+1

Oups miss understanding, excuse my english.

.004 is way too tight on an open cooled marine SC engine in my .02. That would be the root of the problem. Easy fix though with some new pistons. If you are going with a bigger blower, than the new pistons give you a chance to lower compression a bit anyway. The little blower needs the squeeze though, so if you are staying with 174 I'd go up in compression. And of course you need to set the quench to .040 :whistle:

I had very bad luck with the 250 B&M blowers and I NEVER will run a blower without duel carbs. Ever. Period! Always leaned out Cyl. 5 and 7. Too much heat.

I agree litle blower make much heat but the B&M 250 depending on the engine ci and blower rpm could make some great power at 7-8 lb boost non intercooled.

I also had issue with my B&M 250 blower on 454ci with single carb setup and offset carb plate, all 4 rear cylinder was lean. I will run twin carb setup now.

Has for the wrist pin i have notice on my engine that i had a tight pin in the melt #6 piston, that is normal after a melt piston, but if lil red have 7 on 8 tight piston wrist pin and had only 3 scuffed piston (surely due to a piston wall clearance issue), no sing of detonation what can make that tight wrist pin???

Can be not enought clearance between wrist pin to clearance in piston during the assembly??

That is one thing I'm still wondering, I did not have a chance to call dean yet, speaking with one engine builder, they had found too smooth of a cylinder wall finish to not retain enough oil and cause issues. This was on high ductile dart blocks that clog up typical fine honing stones and should not be an issue with an oem block

The upper rod bearing will tell you long before the piston will if you are detonateing in a boosted motor.

Are you saying you need umbrellas on your pistons? Or its raining out on your terrace?

Lmao damn smart phone!!!
Not retain enough oil!!!!

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: