engine clearance question

[racinjason29]

Im building a 350 sbc, steel crank, good 5.7 rods, good pistons, solid cam, world heads, approx 10.5:1, King bearings
My question is what is the recommended clearance on all the bearings, a guy told me .003 on the mains, that just seems to tight to me, any info is greatly appreciated

[joedoozer]

I wouldn't call that tight. You will get responses from .0015 to .004 or more. I have run mine from .0025 to .003. General rule of thumb is .001 per 1" of journal.

[perfconn]

.0025 on the rods and. 003 on the mains if they are 2.100 rods and 350 mains.

[TBRE]

bearings

I agree with Perfconn. But, what are you measuring with. A dial-bore gage is the way most engine builders use.
If.......the the rod bearings don't "fall off", 5 to 7 tenths, at 45 degrees, go to 2.9 clearance. The reason the mains run a little more is because the crank flexes, and it rides on 5 different bearings, which is almost humanly impossible to keep in perfect alignment (I'm talking in tenths, in a 2 foot line).
If you ere (spellin'???), ere on the loose side, it wont gall anything. But, if you can keep everything lined up straight, keep your clearance on the tight side, because the rings have to control the extra oil you sling on them with excess clearance.
This is where engine builders make they're worth, knowing what clearances to run, and how much oil ring tension they need to control the bleedoff.TB

[dynoman14]

Tony is dead on with his thoughts on the mains being in line.

There is a reason most engine builders keep a tool box with sized mains and rods to the .0001 in uppers and lowers. Even if you think you have mains perfect in line, your crank may not be perfectly straight. If you put a .0001" dial indicator on each main even the best cranks if they have been run will be out of round. Back in the day folks used to straighten cranks but I guess it is a lost art. I have a 20# sledge hammer and a blown up block that I use to straighten my cranks if they need it.

I had a couple folks freak out when I started banging on he crank, I also have a small air hammer I use for camshafts and sometimes on the crank. Some have to be heated and pressed in a 10-20K Hydraulic press, all of these ways work but best not to try unless you have a crank to trash out whilst learning.

I like to be able to spin a crank by hand with all the mains in and get 4-5 revolutions on it with assembly lube on the main bearings. If I don't get that I go back to swapping bearings in and out.

Ring drag is a whole nuther ball game. I have a couple pistons I drilled holes in the top to fit most ring sizes I use. I installed a bolt with the round ring on it ( forgot what they are called ). I pull the piston up the bore a couple times and look at the needle reading. I thought I would get trick with a digital one but it sucks...you can't hardly read the thing but it is good for calibration to make sure your dial one works proper. I had to be able to justify the $50 I spent on it at Bass Pro Shops...so I use it as a calibration tool.

I think it is directly related to the rings you use, ring finish, bore finish, bore material, and the type of engine you are running. NHRA Super Stock with different requirements than a NASCAR Late-Model Stock, different than a Hi-vacuum Super Late-Model.