[Jack21]

Nick, here's why. Essentially how much error are you willing to accept when you assemble your engine, particularly when you can control it by checking it? It's kind of a pay me now, or pay me later proposition.

The crank keyway notch, timing gears and chain, and cam, are all manufactured with machining tolerances. These tolerances can add together to install your cam in quite a different location than it was designed to. Using a modern replacement timing chain set, you can adjust any inconsistencies to install the cam exactly where it was designed to go. That's why the different keyways and offset keys are there. Some guys even install a cam with 2 deg advance just to allow for timing chain stretch.

With a grocery getter rebuild, you can assemble everything on the marks, and the engine will run (hopefully). You can also take the rings out of the bag, install them on the piston, and if the piston slides in the bore, the rings are fine. If the crank turns when the bearings are installed, they're fine too.

HiPo street motors need more checking. If a cam isn't dialed in, the valves could hit pistons, or the engine will perform poorly, and you'll never know why. Some guys not only degree the cam, but degree each lobe, and send the cam back if it's off.

Some guys only use file fit rings, and set each ring with the minimum gap. Some guys will send a crank back if one bearing is too loose or too tight.

And it's always the one thing thing you didn't check that haunts you later.

It's your time, your money, and your engine. If you spend the time checking the clearances on the engine while you're assembling it, and adjusting things that aren't right, it will run better and last longer, and minimize the chances of having to pull it back out again to do it over.