Model 95 replacement piston

[HenrysHodo]

My last race at Millcreek, AL revealed that the motor on my CW is just worn slap out and out of power. I took it apart last night and measured my clearances and I would like your opinion for a course of action.

The cylinder was measured using T-gages and a micrometer three times at each area: top, center, bottom from front to rear and side-to-side. All measurements were consistent at about 2.115" some variation existed but that was no more than .001. The piston skirt measures at 2.110" ~ 2.111". There is no scoring or defects in the cylinder walls.

My question is, can I get away with installing a new ring with a light cylinder cross hatch? Alternatively, replace the top end without a rebore in hopes that a replacement new piston of the same specification is .002 to .003" larger to close in my current .005" piston to cylinder clearance? What is the size of a new replacement piston? I am assuming that the piston I have is a +.020.

One more question: When I pull in the clutch at a stop with the engine idling transmission in neutral the engine rpm slows. Is this a pending problem with crankshaft side play or not an issue? I am using a PVL ignition.

Thank you

[Zyx]

Standard bore is 2.20" (56mm). Doesn't sound like the bore is worn out, and it isn't 20 over. Without seeing it, it sounds more like the piston and ring are worn. Assuming your measurements are correct, replace piston and ring with standard units. Piston clearance can be 0.004" and be considered normal, but some like a tighter setup. Depends on who's piston you are using. Did you measure piston skirt clearances with the piston in the cylinder, or just with micrometers? Mathematically, it sounds not worn out. But put the piston in the bore and verify skirt clearance with a feeler. You might find clearance is more like 0.006", in which case replace the piston and ring. Bore in good shape, just do a deglaze.

[Bullfrog]

By my calculation, 56mm = 2.204" -- Though I don't recall whether or not factory bores were exactly that dimension (certainly there were tolerances). So you aren't working with a "20 over" bore. It seems like the measurements must be re-done since you came up with a bore which is smaller than standard bore by about .009". That just doesn't seem reasonable.

Taking your identified clearance of .004"-.005" at face value (even though I do not trust it) - - I would have to wonder if something else has caused your drop in performance. At that clearance, ring replacement is certainly do-able if you want . . . and piston replacement is not yet indicated.

However, please recheck your measurements.

Ed

[HenrysHodo]

Thank you for your prompt feedback. I will go back and recheck my measurements and repost tonight. Thank you all

[Zyx]

56mm = 2.2047244" but that is assuming that when the factory states bore is 56mm that it is EXACTLY 56mm. Factory specs stated in inches is 2.20" which is correct if only taken to two decimals. But if standard bore is fractionally larger or smaller than 56mm, making decimal references more than two places is guessing at best. My NOS model 03 cylinder measures 56.00mm at the skirt, and 56.04mm at the top of the bore. I have no idea what factory tolerances were, but clearly, the bore is simply something like 56mm standard.

I agree this is not a .020" overbore, and also don't trust measuring piston clearance by math using a micrometer. It is too easy to find a spot not exactly on the diameter of either piston or bore. Best bet is to put the piston in the bore and let the real world inside your engine tell you what the clearances are. I am guessing your bore is good, your piston shows normal wear, and that performance loss is from something else unless the ring is totally shot. Check ring end gap in place in the bore. Verify timing, etc. check the exhaust system for bird nests, rat nests, wasp nests, or whatever.

[junker2k]

56mm = 2.204724409

[HenrysHodo]

Thank you all for your input - Reality check! I found that my micrometer (purchased at a swap meet) was way out of calibration. Lesson learned.

Here are my new measurements: Piston=2.217 Cylinder=2.224. This was verified by using a feeler gauge between the skirt and piston which shows exactly .007 clearance. With these numbers it appears that I have .020 over piston worn about .003. Since I do not have a new .020 piston in hand to measure I think prudence would necessitate going .030 over and start fresh.

I know Wiseco recommends a specific clearance but do you use anything else based on personal experience. I will go back through all checks as stated to check the reasons for power loss. Once again your responses are appreciated. Hopefully I will see you at Barber. AHRMA #60L.

[Zyx]

0.007" clearance would yield a noisy piston and probably low compression. I have one with 0.008" and while it runs, it is way off its peak.

I believe clearances do differ some as between cast versus forged pistons. A manufacturers recommended clearance is a broad guess at correct clearance, and is not an absolute. Modern water jacket two strokes would do better with a different clearance than air cooled, even using the identical piston. Lots of folks here are using Wiseco so ask what works if that's what you intend to use.