ORIGINAL: downunder

One thing that I and a few others have found is that a piston can be quite free of any pinch without the head on but as soon as it gets tightened the liner distorts and you feel a pinch. This doesn't happen until the screws are being tightened beyond the initial barely tight stage though. Each time I tighten a screw a little more I turn the piston over TDC to make sure it still feel the same. If it goes tight I'll back off that screw and try the next in line.

Seeing your engine still goes tight after adding extra shims it won't be the piston hitting the head so try tightening the screws in very small stages and check each time you tighten one.