So that's great you say. My rod bearing clearances are all about the same. That is certainly a good thing.
But how do the values compare to what is generally recommended? On a Chevy racing engine Smokey Yunick
recommends 0.0025" - 0.0030". But Chevy bearings are big diameter, and clearance is
proportional to diameter, so it makes sense that my clearances would be less than on a Chevy.
According to Bob Goppold, BMW recommends two sets
of rod bearing clearances (red/blue dot scheme): single classification 0.0012" - 0.0027",
and double classification 0.0008" - 0.0022". I've never been a fan of the BMW "dot" system.
To me a bearing is a bearing and clearance is clearance. But in any case, my rod bearing clearances are
well within the BMW spec's for either classification.
Then there is the formula for bearing clearance based on journal diameter:
Recommended Clearance (in) = (0.001) x Journal Diameter (in)
In this instance the formula yields 0.001 x 1.8907" = 0.0019", which is a little larger than the values
that I measured. I pondered this for a while. The only way to change it is to buy undersize bearings
and then have the journals on the crank ground down to produce the desired bearing clearance. The
crank must be heat treated again as the surface hardness will have been ground off. I figured all
this might be a bit much. As it turns out Jim Butterworth, while building an identical bottom end to mine,
measured his bearing clearances and came out with values slightly larger than mine. This highlights the fact
that the measured values are operator dependent to some extent. A small change in technique can easily
vary your measurement by a couple ten thousandths of an inch.
In the end I took solace in the fact that I had roughly equal bearing clearances on all for rods, and that my values
were within BMW specs, and left it at that. Bearing clearances are a sensitive thing. Too tight and you choke off the supply
of oil. But too large and the oil just flows away and does not build up any pressure. I have not had any problems
and have good oil pressure (80 psi at 4000 rpm).
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