Shimming main and rod bearings?? Am i crazy?

[Che70velle]

Well...that's a funny subject to talk about among engine builders. Longevity, versus high rpm/power. You can't have both. It's a one or the other deal.
A race engine, for instance, will have an oil pump that will make up for any loss of oil pressure caused by setting up bearings a "little looser". Adding more bearing clearance does free up power, and helps keep oil temps down. Now if I'm selling engines that are supposed to go a couple hundred thousand miles, and I give a crap about my reputation as an engine builder, then I'm gonna set up my bearings to the tight side of spec, because bearings wear over time. If you add boost into the equation, or a high compression naturally aspirated engine, it changes things as well. These type of engines are hard on rod bearings, which is why you often see aluminum rods used in extreme applications. The aluminum rods take a lot of shock from the rod bearings, and crank as well.

[David Pozzi]

I'd carefully sand the main cap on a flat surface. I don't think it would take much to remove .005" & increase bearing crush. I'd look into the bore specs & see if you can safely reduce the bore a tiny bit.

[TheJDMan]

I would consult a competent engine builder first but to tighten the clearance by .0005 should not take much work. Like David said, carefully sanding the mating surface of the main caps should allow more crush to the inserts. The alternative is leave it as is and run a heavy diesel engine oil like 15w40

[twentyover]

Thanks all for the input