Whether it's your first engine build, your 50th, or a simple remove-and-replace procedure, bearing clearances are always going to make the difference between a motor that lasts and performs, and one that rattles then explodes. Small-block Chevy bearing clearances tend to follow the rule of thumb for most manufacturers, but there's a little more to think about than a quick snug and a bit of plastic bearing checker.
You've probably heard stories about professional engine builders who have special temperature-controlled, engine-building "clean rooms." Those aren't just to keep the builders comfortable. Metal expands and contracts with heat and cold, so manufacturers have to specify clearances at a certain temperature. That's true for micrometers, too. All quality micrometers are designed to be used at 20 degrees Celsius, or 68 degrees Fahrenheit -- that's the best temperature at which to check your bearing clearances, even if you're not checking them with a micrometer. Between 58 and 72 degrees, metal can tighten or expand by 0.0005 inch or more.
The general rule of thumb for bearing clearances is 0.0010 inch for every 1 inch of journal diameter. That's true for mains and rods, and applies to almost all engines. For the small-block's 2-inch rod journals, that means 0.002 inch of clearance between the rod journal and bearing, by the rule of thumb. The GM-specified tolerances are 0.002 to 0.0025 inch, which lines up well with the rule. Side clearances come in at 0.010 to 0.020 inch, as specified by GM. The clearance rule applies to most engines, but it's generally best to avoid going below 0.0020 inch on any bearing, regardless of the journal diameter.
A narrower bearing clearance will make the engine a little stronger by spreading out the load on the bearing, but will squeeze the oil out of the bearing and make it prone to overheating. If you're going with minimal clearances for a really high-performance engine, you need to remember to always use a high-quality, light-weight synthetic oil that can stay in that narrow gap and provide lubrication without breaking down. Wider clearances will give you a bit of extra power by reducing parasitic drag loss, and allow you to use thicker and cheaper oil. The real drawback is a reduction in strength and bearing life in high-horsepower or very high-rpm applications.
On the main bearings, you're looking for 0.002 to 0.003 inch bearing clearance, following the same clearance guidelines as above. You're looking for 0.005 to 0.007 inch crankshaft endplay. On the wristpin, you're looking for 0.0006 to 0.0008 inch inside the piston. If you're using a full-floating rod, that'll be a slightly wider 0.0008 to 0.0010 inch inside the rod and 0.0-to-0.005 inch wristpin endplay. Maximum piston-to-bore clearance measured at the centerline of the wristpin should be 0.0055 to 0.0065 inch; however, a lot of aftermarket pistons take a tighter bore, so check with the manufacturer. Minimum ring gap is 0.022 for the top, and 0.016 inch for the second and oil rings.
