Model 95 new piston rings and bore

[HenrysHodo]

I am doing a top end on my CW using a Hodaka piston and ring. I would like recommendations on what clearance I should put between the cylinder and piston? Ring end gap?

I use this bike to race the AHRMA circuit, which means I ring its little neck out every ride. The last top end lasted about 15 moto's before it ran out of power. I am not rebuilding due to seizure or mechanical malfunction. In fact, I found the piston and cylinder to be remarkably clean and free of any scoring upon disassembly. I do not know clearances I started with because I sent it off for machining last time. I use 40:1 Yamalube with 90-octane ethanol free gas.

Does a 15-race life span sound about normal? Recommendations?

[Bullfrog]

Using a Hodaka (ART) piston in a Combat Wombat. Here's what I do. If I need to put it together and go to the track and race, I ask for the clearance to be set up at .002". If I have time to do proper break-in (several cycles of bringing the engine up to full operating temp - including some quick excursions to max rpm . . . with cool down to ambient temp. prior to the next cycle), then I ask that clearance be set up at 0015".

Ring end gap? You'd better check me on this (don't have my manuals handy) - - .006" is what I remember.

Ed

[hodakamax]

I am doing a top end on my CW using a Hodaka piston and ring. I would like recommendations on what clearance I should put between the cylinder and piston? Ring end gap?
Does a 15-race life span sound about normal? Recommendations?

Rings usually wear out faster than pistons. I'd recommend looking at the ring end gap every few races. With a good air cleaner and seal, piston life should be a few seasons or even longer. Replace the ring any time you see excessive ring end gap.

Recommended ring gap on a Wiseco piston is.004" per inch of bore. As Ed says, better check the specs for a Hodaka piston. (I can't seem to find the info either.)

My humble opinon,

Maxie

[Zyx]

Original specs as shown on the website:

Piston skirt clearance 0.003"
Ring gap 0.004 to 0.012"

This is assuming using stock Hodaka piston. Skirt clearance is a bit broad by modern standards, and may have been driven by the oils then in use. Ed's suggestions sound right. 40 to 1 is a bit lean but if it works for you... As the premix sits around it will richer up some anyway unless you only mix what you need for the weekend.

Ring end gap should not be different now as opposed to then. Gap is there to make sure the ring does not swell enough to touch ends, which leads to seizure. Too much gap can also cause a seize point due to blow by. I would think 0.004 to 0.006 to start should do.

[racerclam]

Ring gap shoiuld be .003 to .004 per inch of bore regardless of what your working on so .oo4 would be a no no

Rich

[Zyx]

Rich,

I am only repeating what is shown in the Hodaka spec sheet for the CW. Expansion of rings is a function of several things, from ring style to mass to metalurgy to riding style. I believe that regardless of your build target, no one wants a ring to expand to the point of end contact. What that gap needs to be could be a matter of individual application, but Hodaka would not be able to control the variables, so no doubt their spec is conservative in that if set to 0.004", over expansion is pretty much precluded. Less than that may put the builder in the experimental engineering world. You have built more engines than I have and I don't doubt that a smaller gap is doable, but is it necessary? I don't think I would go so far as to say that Hodakas stated minimum gap is a no-no. For the average builder it might be considered tight.

Then too, it takes patience and practice to file ring gaps correctly, so even if the gap is, at its closest approach, 0.003", if not cut squarely, the average gap could be greater or leak compression a bit even if the gap appears to be tight. Some leakage is acceptable. Bound compression rings aren't.

Also consider that Hodaka's stated maximum is 0.012", which is way more than 0.003". I would consider such a gap excessive, or a sign of wear if it wasn't that gap to start with. I don't know for sure but would guess that Hodaka was suggesting setting the gap at 0.004" and pulling the ring from service if it got to 0.012" during use.

Factory specs would never be intended to define the ragged edge of performance, but instead offer guides that, if conformed to, would offer good service and long life, with minimal failures or warranty claims.

[racerclam]

The package from Hodaka with the orange label made from poppy actually specifies .003 to .004 per inch of bore that is also the recommended spec from Wiseco. The Hodaka manuals ar known to have mistakes , one is the 175 inch lb torque spec for the super combat Harry had told me its bogus and should be 225 in lbs . Another one my Thunder dog manual says the bike is reed inducted Definatly not true. I personally set my gap at .008 on the 125

Rich

R

[Zyx]

I have always used 0.004" on the 125 ring, both Webco and Hodaka.

Guess I missed the "per inch of bore" part of what you said. I would think that to be too wide.

