If you notice those pistons have the small grooves machined in the skirts to retain oil between the piston and cylinder. Those really aren't that great of pistons but I don't think that was the problem. In my opinion it was all due to poor machine work. If you have the ability to double check the specs of the machine shops work before assembly than you know everything should be fine. Most people don't think twice about doing this. Here's the piston clearance specs from the merc manual.

The pistons are not the problem, I had run those pistons in my old cigarette with bm 250's and single 850 's making 5 lbs of boost for over 600hrs and no failures. I have also done numerous motors with the same pistons in a blower marine application that have lasted for many hours. I do not think your getting 135-140 deg water temp without a thermosat in the northeast waters , probably more like 120. Thus you are not getting the block to expand enough with your current piston to wall clearance. I think if you go to 6.5 that u said you were going to, you should be fine.

Tom thank you for the input, I had restrictors in the intakes to get a little temp in them and the gauge registered 135-140 but I never verified with any other gauge or laser temp gun.

hey Jamie, you should also enlighten the readers as to the fact that you are upgrading to Mega Blowers (420'S) which is almost equal to an 8:71, with twin DP Holleys

like this ?:whistle::party-smiley-004: