87Stallion

I have to use 1 half of a .001 undersize rod bearing to get the clearance needed.

When end does the smaller bearing half go in or does it matter?

67mustang302

Um, the rod bearing clearance is a range that's larger than .0005"

87Stallion

Ok Im at .002 now and I need .0025 according to Woody. So I bought a set of .001 undersize bearing which when I use one half and one half of standard bearing will give me the additional .0005 clearance I need. THis isnt a pissy 300 horse motor so things have to be precise.

67mustang302

Um, no...do NOT use different sized bearing from different packs. That's a guaranteed way to f your bearings up. Bearings are matched.

Does it have to be a min .0025? If so, then get a bunch of bearings and select the bearing pairs that give you the clearance you need, and don't use the ones that are tighter.

And just fyi, undersized bearings will make it TIGHTER not looser. You'd actually be at .0015....of course with mismatched bearings you have all sorts of other major issues.

uedlose

I have been running a half shell .001 oversized mixed set for years it hurts nothing. seen 7500 rpm and has seen well over 20psi of boost. Mine is on the mains bearings. No it did not matter were it was placed other then the right journal.

AdderMk2

if its going in a n/a car, i'd put it as the rod side and measure the deck clearance. make sure the piston doesnt poke out of the hole.

if it does, then I'd switch them

87Stallion

I call it undersize as thats what Im used to seeing but I know what you mean.

Ive heard of tons of guys running bearings this way with no ill affects.

67mustang302

Personally, I'd never run different sized bearing shells, even if only .001. Engine bearings are hydrodynamic bearings, and rely on controlled fluid flow/drag generated by the rotation of the crank. A slight "step" created from different sized bearings could potentially screw up the nature of that fluid flow and collapse the oil wedge. It's not likely to cause a problem in something with loose tolerance, but could become highly problematic in a tight tolerance setup, particularly newer small engines that run .001 clearance +/- a few 10ths. I wouldn't want to take the chance though.

If you can't find the right sized bearings(including mixing +/- .001 shells if you want to) to get you where you need, maybe torque the rod cap on with what gives you .002 and use something to very lightly hone the bearing out until you get the .0025. You'd need something that can remove the bearing material in small amounts without tearing it up though.

Something on how engine bearings work, in case you're interested.

http://www.epi-eng.com/piston_engine...e_bearings.htm

87Stallion

I thought about doing what you mentioned to the bearings but all you hear from the bearing companies is to NEVER scuff,sand or polish a bearing. But again I know alot of the old timers did this. But I dont know it I want risk it on a 10,000 $ motor.

67mustang302

Have you called the manufacturer of the bearings you're using to see what they say?