Have you ever wondered why engine main or rod bearing clearances always seems to be around 0.0025-inch? We wondered the same thing and launched into a search for an educated 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. This jives nicely with that 0.0025-inch clearance. That engine’s rod journal is smaller at 2.10-inch so, in theory, you could run a slightly tighter clearance. Most engine builders set their clearances close to 0.0025 for both as a matter of convenience.
All of this is 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 stating how the liquid oil keeps 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 journal engines such as a 455 Oldsmobile, then a thicker oil such as a 20w-50 is generally used to maintain the proper lubrication barrier.

This chart created by Driven Racing Oil offers oil viscosity recommendations for engines based on cylinder block material, bearing clearance, and maximum oil temperature. An iron block, drag race engine that barely sees 150 degrees-Fahrenheit oil temp could use a much lower viscosity oil than the same clearance engine used in a road race application where the oil temperature could see 220 degrees or more.
One point worth mentioning is that it’s not a good idea to arbitrarily choose thin viscosity oil based on a whim without knowing the consequences. A thin oil used in applications where oil temperature exceeds 225 degrees (which is not unusual even for street engines under high ambient temperatures) could expose the bearings to additional wear that could have been prevented with a thicker viscosity.
There are a ton of variables that accompany 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. As an example, an engine with an iron block with 0.0025-inch main bearing clearances that sees oil temperature in excess of 220 degrees would be best served using a 10w-40 or 15w-40 viscosity oil.