Tired Combat Wombat

[dcooke007]

Well here are the pics. We had run the bike during testing at my home, Buds Creek on a Friday which was a practice day, and one race prior to the event. The race prior did not have the long wide open run and was very muddy. Engine survived and performed well. On this last race the temperature was in the 40's where as the prior race temp was in the 70's. Will participated in the practice laps during this race with no noted concerns. During the race he thought he noted something on the first lap. Second lap on the extended wide open run is when the engine fully seized. Bottom line looks like it was just to lean on top for the air temp and extended wide open runs. The video does not capture the moment the seizure occurred. Camera stopped working around the first lap. The color of the plug is not terrible but you can see the insulator is white at the bottom. Picture makes it look more tan. Also don't know the running condition the last few moments of operation.

Danny

[hodakamax]

I think all of us mechanical minded people feel the pain of machinery--at least I do.--Ouch!

Maxie

[dcooke007]

If it could only speak!!!!

Danny

[Zyx]

Plug doesn't look lean. Down at the bottom of the insulator is the coldest part of the plug. White probably due to limited run time. Color otherwise is normal. Seizure pretty general all over. Gotta ask, what oil and mix ratio? If leanness caused this it would be oil lean not fuel lean, I think.

[taber hodaka]

A different opinion The plug does not look normal to me. I say lean fuel. with the bike running it sounded dry and lean, wound very high. If you are jetted for good oil you can't run a poorer grade of oil. In my opinion you jet to what you are using. The air fuel ratio determines rich /lean. If you have to mix 20:1 you need a better oil. Yust me ----------Clarence What piston??

[Zyx]

Not saying the oil made the fuel mixture lean (although presence of oil displaces fuel and can affect mixture, but that is another days discussion). Saying that the plug is black to brown, which does not suggest lean fuel. I was suggesting that the oil mixture was too lean for conditions. As in, 40:1 when it should have been 32:1, or 32:1 when it should have been 26:1. I didn't hear it run, so can't comment on whether it sounded lean, but while you can hear lean run due to jetting (if it is excessively lean) you can't hear oil lean conditions until the bike goes quiet.

This looks like a lubrication failure from running too little oil for sustained high rpm running. Assuming the piston was set up tightly, as it should be, at high temperatures, pistons often will measure a slight press fit in the cylinder, and the only reason they don't go metal to metal is the oil film. If that film fails, seizure ensues in only moments. Which leads back to my question, what oil, what ratio? The plug itself doesn't look lean at all. In the photo it looks like a black insulator tip, and a brown ring. I would rather see the colors reversed, but if I had to guess on jetting looking only at this plug, I would say slightly rich to normal. The only other variable that comes to mind is insufficient ring end gap, but the displacement of metal isn't limited to the rings or the area below the end gap, so I am thinking oil related failure.

[taber hodaka]

The plug doesn't look black to me it looks silver grey the color it would be if I fried my motor?? And I would ask what piston old stock or new and what was the ring end gap?------Clarence

[Charlie R]

Danny, I know you didn't ask but I was reading about the conditions on race day. Do you suppose you're seeing a cold seizure? I used to race TZ motors that had to be carefully warmed to temp when the ambient temps dropped. Just thinking out loud.

[hodakamax]

I shouldn't even get started on this one but; Hey gang, It seized because the aluminum piston heated to beyond its melting point and began to stick to the cylinder. This is almost always caused by too lean of mixture or lack of clearance. Too lean is not enough fuel AND not enough oil. Lean means not enough fuel to cool the system--It got too hot. Reasons for catastrophic seizure could be clogged cooling fins, obstruction of jets, lack of clearance, not enough break-in and/or too lean of jetting or all of the above or something I haven't even considered. 32:1 should be plenty with this engine and a slightly rich mixture with proper break-in. As for the spark plug, to me it means nothing, the evidence is the melting of the piston and the resulting seizure. Again, it got too hot. I must say and we all know, it's hard to tell when an engine is "broken-in" and first races should be on the rich side. Danny is no "babe in the woods" when it comes to engine building and I'm starting to suspect an obstruction of the main jet or perhaps a too short of break-in. We have lots of demands on race engines sometimes exceeding the limits of design.
The last picture of the piston appears to show the ring gap. If so it does seem to be enough. Just thinking, we've all stuck one it seems and we always end up with a bigger main. All just opinions of course!

Maxie

[dcooke007]

Hey guys, thanks for all your input. All your thoughts are appreciated and we don't have to agree on everything. The discussion alone peaks my curiosity and makes me think.

