Piston Clearance

[Brett]

I went for a ride on the weekend and lost third gear.  Rode home and pulled it apart and had seemingly smashed one of the circlips which aligns the third gear on the output shaft.  I thought while it all disassembled would check out top end clearances.  Motor has 86.5 bore (has been sleeved) of Wiseco piston type. I measured the piston and found that it was out of round in the wrong direction. (its biggest dimension was on the non thrust axis) Then i measured the piston to bore clearance and was astounded at it being 190 microns (.19mm). I have never dealt with pistons of this diameter before but i am sure this piston has collapsed.  I would have thought the clearance would be about 80 microns.  The fact that the rings only have .4mm gap shows that the piston has worn excessively.  Could someone please advise what the proper piston-bore clearance should be and what ring end gap should be.  Also are there any piston manufacturers that do o/s pistons besides Wiseco.  I would prefer a cast piston as they are more rigid inherrently in design.

[John]

The piston clearance is different depending on make and size. Your best shot would be to look at the origninal box or call Wiseco Technical Assistance 1-440-951-6600.

//John

[hughes]

Remeasure your piston. You need to measure the area closest to the wrist pin hole and mic it. Next move to the intake and exhaust side of the piston and mic the skirt area. Their is more material in the wrist pin area which lets the piston grow as it is heated. I'm out of town now and do not have my manual with me. Sorry.

[Brett]

Sorry to sound like a wanka but i actually manufacture pistons and therefore know which way the piston should be ovaled.  (We have a family business making racing pistons for 100cc two stroke gokart motors in australia) The piston i removed from my motor has definately collapsed and from speaking to a few people in the trade they inform me that wiseco pistons are renowned for collapsing in the larger bore motors.  This i have no doubt is due to the forged design rather than the materials or quality of manufacture.  I have since bought another piston which is as all twostroke pistons should be oval with the greatest diameter perpindicular the the gudgeon pin axis.  From the box i have found out the standard clearance for wiseco piston is 100 micron.