[racerclam]

I think you have been lucky , if ran hard enough to get really hot it wouldnt be good , fortunatly un like nicasil and iron bore will widen the gap kinda quick.. But now you know and it will be different , just like the coyete chasing the road runner off a cliff , he don know he is in trouble until he looks down. When working at the shop in vegas I came up with the term coyete syndrome for things I have seen customers get away with just because they were un aware if things I could never get away with , weird !

Rich

[Bullfrog]

I finally got my hands on my Official Hodaka Workshop Manual for the Wombat (94) and Combat Wombat (95) to look up the ring end gap specifications. Ring end gap for both models ("as built" using ART pistons and rings) is listed as .004" - .012". My interpretation of that is that you shouldn't set up ring end gap tighter than .004" on a new bore . . . and that if you get into your engine and find ring end gap of .012" (or more) it is clearly time to at least change the ring. Of course, many folks will want to change the ring well before reaching .012" end gap -- and if you find .012" gap, you will almost certainly find excess piston clearance issues too.

So, my memory was not correct in my earlier post. And you may rest assured that I have followed the .004" ring end gap specification on my own Hodaka 125cc engines - without any problem. NOTE: My basement parts mine has supplied all the pistons/rings I've needed - so I am still running ART pistons/rings (or mayhaps "Poppy" rings).

Ed

[Zyx]

.... if ran hard enough to get really hot it wouldnt be good......Rich

Don't know what to say. But running hard and getting hot have been about half the time I ride. It's not like I baby the bike. I finished the first moto in Athena this year pegged out in third gear for the last 150 feet. Can't run harder than WFO in third gear. Finished the Daniel Boone Hare and Hound in 1974 in 8th place after starting about 400th in the pack, which suggests a bit of hard running somewhere along the line, and bike and cylinder where packed with mud. I have always followed the Hodaka book and set the gap at 0.004" to start. Can't say what it opens to after a breakin because I have never thought to check it. When I pull the top end off the 97 this fall I will check clearances.

But if I haven't had problems, and if Ed hasn't, and if others following the manual haven't, maybe it isn't an issue to set the gap at 0.004".

[racerclam]

Maybe but I wouldnt get away with it , do what works , note My friend at wiseco has told me that on the dyno they havnt found a measurable loss at up to .050 gap. But that dont mean wore out to that but set up at that , big difference.

Rich

[hodakamax]

A new bore opens the ring end gap fairly quickly as parts match together and the cylinder walls smooth up. .004" seems too tight for a properly broken-in engine. On our short track race engines the ring gap was restored to .008" between race events. Sometimes a one oversize ring was needed to accomplish the requirements. As I recall, the ring was changed about every 2 to 3 events when the gap reached about .010".

On a new re-bored cylinder you probably could get away with .004" with a proper break-in whereas on a broken-in cylinder I would go with .008" end gap. It would be an easy and interesting experiment to test the compression with different gaps to see how significant the gap is to compression loss.

Only my opinion--

Max

PS--I see that Rich has mentioned such a test while I was typing. That test would have more meaning if we knew the bore.

[racerclam]

I dont know the bore size they were playing with

Rich

[hodakamax]

If the test was done on a 427 chevy engine with a 4.125" bore, the proper ring end gap would 4.125"x.004"=.0165", (.004" per inch of bore.) exceeding the maximum recommended end gap for the 100cc 2 inch bore of .012". I doubt a 100cc engine would run very well with a .050" ring end gap as mentioned in the Wiseco test.
Just an observation--

Maxie

[taber hodaka]

On the ace 90 or 100 I shoot for .005 ring gap. when the cylinder is worn I file down a over sized ring, I am talking a L dikes ring here art racing piston. I just pull the head and check to see how much the ring is worn. When I need a new ring and the cylinder is clean I do not hone. I really didn't break in my race engines and started all my races with a cold engine no warm up. I do not recommend this to anyone else. But for me with about 75 trophies motocross and cross country with -0- engine failure it wernt bad. Clarence

[Zyx]

Couple of observations for what they are worth:

Wiseco pistons these days are forged, not cast, and the design for their modern pistons is targeted at current production motorcycles, all of which are water cooled. Forged pistons require different clearances than do cast pistons due to metal density differences, and water cooled engines create a very different operating environment than do air cooled vintage engines. Moreover, Wiseco recommends the same gap (0.004" per inch of bore") for both two stroke and four stroke applications (and apparently cars as well) and does not differentiate for air cooled applications, probably because they don't really make pistons for air cooled applications anymore. Rather, they adapt modern designs to vintage bikes. Further, Wiseco is taking a conservative stance in their public recommendations with regard to application clearances because, as they acknowledge in their FAQ section, rings that close the gap to the point of contact will cause engine failure, and a larger gap is preferred to one that is too narrow. All true.

