Main Bearing Caps
The main bore holds the crankshaft. When measuring the bore the bearing isn't installed so your just measuring the inside of bore for roundness. These bores will become more oval than round, more like an egg than a circle. So we measure the inside of area were the main bearings sit to make sure the bore created by the main cap is round. This phenomenon usually happens to older engines so if your working on newer vehicles at the dealership you may not have seen this.
There are different kinds of gauges for measuring the inside cylinder or bore. A bore gauge is a popular tool that is used in the process of accurately measuring holes. A telescopic gauge is used along with and outside micrometer or caliper for similar results. Either way it's important to understand why were taking the measurements and it's really because the main cap can become oval from the intense downward pressures imposed on it by the engines crankshaft. The block can become distorted as well. One last thing to remember is to keep these caps in order, they are usually numbered with arrows pointing toward the front of the engine. Whenever working with an engine part arrows or indented dots point to the front of the engine. Always check the appropriate manual.
Thrust bearings affect the crankshafts end play or front to back movement. If the thrust bearings wear too thin the engines crankshaft will have excessive end play. This will result in engine block and main cap wear. This is because the metals used to create crankshafts are much harder than those used to create the block and caps. When the crank comes into contact with the cap and the block, they end up looking machined smooth by the crank.
Crankshaft end play is measured with a dial indicator mounted on the engine and the indicators tip on the crankshaft flange. Use a pry bar or a large screwdriver to move the crankshaft back and forth. It's important to check the measurements before and after a rebuild and clearance should be minimal, a few thousandths at the most. You can't just replace the cap with a new one. A stretched main cap is repaired by line boring all of the main bearing bores.
