Different oversized bearings for main and big ends 4G63

Hi guys,

Would it be okay to use 0.25mm oversized bearings on the main journals and 0.025mm oversized bearings on the big ends as long as i get my clearances right?

I measured 3.8thou clearance on the mains using a STD bearing and was aiming for 2.5thou, that's the reason I feel the need to go 0.25mm oversize, even with the .025mm bearing I still won't get to my target clearance.

Thoughts?

In principle, that should be fine - just make sure you have a machinist who can grind and finish the journals to the exact size you want, you don't want to have the regrind giving even more clearance.

Shoti, André , and others have a lot of experience with these engines, so I'd go with their recommendations.

That said, exactly what sort of power, torque, and rpm do you expect the engine to be built for - their experience will confirm your clearances or suggest an alternative that should work better for your application?

The rotating assembly will be good for 600bhp but i'll be running it at 400 brake for now at a maximum of 7500rpm

Going by the 0.001" per inch of journal rule, I intend to keep the big end clearance at 0.002 and the mains at 0.0025

I've added roughly 0.0003 of extra clearance just to be on the safe side as i'm not too confident with the quality of work from the machine shops available to me.

Do those clearances seem okay for my power and rpm targets?

Using 0.025 mm oversized bearings will result in 0.07mm clearance which is a bit too much and will require usage of very thick oil to ensure proper oil pressure. 5-6 would be just fine.

Thanks for the replies guys. So you suggest 0.06mm clearance for the mains?

0.05mm would be good for the big ends?

Absolutely. That is what i use on my engines.

Interesting. I read on another post in the forum Andre uses similar clearances on his drag Evo that makes lots of power.

Was curious to know whether the same bearing clearances would be acceptable for my 400hp engine

Yes, if you want to stay on a safe side in case of uncontrolled overboost

Your help is much appreciated.