But here we are dealing with a single application, factory OE piston and ring designed by their engineers and offered as factory replacement parts. Hodaka could afford to be very specific about clearances because they were dealing with a very limited very controlled set of circumstances surrounding an engine they had designed. It wasn't necessary for Hodaka to suggest a clearance based on broad uncontrolled applications, and it was thus not necessary to take an overly conservative position, or to state a general rule that kept everyone out of trouble no matter what they put the parts in.

For example, in the Ace 90, Hodaka suggests a top ring clearance of 0.006" to 0.014" and a bottom ring gap of 0.004" to 0.012", and suggested these clearances for the 100's and 125's of similar generation (that is, up to the model 94). By the time the newer generation of machines came along, clearances had tightened to 0.004" ring end gap because cylinders had changed from iron to alumiferric. These clearances have been working for Hodaka engines using Hodaka parts for a long time now.

But Wiseco doesn't make motorcycles or engines, just pistons, and so unless they want to do engineering studies on every conceivable application, they would need to find a recommendation that was safe no matter which engine their piston may be used in. That's why they have come up with what they have. Wiseco has no way of knowing what the lower limits of clearance might be without doing a great deal of testing, and there isn't a reason for doing that since, as they say, you can get away with more gap without risk.

So if I was building a Hodaka with a Wiseco piston I would tend to follow their recommendation. If building with genuine Hodaka parts, I would follow the Hodaka recommendation. If I was building a 5.0L engine using OE Ford forged pistons, I would follow Ford guidelines, and if using KB hypereutectic pistons I would follow KB guidelines.

So I don't fault the idea of using 0.004" per inch of bore when using Wiseco pistons, I just don't think those same guidelines pertain to OE Hodaka cast units.

[Darrell]

There's been lots of back and forth about ring gap. That information certainly applies to complying with rebuild specification, but what about compression testing before tearing down the top end and physically measuring the rings for wear?

The last time I compression tested my 94 and 70 the reading was around 150 psi each - though that probably ballpark as calibration amongst different compression testers probably varies a few pounds up or down.

How low of a compression reading indicates that it's time for a top end tear down and inspection? I don't recall that my Hodaka manual contains that info.

[racerclam]

Ok guys I just spoke with my friend at wiseco and the test with rings gaps was done on a Honda CR125 and he said that it was betweeen .045 and.050 where they bagan to see a power loss. Also here is a real HODAKA ring package with the info printed on it for ring gap

[racerclam]

oops! Here it is

[Zyx]

Not an official source I suppose, but my Clymers manual from 1980 suggests compression should be from 155 to 170 psi. Also suggested is the old trick of putting a squirt of motor oil in the spark hole and testing again if the reading appears to be low. What this does is help to seal pressure bypass at the rings, and as a broad rule, if a squirt of oil makes pressure jump up significantly, it is time for rings.

Compression reading are subject to variation due to slow kicking of the engine, atmospheric pressure, engine temperature, clogged exhaust, leaking base and head gaskets, and probably a few other things that don't come right to mind. Test compression with a warm engine, so run it until full operating temp, then test, and keep in mind that compression with the engine running, especially at high speeds, is going to be quite a bit higher than static compression, so even a 150 psi reading isn't all that bad for general use bikes because they will operate at more like 160 or more.

I don't find compression testing on high performance two strokes as valuable a tool as it is on four stroke motors, simply because the two strokes are piston ported which means that the cylinder only makes pressure during a relatively short portion of the full stroke due to uncovered ports and overlapping port timing. Usually there are other symptoms that arise if compression is too low. Poor idle, hard starting, repeated plug fouling where it didn't used to do so. By the time you get to that point, your compression is probably lower than 150 anyway, and it is time for a top end overhaul.

Also, if you are not building a special use monster, high compression isn't necessarily your friend. It demands higher octane fuel and shortens service life. So if your compression tests within a normal range and starting and performance are good, I would not attempt to boost compression just to get a higher number. The amount of increase in power attributable to just a higher compression ratio is negligible. You have to do everything together with high compression to get all th benefit.

[Zyx]

Even that gap is smaller than Wiseco parameters.

[racerclam]

THat ring package I just posted is not wiseco but actual Hodaka . Ok Im done now

Rich

[Darrell]

Arizona,
I compression tested my Hodaka's while cold, dry - no added oil, and with the throttles open. Is an open throttle as necessary to obtaining an accurate reading from a two stroke as a four stroke engine?

Both bikes have 3,500 miles on the original top ends now.

Thanks!

[Bullfrog]

Yes. Throttle should be wide open when doing a compression test on two strokes also.

Ed