There sure are differing amounts of the pinch, my Irvine .53 and the above mentioned Fox .74 never had any pinch, my Russian Zeus .61 had so much that it could not be flip started, it would fire and the piston would stick on the next compression stroke. My MVVS .61 will push the liner out of the block as soon as the piston gets past the exhaust port. My Fox .46 and MDS .48 have a lot of pinch also. And the amount of pinch or the lack of it doesn't seem to effect performance or longevity. Based on my "sperience".