Can someone tell me what the main and rod bearing clearances should be on a gen IV bbc? Thanks

Federal Mogul specifies 0.0005-0.0034 in. oil clearance for mains 1-4, and 0.0008-0.0038 in. for main 5.
Rod bearing oil clearance should be 0.0007-0.0032 according to Federal-Mogul.

From "Big Block Chevy Engines" by Tom Wilson.

.0025 is a nice round number. the rear main is usually a little looser.

.0025 is a nice round number. the rear main is usually a little looser.


Tom..is the .001" clearance for every inch of journal diameter an acceptable guideline for most street engines?

Thanks for the info. I should be ok then.

Federal Mogul specifies 0.0005-0.0034 in. oil clearance for mains 1-4, and 0.0008-0.0038 in. for main 5.
Rod bearing oil clearance should be 0.0007-0.0032 according to Federal-Mogul.

From "Big Block Chevy Engines" by Tom Wilson.
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I would highly recommend that you not try the lesser of those specs in any BB & especially not in any performance build ;)
Anything less than 2 thou would not make me real happy
In general 2-3 thou will be fine for almost any BB combo with .0025-.0028 preferred for most performance use.
Very high HP or drag race only will usually want a bit more.

Mike, would the exception to this be if a block was not line honed, but instead was running with the production (GM) line bore/hone. I would assume, a tighter the tolerance on the line hone, would allow for tighter bearing clearances. Don't know if this is the real reason, but the lore of the day was looser clearances allowed the bearings to live when the caps walked (assuming that all caps walk some on race motors - some more than others). Especially back then, we didn't have billet caps with splayed bolts and aftermarket blocks with beefed up webs.

My motor builder likes to build em loose, but then again he is an old drag racing engine builder. I specifically had him build mine at .0025 on the rods and mains. My motor will be primarily street driven with a few trips to the track. Smokey Yunick specified .0026 - .0028 on endurance racing engines.

Bearing manufacturers tend to specify a fairly wide spec range for vertical oil clearance. In this case (Gen IV BBC), Clevite specifies .0012" to .0037" for the rods and .0007" to .0032" (position 1-4) and .0009" to .0039" (flange) for the mains. In my opinion .0025" to .003" is the optimum clearance for these engines for all applications.
This is another of those instances where you will want to go with the recommendation of your machinist or engine builder.

Mike, would the exception to this be if a block was not line honed, but instead was running with the production (GM) line bore/hone. I would assume, a tighter the tolerance on the line hone, would allow for tighter bearing clearances. Don't know if this is the real reason, but the lore of the day was looser clearances allowed the bearings to live when the caps walked (assuming that all caps walk some on race motors - some more than others). Especially back then, we didn't have billet caps with splayed bolts and aftermarket blocks with beefed up webs.

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It is more of a you never get the oil to temp & you are making a lot of power with a lot of RPM thing
In these cases I like to err on the high side
Loose clearances will for the most part never hurt you within reason of course but just a little to tight will most likely ruin your day

I run most small blocks in the .0022-.0026 range with .0018-.003 acceptable & have run under 2thou to over 9000RPM with Honda rod journal sizes
BB's I don't like to go below .0022 but have run .0020 with no issue, prefer .0025-.0028 most of the time
Both rods & mains.
This does not include 8000+RPM BB engines :D

This does not include 8000+RPM BB engines
Mike it sound like you like to build those big cube race motors loose like the guy that built mine. On those race motors he recomends Amsoil 60 wt racing oil after initial startup breakin:D