Have you ever wondered why the engine main or connecting rod bearing clearance always seems to be around 0.0025 inches? We asked ourselves the same question and set out to find an informed answer. The classic rule of thumb has always been 0.001 inch for every 1.00 inch of journal diameter. With a typical 350 small-block Chevy, for example, the main journal diameter is 2.448 inches. It pairs well with that 0.0025 inch clearance. The connecting rod journal on this engine is smaller at 2.10 inches, so in theory you could have slightly tighter clearance. Most engine builders set their clearances near 0.0025 for both for convenience.
This is all a generally accepted theory, but what few people consider is that these bearing clearances rely on a given oil viscosity to maintain the proper hydrostatic clearance, which is a fancy way of indicating how the Liquid oil pulls the crankshaft journal away from the bearing. In the case of this typical 0.0025 inch clearance, conventional wisdom calls for a 10w-30 or 10w-40 oil. If the clearances are larger – around 0.003 inch or more for larger journaled engines such as a 455 Oldsmobile, a thicker oil such as 20w-50 is usually used to maintain the proper lubrication barrier.
A point to mention is that it is not a good idea to arbitrarily choose a low viscosity oil based on a whim without knowing the consequences. Thin oil used in applications where the oil temperature exceeds 225 degrees (not unusual even for street engines at high ambient temperatures) could expose the bearings to additional wear that could have been avoided with thicker viscosity.
There are a ton of variables that come with this discussion. Driven Racing Oil has created a generic chart that recommends a given viscosity based on block material (iron vs. aluminum), bearing clearance, and engine oil temperature. For example, an engine with an iron block with 0.0025 inch main bearing clearances that sees an oil temperature above 220 degrees would be best served with 10w-40 or 15w-40 viscosity oil.