Bbc main clearance

[jpankey]

Guys I've got a bbc showing .005 clearance . If you use .0015 it's around .004 at .005 will it work or is it a concern

[jpankey]

.0015*2.75 crank journal=.004125 so is .005 a detrimental main clearance ? Please help with replies

[MadBill]

I just put mine together at 3.2, 3.2, 3.2 & 3.5, all 0.001" under and coated, to close up a couple more tenths. The old rear main was 0.002" under but that size appears to be unavailable now. Haven't run it yet, but i'm not concerned.

Someone posted on S/T a while back (paraphrasing) "If you go a thous. or so loose, no one will know. If you go a thous. tight, everyone will know."

Here's a heretical thought: standard grade aluminum foil is exactly 0.001" thick...

[cgarb]

People used to shim bearings all the time back in the day on older engines and farm equipment...If it were a total slap together job I would maybe think about it, probably not a race engine.

[jpankey]

Mad bill I would feel better if mine wasn't over .004 but it is with all the work done but assembly

[MadBill]

People used to shim bearings all the time back in the day on older engines and farm equipment...If it were a total slap together job I would maybe think about it, probably not a race engine.

"What Could Possibly Go Wrong?" (Recall there's a 0.001" tolerance on the main bore diameter.)

[DaveMcLain]

Was the block shot blasted when it was cleaned? Sometimes when they do that it'll pound on the register where the cap sits in the block and raise metal. This will increase the housing bore sometimes quite a lot. Use a fine file to dress that area and the bottoms of the caps and then re check the clearance. I have seen this change the clearance .002 before in a bad case.

[wyrmrider]

AND IF you don't do the file check and the surfaces are raised they will flatten and loosen all on their own

[cgarb]

A .001" shim of aluminum is very easy to bend and bunch up. I'm sure with careful assembly and trimming it would be fine. If it were mine and a new engine, I would rather fix the problem now, if that would be recutting the mains if out of spec or order undersize bearings if its the crank, or have it dressed to -.10 under to get it where it needs to be to not have to deal with shims. My personal preference, If money was tight and I had to shim...sometimes you have to do what you have to do to race. I think we all have been there.

[MadBill]

When we tore it down for the first time, the engine of a Pro-built 1984 Mustang Trans Am car I worked on had a 0.001" flat shim under the rear main cap.
I was told this was a standard factory procedure in the era.

[cgarb]

If it works it works you know...can't argue with results. I am a machinist by trade, not on engines but general industry. When I build engines, I check everything I possibly can with a dial bore gauge, micrometer or whatever means I can. I'm a machinist, its my job to sweat the small stuff. Its good practice because I find a lot of issues that way, it cheaper to fix them before rather than sweeping them up later. When building my 377, I checked the main bore diameters, I just had billet caps put on the center 3 and a fresh line bore/hone job. I cleaned everything and torqued to spec and measured the mains. All were good except the front cap measured .001" big. My guess was that the center 3 being made of steel and the rear cap was a wider journal made those hone more difficult than the thin iron front cap and it cut oversize. I fretted about it for a while, but eventually decided to take the cap on a granite plate with some emery cloth and I sanded till the machine marks smoothed out. That put it right on the high limit with the rest of the caps. It was a bit out of round, a few tenths but I went with it. I haven't tore it down to do a bearing check yet but will in a few weeks. Hopefully all is good. I could have made a deal and wanted it line honed again, but I chose that time to do the quick fix.

[jpankey]

Crank was purchased used turned .010 block was line bored and is on the high side of specs and crank is .0005 below the low side .

[statsystems]

Crank was purchased used turned .010 block was line bored and is on the high side of specs and crank is .0005 below the low side .

Why was the main line finished to the high side?

[Jeff]

You could buy a set of .o11 bearings

[jcisworthy]

I would not be happy with .005

.011 bearings, maybe coated, would be the easiest solution if the clearances end up where you want them.

Another option is to get .020 bearings, install them, mic the id and have the crank re ground to the numbers you give the crank grinder for the clearance you want.