You didn't compair the new Pro-x to the Wisco BEFORE installing it? That is what I always do, compair the new part to the old to make sure they are the same befor putting it on the bike. If you had, you would have seen the difference and should have investigated further. Say like mabe ordering another Pro-x and seeing if it was the same or not, could be you just got a bad one. If it wasn't off by much, you could have ground down the skirt to match the length of the Wisco. IDK just saying. I've used both kinds and never had issues with either. My good friend who passed away 2yrs ago, was a kawasaki factory mechanic for over 30yrs, he absolutely hated wisco pistons and used pro-x exclusively. He was also a AA pro Trials racer/rider. He never had a job come back to him, so IMO, Pro-x must be doing something right. When I buy a new piston myself, I look to Pro-x first then wisco, then some of the other not so well knowns...usually buy whatever is on sale at the lowest price. Like I said, I've had good luck with both. I'd also check your cases for mismatch or damage.