At the heart of your powerplant is the crankshaft, and this baby not only has to endure the entire rotating mass, but it is subjected to countless spin cycles at varying rpm. It's a tough life, which makes proper installation that much more critical. It's nothing tricky, but taking the time to check tolerances can be the difference between a long -running powerhouse and a short-lived paperweight.
But what is it about bearing clearances that's so important? Simply put, if the main bearings are too tight, they'll burn up, and, conversely, if the clearances are excessively loose, you'll end up with a big drop in oil pressure at operating temperature. Let's not forget about the thrust tolerances. If you don't have enough crank endplay, you could burn the thrust up, which will eventually wear out the thrust on the crankshaft. If this should happen, you can expect to pony up some serious green for major reconstructive surgery, by having to reweld or even replace the crankshaft. All said and done, take the time to measure every journal on the crankshaft and each main bearing, and make sure to have your crank endplay in check.
The better it's matched the more you're freeing up available power. It's cheap insurance to know everything is working in unison and that the rotating assembly isn't fighting itself. We all know engines aren't cheap, so do it once, do it right the first time, and you won't have to worry about your mill giving up the ghost to a major catastrophe.