What is the most you have run in your 340?

I run .003/.003 on the mains and rods for a race
set up.. I have the same on my 416 street/strip
engine but I should have gone a bit less with all
the oil losses I have built in(EDM roller lifters,
PR oiling, plus I have bushed lifter bores but went
with a .062 hole in them.. I was suggested a .040
hole and I believe I should have to.. reduced the oil
loss from all the built in leaks).. I know a few guys
that have run the mains at .0035 (all these numbers are
for steel rods)

Way to loose(.0035+) is a bunch better than a tiny bit to tight I've built and ran NHRA stocker motor with .0035+ on a lot of different motors, so have friend of mine that ran those cars also. What type, weight and synthetic or non synthetic oil are you going to use in this motor?

if you have loose clearances, you know it. If your too tight everyone will know it.

What are you measuring now

For sure what has been said. .005 is better than .002

I feel the main bearing design also makes a difference. The quality of crank also. As well as your choice of weight in oil.

I run 30wt max oil. An HD bearing and .0025" on a 2.500" main. 360mopar and 351ford mains I run .003".

Engines running full groove bearings with 30wt I'm not scared of .0025".

Big block mopars move around more and have heavy assemblies. Does that reason larger clearance?

My 360 is between 3.5 and 4 on my turbo motor. Oil pressure isn't that great.

Well with the king bearings between .0038 and .0041. Clevites were a smidge tighter but I misplaced my notes so I'll have to put em back in and check em again.

Anybody know if there is a bearing that runs a little tighter?

I have run .004 or more on a lot of stuff and never had a problem

You can buy Kings in .001" over/under.

Well... That solves that problem then. Thanks, I'll look into that

Yep, buy a set of .001's, not X they're for a thou more clearance. Also, you can use a standard upper, or lower, and the other half the .001 to get where you want to be. You might want to look at the housing bore size too to make sure you have enough crush. I'd still go no lower than .003, but that's just me.

This isn't technology. The suggested plain bearing clearance for a spinning journal was .001" per 1.000" of journal diameter 100 years ago.

That's what I did, mine was a little tight so I ordered a set of .001" overs and used the cap half.

I'm running 0025 on both the rods and mains with full groove bearings with 30 weight oil.

No problems yet....

Problem with big clearance and a LA Mopar is delivering enough oil to the mains. If you block off the DS lifter feed you increase some. Drill all the main feeds helps. Restrict the heads also helps. Deep pan to accommodate all the drainback.

Easiest thing is don't over clearance the mains. .025" is minimum on a 340 main imo. But, .003" will keep oil blowout to a minimum and feed the larger rod journals that are likely overclearanced to. I have had many shops laugh when they saw my P bearings in a high HP price. But, after seeing the condition of my bearings was told don't change a thing.

My theory... They oil the rod 360°. This can also cause oil loss. But, I never had hurt a main. Hurt rods but not mains in early builds. I dealt 360° rod oiling cooled the bearing. If the bearing runs cooler the clearance can be tighter.

Just what has worked for me.

Like someone said already. The crank type, oil weight, and power comes into play if you're trying to hit an optimum clearance. Big power, and flexing parts need clearance beyond the "rule" which then requires thicker oil.

Look at a blown alky making power. +.005 on rods, and mains with 70 weight.

I've had motors that .003 was cool till the power went over 1500. Bearings looked bad, went to .004 problem solved. Crank flex, or block flex from no mid plate maybe, who knows.