falls within this range. Further, rings made in the 70's when stock gaps were published were cast iron, didn't have a lot of spring to them, and wore in fairly quickly. Wiseco rings these days are really hard, have more spring, and wear in slowly, according to Klemm Racing. Klemm also points out that Wiseco rings are factory gapped too closely, suggesting to me that using their wider factor isn't that bad an idea. Bottom line here is that there is little to go on in setting end gaps other than manufacturers suggestions and long standing generalities, but most agree that a little too wide beats a ring that closes to zero in use.
The downside to wider ring gap has already been mentioned, in that combustion gases can bypass the gap and degrade oil film in the immediate area. However, all rings operating correctly have an open gap and pass these combustion gases to some extent. A piston that has never seized will still show discoloration under the ring gap due to gas leakage. The answer to this is to use a two stroke oil with a high film strength and high heat tolerance, mixed at an adequate ratio. If you use high quality two stroke oil known to work in air cooled bikes, and mix at conservative ratios, the ring end gap is not a problem even when the gap widens in use.
The reality is that the ring has already been gapped to 0.012". The choices are to use it or throw it away. My choice would be to use it, use good air cooled suitable two stroke oil, mix at not more than 32:1 for long term, use 20:1 or 24:1 for breakin, and give the engine an extended breakin before putting the engine under stress.
GMc
Re: Seized new Combat piston
You present your case logically, well supported and allowing for a good margin of error! ; D Victor
1978 175SL
1976 03 Wombat
1975 99 Road Toad (2)
1973 96 Dirt Squirt (2)
1973 “Wombat Combat”
1973 Combat Wombat
1972 94 Wombat (2)
1972 Super Squirt
1971 92B+ Ace
1970 92B Ace 100B (2)
1968 92 Ace 100
1966 Ace 90
; D Victor
Re: Seized new Combat piston
That's because I thought I was wrong once, but it turns out I was mistaken.
GMc
Re: Seized new Combat piston
Ah. I see you met my Dad before he passed away. He used to say the same thing! ; D Victor
1978 175SL
1976 03 Wombat
1975 99 Road Toad (2)
1973 96 Dirt Squirt (2)
1973 “Wombat Combat”
1973 Combat Wombat
1972 94 Wombat (2)
1972 Super Squirt
1971 92B+ Ace
1970 92B Ace 100B (2)
1968 92 Ace 100
1966 Ace 90
; D Victor
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