1. I only have pictures of the piston at this point....same ones you are seeing. A closer examination may give some more info. Bike still not here. The piston and ring combination is one of Strictly Hodaka's new version Wiseco. Cylinder to piston clearance was .003 and the ring end gap was a little loose at .017. I did not bore the cylinder as seemed to be ok. Port openings were chamfered and ball hone used to re-surface cylinder. The coating on the crown of the piston appears to have come through in good condition from what I can see from the pictures. The skirt coating seemed to do a good job also up to a point. I have to wonder how it would have looked with out the skirt coating. Most of the seizure occurred toward the left side of the piston. Exhaust side from about middle to left. Intake on the left side only.

2 Spark plug color on the end of the insulator is as I would like to see and looks a little rich. Some things I do not know and some things noticed. Did the engine continue to operate for a few moments before completely failing and maybe affect the plug reading? Did Will back out of the throttle when seizure was noted? The base of the plug insulator is white and "may" be a better indicator of lean condition at wide open throttle if the engine was operated at less than wide open throttle for a few moments. Just wondering. We were running an 8 heat range plug instead of a 9 to keep the plug clean at low speeds.

3. After cleaning the cylinder I did notice the cylinder looked very glazed for the modest amount of run time on the engine. The bore was glazed in the piston skirt areas below ring travel also. Cross hatching still existed in the bore areas to the sides of the piston below ring travel. The glazing does bring into question the quality of the oil film and maybe engine break in. I know Will is using Repsol at 32/1. Not sure what version of Repsol. Not a fan of 20/1 ratio myself. If an oil film failure was the sole cause I would expect to see the seizure closer to the center / thrust line of the piston and over a larger area. The ability of the cylinder to hold oil is reduced with the glazed / worn cross hatching. So with out having the piston in hand I think the engine was to lean for the operating conditions, piston expanded and seizure occurred. Glazing / cross hatch wear indicated oil film was none the less marginal for operating conditions and contributed to the failure. At this point the plan is to bore the cylinder and replace the piston assy, use different oil, break in carefully and re-address jetting. Some modifications are in order for the piston also. I plan to drill a couple of holes in the piston at the four corners. Between the intake / transfer port and exhaust / transfer port. Sort of like the holes drilled for a bridged exhaust port. This idea came about after speaking with Rich. Dimpling the piston skirts also.

Will got off to a bad start on this race and was really winging the motor to catch up. I had considered a cold seizure also since he had to use the choke to get going and then was full throttle. Rich does not seem to think that is a real possibility due to how long the engine continued to operate. As has been said before this is racing and we just need a little more run time to work out the bugs. Don't you just love it.

Danny

[hodakamax]

Danny, As always, it sounds like you are on top of it. I like to compare these events to an airplane crash. It's always a series of causes, not just one thing. And just like an airplane crash we're all just looking at wreckage. Sometimes it's just hard to tell what happened. All the gang sounds to be on track and I think we're all just trying to console you!

Keep up the good work!

Max

[dcooke007]

Max, thanks for the consolation but I am way past that. It was great seeing how strong it was running and surviving the thrashing......at least for a while. The engine really pulled very strong with the modest port work and reed valve.

I am moving on now to getting this thing going again and helping Will. It looked like he was bouncing around quite a bit also and may play with the fork oil weight also.

Again, any thoughts you guys have are appreciated.

Danny

[bchappy]

A lot of experience and thought are going into analyzing this failure and I have a lot of respect for everyone that has commented. From me experience in seizing and burning holes in probably a bushel basket full of pistons back in the day here is my two cents. In listening to the sound in the video the engine was being really wrung out on several straight-a-ways (probably should have used a little higher gearing.) Therefore my guess is the main jet was one size to small for the conditions. I believe Danny has already mentioned that. I can remember running a cool Saturday night race and taking the same bike to a race the next day when the sun was out and the bike would just blubber on top end. So the altitude and weather can really make a difference.

Keep us informed Danny.

[dcooke007]

Bill, you are on the money about the gearing. Could have used a little less gear. I think Will was pushing himself and the bike as much as he could. He passed the one guy and if you pay attention towards the end you can see another rider ahead he was catching up to. I think he had him in his sights and wasn't going to let him get away. Will did report he was catching up and his heart sank when the engine failed.

I had considered using the wombat 19 tooth 5th gear instead of the 20 tooth combat wombat 5th gear. Used the 20 tooth in the end.

Danny

[dcooke007]

Almost forgot the most important diagnostic issue. Too much flux capacitance from the tank emblems!!!! That would also explain why the engine had so much power with the minor port work and reed valve. You know that would certainly require a lot more fuel to live! GMC, I should have taken your 42 extra horse power info more seriously.

