Piston Nightmares!

Just trashed a set of MTC 1075 pistons. Still not sure what happened. Bad scoring, lost compression after a short run. If debris got in there I don't know how. Piston clearance was .001" to .0012" (MTC says .0015")

I'm not doing MTC again, had problems before. Got the Wiseco set. Problem now is after honing the scores out of the bore I'm looking at .0028" clearance (Wiseco says .002", Kawasaki says .0016" to .0027"). I only honed the worst looking cylinder so far and didn't mic all pistons yet. So I may be able to get them a little better juggling, but nowhere close to .002"

There is no problem with MTC pistons. Have set many of them up. Sounds like the ring gap was too tight and they locked up. On a 1075 you would want to see a minimum of .008" gap.

Bore clearance should be 0.051 mm = 2 thou of an inch.
This is what we use on both MTC and Wiseco but as Jay suggests, the ring gaps may have been too tight.

Ring gaps to tight nothing wrong with MTC Pistons.

End gap was definitely not too tight. The formula says .012" top .015" 2nd for a street 72mm bore. I even asked MTC directly and they said bigger (.016" and .022" 2nd). I compromised and set them at .015" top and .021" second, Plus, these rings in a true 72mm bore (which mine are) are pretty much plug and play., none less that .011" out of the box.

I guess the opinion is I shouldn't use this block if I can get tighter than .0027" clearance. I could with the MTC pistons because they're .001" bigger than Wiseco, but I really don't want too.

Something else I noticed. Piston measuring point (widest spot) on MTC pistons is directly on the bottom edge of the skirt. Meaning the edge centered in the mic anvil. Wiseco is 1.3" down from the oil ring groove which puts it above the edge. So if you mistakenly measure an MTC piston at the Wiseco spot, you'll get about a .001" tighter clearance than you were shooting for.

.0027" clearance may not be optimal, but it won't cause any problems either. Main thing is for the bores to be round and with minimal taper top to bottom.

Bores are perfect. 72mm straight and round. Only a few miles since bored. Just worried about going on the high end of clearance to start.