Main Bearing Clearances

[Sunnyledge]

I am a first time engine builder and I’ve just started assembling a new M10 motor stroked with a 2002 crankshaft for my 318i and I’m at a critical decision point. The crank checks out per my micrometers but is on the lower end of the spec range. I’ve inserted a set of standard main bearings in the prepped block and checked clearance with a dial bore gauge. It turns out that my main bearing clearances (0.0025-0.0028) are right on the edge of clearance spec range (0.0008 - 0.0028”). I picked up a set of standard size “Blue” BMW main bearing shells which supposedly will tighten up clearances by a few ten thousands. Installed the blue bearing clearance in my worse site is now 0.0021”. So my question is do I just go ahead and assemble it and run some 50w oil with the original standard bearings, consider installing “blue” BMW bearings ($400) throughout or do I have the crankshaft turned .25 under and work with oversize bearings with tighter clearances?

[TobyB]

I would not hesitate to run 0.002" main bearings in an M10. That's about optimal. The rods can be a little tighter, but I personally wouldn't
go much tighter than 0.0015", just to keep oil flow up a little.

You can certainly run up to 0.003" as long as your oil pump's healthy.

These engines are stout.

t

[moatilliatta]

You could look at aftermarket bearings for +.025.

Rule of thumb .001 clearance per inch of journal.
You can also run different sizes on each side to get closer to your spec.

[Sunnyledge]

Thanks for the responses. I have a new factory BMW oil pump and so oil pressure should be fine. After sleeping on it for a few nights I’m going with complete set of BMW blue bearings. This should get my main bearing clearances into the 0.002” +/- 0.0001” range. The rods bearing clearances measure 0.0019-0.0021 with STD KS bearings. While running the wider clearance would probably have been ok, I figure I’ve come this far, not the time to cheap out and cut a corner.

[TobyB]

I too would sleep better at 0.0020"

t