So what do you guys do when you have to much flux capacitance? Tape over one emblem at a time? Or are the emblems so powerful they have to be completely removed? I think I will re-attach these with Velcro so I can remove one or both as required depending on conditions.

Danny

[Zyx]

One more set of comments, and then I will let Danny do his magic.

Pistons are not round, as we should all know, they are an oblong shape because the metal thickness differs front to back versus side to side. Wider front to back when cold. So it isn't surprising that the crosshatch was still visible in some aspects and not others. I would have to ask where the ring gap is located on that piston. Can't tell or didn't see it in the photos, but the question is whether the ring gap is on the left side. If it is, the wide ring gap could be a catalyst for failure by letting combustion heat and byproduct through the gap, raising temps in that place, causing a cascade failure in the oil film.

As for the piston getting too large for the hole, I doubt it. 0.003" should be adequate, and anyway, engineering research shows that under normal conditions, the piston grows to a size greater than the bore. This may sound counterintuitive, but if the piston were smaller than the bore all the time, you would hear excessive piston slap, and the wear patterns would show excessive cocking on front and back of the piston, with similar scuffing on the bore at the top and bottom of the stroke. Another clue that the piston was metal to metal only at the time of failure is the scuff pattern below the ring wear area. Clearly the piston did not grow to the point that the sides of the piston were wearing on the bore, so I don't think it was a size issue.

Not familiar with the oil mentioned, but 32:1 should be fine for sport riding. Question is whether is was sufficient for full throttle conditions on that day. I don't think so or it wouldn't have seized. Kind of obvious. While I don't like to ride full throttle for extended periods, I am also not a pro MX rider, and in racing you do what you have to do. First time I ever experienced a rev limiter on my 5.0 HO motor in my off-road racer was during the start of a night race in Australia running head to head with someone else. The Hodaka doesn't have a rev limiter, except the one in our noggin, and during a race, that feature doesn't always work.

Assuming as I do that hot pistons operate slightly press fit and do not routinely seize solely because of the presence of the oil film, that leaves the question here of whether this was a failure of the oil film. I say it was. What precipitated that failure is a combination of over revving, lack of break in time, possibly the failure to rebore rather than ball hone the bore, and a too wide ring gap. The bore could have been out of round. Was it checked for this before returning it to service? Mixing oil at a richer ratio for racing is just insurance. Whether that insurance would have saved the day here, I don't know but it is worth considering.

[dcooke007]

GMC,
Certainly the bore cross hatch pattern wears more in the piston ring travel area and the piston skirt area. However the wear noted is more than would be expected for the modest amount of time this engine has operated. That wear, to me, would indicate there is room for improvement in the oil film / oil.

I still do not have the piston yet but I "think" ring gap is on the left intake area, same area as the intake side seizure....I think. Part of the process of determining what caused the failure is to determine where the failure first began. I do not believe this failure started on the intake side of the piston. The damage on the exhaust side of the piston is greater and more extensive than the intake side and would indicate the initial area of failure. If the failure started on the intake side at the ring gap I would expect greater damage in that area....but it isn't. I do not dis-agree with your conclusions about excessive ring gap, but in this case I do not believe it to be the cause of failure for the reasons stated.

The cylinder bore was measured for diameter, roundness and taper in several areas. Nothing stood out as a possible concern. The metal transfer in the exhaust port area from center to left raises a question as to why the metal transfer was not more centered on the port opening. Perhaps there was a high spot in that area of the bore or a higher concentration of aluminum in that area of the piston. At this point it is not possible to tell.

In the end all failures of this type are at least partially and sometimes totally a result of oil film failure. No oil / ratio I know of is capable of with standing any and all types of load. The operating parameters of the load applied must not exceed the capability of the oil. Some oils are better than others.

My opinion is the failure began in the area of the exhaust port due to excessive heat. The resulting failure whether caused by reduced clearance exceeding the limits of the oil film or exceeding the melt point of the piston, your guess is as good as mine. As has been mentioned everything goes down hill from that point. Cause? Lean fuel mix combined with very aggressive operation and maybe a high spot in the bore diameter in the area to the left of the exhaust port.

You know what they say about opinions.

Danny

[michael_perrett]

I set up a model 98 Rat for my son David to race in motocross. Other than lightening the bike, the engine was stock except for a Harry Taylor pipe. At one of the last Mid Ohio races we attended, David was entered in the intermediate class. In his race, they started four expert riders 20 seconds ahead of the group of 15 or so intermediate riders. David got a mid pack start and worked his way past the intermediate riders. Since he had time on his hands, he decided to go after the four experts, which he passed.
The reason for stating this, is that at the beginning of every race I told him to "run the bike as hard as you can, you won't hurt it". We run ordinary Pennzoil 2 cycle oil mixed at 12:1. We never had a seizure.

