How to measure a piston

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JimKlaas - 07/01/12 at 4:39pm

1. What is the correct way to measure a piston? Top, middle, bottom? should they all be the same? Front to back or side to side? Should they be the same?
2. Same question but for the cylinder?
3. I would also like recommendations brands for"
a. Ring Groove Cleaning Tool?
b. Ring Compression Tool?
Extra Credit.....what is the Stock MM on a 94 Wombat early frame number? I see pistons listed for +1 etc but I don't seem to be able to find the starting point.
I measured mine front to back, FRONT being the way the arrow points on top of the piston.
Front to Back at the:
top 55.74
middle 55.90
bottom 55.99-56.0
so....ummm what size piston is this or was it...I am a little surprised the measurements are so different top to bottom...
Jim Klaas

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Bullfrog - 07/02/12 at 1:36am

1. Measure diameter at the bottom of the skirt - front-to-back. (no need to measure anywhere else) All two-stroke pistons are made with built in "taper" (smaller OD at the top where the piston gets hottest and "grows" more).
2. When measuring the cylinder for wear (to determine whether or not it is time to bore) - the primary thing to check is bore diameter near the bottom of the spigot (front-to-back) AND bore diameter just below the upper limit of the ring travel (front-to-back). If the difference in these two measurements is .008" or more - it is time to bore due to excessive "taper"/wear.
When measuring for piston fit upon completion of a new bore job, the primary cylinder measurement is front-to-back near the bottom of the spigot - If you don't want to assume the bore is straight and round, you can take multiple measurements where-ever you'd like.
3. I have no recommendations. I've always used a broken ring for groove cleaning . . . and have never used a ring compressor tool (but I couldn't do that without the "U" shaped piston support tool which "straddles" the rod and provides a "square" surface for the piston to rest on as top end assembly proceeds.)
Extra Credit Question: Std. bore (cylinder) is 56mm for Hodie 125cc engines. (50mm for hunnerts)
1st over is 56.25mm
2nd over is 56.5mm
3rd over is 56.75mm
4th over is 57.0mm
(each size of piston ought to be slightly smaller than the above numbers)
Ed

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JimKlaas - 07/02/12 at 12:25pm

(but I couldn't do that without the "U" shaped piston support tool )
Ok...what is that called and where can I get one?
Jim
PS Broken ring works GREAT! I didn't have one when I asked but I do NOW!!!

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