PM T2 wrote:regarding the piston fit - I'd suggest measuring the bore with a telescoping gauge and micrometer, then measure the piston skirt to verify the clearance. Using a rule of smashed thumb, I usually suggest it should be in the vicinity of 0.002" of clearance per inch of bore diameter.
if the piston to bore clearance checks out, look at the radial thickness of the rings next. The gap might be correct, but if the radial thickness is excessive, the sealing surface of the ring will be proud of the ring groove surface.
Checking the fit of the rods is a simple enough thing to do. With the rod removed from the piston, clamp the rod onto the crank and tighten to torque spec. With the crank laid into V-blocks (or held with the crank axis parallel to the floor) put the rod at the one o'clock position. If it falls quickly to the bottom and swings back and forth, its too loose. If it doesn't fall at all, its too tight. Proper fit for operational purposes is attained when the rod swings from the one o'clock position down to six o'clock at a controlled rate, and doesn't flop around loosely at the bottom.
Hope this helps.
Thanks for the advice. Relieved to report that everything checks out OK dimensionally using your recommendations. I also checked the dimensions on the donor block's cylinders and crank journals and they match my project motor (I believe the donor power head came off a running motor).
I did find some carbon buildup in the ring grooves under the rings. I think that might be a big part of the problem considering the width of the rings. I think cleaning up the grooves and some more bore polishing might help a lot. I'm sure I got all of the grit from the honing stones out the last time, but I'll make sure I really scrub it this time.