Let's say that the inside diameter of the rod bearings measure 2.0022 inch. Subtract the rod journal diameter from the bearing id and this will give you the true bearing clearance (2.0022 - 2.0000 = 0.0022 inch). Typical rod and main bearing clearances vary between 0.0020 and 0.0030 inch with the ideal clearance closer to 0.0020 inch. You can also try this trick: If you find you have both a tight and a loose clearance with a couple of rod bearings, try swapping the two and recheck the clearances. Often, they will fall more closely in line with the others.
You can also mix and match over- and under-size bearings to customize the clearances. Let's say you are using a set of standard main bearings and a tolerance stackup creates an overly tight clearance of 0.0017 inch. By adding one shell half of a +0.0010-inch oversize bearing set, this theoretically will add 0.0005 inch to the clearance. Often, this doesn't work out this precisely, but this is an accepted procedure as long as you're using under- or over-size bearings from the same manufacturer. A good rule of thumb for rod and main bearing clearance is 0.001 inch for every 1 inch of journal diameter and then add 0.0005 inch just to be safe. So for a 2.00-inch rod journal engine, this would be 0.0025 inch.
Computing Compression RatioLet's make this easy. There are at least two Web sites that offer free computer programs for determining static compression ratio. The two programs can be found at performancetrends.com and kb-silvolite.com which is the parent company for Keith Black pistons. These calculators only require you to input the proper data into the programs to generate the static compression ratios. Here's what you need to know.
Simply stated, compression ratio is the mathematical ratio between the volume of the cylinder with the piston at bottom dead center (BDC) divided by the volume of the same cylinder with the piston at top dead center (TDC). To compute these volumes, you need to know several variables that affect these volumes.
We'll start with cylinder bore and stroke. For our example, let's use a 454 Rat motor with a 4.250-inch bore and a 4.00-inch stroke. As bore increases in diameter, this will increase compression since we're dealing with a larger-diameter cylinder. Stroke has a big impact on compression as well for obvious reasons since this also increases or decreases the cylinder volume. These volumes are generally established in cubic inches.
Next let's deal with the head gasket. The thickness of the gasket creates a space between the head and the block, adding volume. This is usually expressed as the compressed thickness of the gasket, usually between 0.015 and 0.050 inch. If we want to split hairs, generally the gasket bore is larger than the diameter of the cylinder, adding to the gasket volume figure, but this is generally a very small value.
A similar value to head-gasket thickness is piston deck height. The farther the piston is down in the cylinder at TDC, the more combustion-area volume it creates. This reduces compression. Conversely, milling the block deck surface, reduces the piston deck clearance and increases compression. Generally, a zero deck height, where the piston top is equal to the block deck height, is considered ideal, especially for engines with wedge style combustion chambers.