[hodakamax]

In our quest for more power in a race engine we are always trying to burn more fuel/air mixture. When we replace fuel with lubricant we are always losing power but a balance must be achieved. One thing in our favor is that lubricants have vastly improved with synthetic design especially in the area of heat related failure. Two-strokes of today commonly burn 50:1 ratios because of improvements in lubrication qualities. If we pour our best lubricant onto a pool of molten aluminum it's easy too see that we have (as Danny said) exceeded its limits of being a lubricant. I'm sure we all agree that excessive heat was the problem in this case. I also think we all agree that a lean condition is providing less lubricant than a rich one (and less cooling). Less lubricant causes more friction and the vicious circle begins. Once the piston starts to drag molten aluminum even more friction causes even more heat and so things get even worse. I agree with everyone on this case but I'm sure that it was not just one cause for failure but many, but the cause of the seizure was the melting of the piston.

OK, thats my ramblings for the day. I'm just trying to improve my writing skills and this is the current subject!

Later

Maxie

[michael_perrett]

Perhaps you did not understand my posting. Anyway, back in the day, I purchased a used model 97 set up for short track from the Hodaka dealers son. Money was no object for him, but he decided to stop racing short track. Had the bike for sale. I knew the bike very well and also his repeated time of destroying the engine racing. When I purchased the bike, I raced it, I used a oil/gas ration of 8:1 using Pennzoil 2 cycle oil. I did not have any trouble at all with the engine, only changing the ring occasionally. You think I had no power? I have a row of first place trophies to show for it. You need to research an old copy of Cycle magazine, where Gordon Jennings did a test of oil ratios. He found out on the dyno the increased oil ratios not only increased horsepower, but reduced temperatures.
New oils are better? I have recent experience to prove otherwise.
I challenge to anyone with a model 97 to a horsepower test against this bike, which I still own.

[DGardner]

http://www.klemmvintage.com/oils.htm
http://www.bridgestonemotorcycle.com/do ... remix6.pdf

Here's some good reading on oil and ratio's.

[hodakamax]

Hmm--good points and info. I was a Gorden Jennings fan. We also tested different oils of the day and Golden Spectro (I think that's how you spell it) did seem to increase power. Its downside was that it had no rust protection. We suspected that it had something in it other than lubricants. I'm open minded and listen to all theories but it's hard for me to imagine that lubricating oil would have more energy of combustion than gasoline. I don't question Jennings findings, I just wonder how that could be. Maybe again his test engine was too lean in the beginning and did require more lubricant. Lean engines do sometimes have more power, they just run too hot. On the subject of new oils being better I would hate to go back to single weight petroleum base lubricants compared to new designed synthetic lubricants of today. Synthetics have the advantage of design for special purposes. The reason they were designed was to meet the specifications of turbines and jet engines (and 2-strokes). Not only are they slicker but they exceed many other specifications such as viscosity vs temperature and resistance to high temperatures (all good for 2-strokes).
Anyway, this is only Max's opinion today. What's everyone else's opinion on this one? (keep it short AZ) Just kidding! This could be another fun discussion!

Maxie

[dcooke007]

Pandora's Box......

Danny

[hodakamax]

Funny Danny---You're probably right again! (actually, really funny, I'm still giggling).

Max

[Zyx]

Synthetics were developed long before the advent of jet engines (1929, with advanced development between 1939 and 1944), and the reasons had more to do with WWII shortages than high performance needs of radial engine bombers.

A guy puts a two stroke on a dyno and determines that an oil rich mix makes more power, and this is a problem why? Cooler engines make more power. Better lubricated engines make more power. Better lubricated engines running cooler make less power? Why would that be? I don't imagine the "more" part is all that much, but have no problem believing that a well lubricated engine can out power a lean running one. One of Harry Taylor's points was just that same point, that 20:1 out lubes 32:1. I don't see the down side. Plus, assuming you are racing on 20:1 not just putting around the block, you will be creating a need for that oil rich mix by running flat out. An idling two stroke actually doesn't need oil premix. It will run fine with just gas. But running flat out, it needs lots of oil, demand for which in a two stroke is a sliding scale from low to high. Which is why I have suggested that while 32:1 is a fine compromise for sportsmen, racing demands more oil. Doesn't really matter which brand, or how modern.