This is a common problem on the 150 cc and larger Wiseco forged pistons. Larger pistons collapse much quicker than the smaller pistons. On the CR 500, KX 500 and LT 500 it is not uncommon for the major piston diameter to be .003 to .005" inches smaller and the diameter of the piston parallel to the wrist pin to be .003" to .005" larger after one hard riding session. What is happening to the pistons is the same thing that happens to a cardboard toilet paper roll when it is squeezed. (Gets smaller in one direction and larger in another direction.
Wiseco has a list of complaints a mile long from me over this problem on a lot of different types of two strokes. I have not complained much in the last ten years since Dover Diversified bought, Wiseco, JE Pistons, Vertex pistons, Carrillo Rods, and there was talk that they also were going to buy Pro X.
A guy from Dover called me right after they purchased Wiseco wanting to know if Wiseco ever found a remedy for the collapsing problems I was having with their pistons. The answer was a big NO. He said that they were not gong make any design changes on the old two stroke stuff unless I was willing to pay for new forging dies. I think my design would be a little heavier but it would probably solve the collapsing problems they are having with their current 40 year old big bore two stroke piston designs. They also agreed that my design would probably solve the problem but they were not willing to invest any of their money on these obsolete two stroke engines.
Using the numbers they gave me for custom forging dies, and small runs of pistons (anything less than 100 pistons): If I spread the cost of the new forging dies over 50 to 100 pistons, plus the cost of small runs of pistons of around 50 pistons of one size would put piston prices at about $250.00 to $300.00 each. That was 10 years ago when I did the numbers. I do not know what they would charge nowadays for the custom dies and small runs of pistons. If I could do it for less than 300 per piston, guys might buy them because guys are standing in line to spend $250 to $300 for four stroke pistons from Wiseco JE, and CP.
Not long after Dover bought JE Pistons, the key designers left JE and started CP pistons. CP purchased Carrillo Rods from Dover a couple of years ago and now CP is in the process getting Carrillo's products back up to par after being owned by Dover.
