Main caps

[tparker]

@Rich G. that kinda makes sense if you replace one, but would the same hold true if changed them all? If you bored it, they should all be in alignment and the same diameter. No? Luckily this aint my problem.... I hope. Taking my block down shortly, hope all is good with it.

[Brads70]

So, for those of us not experts in machine work and such, why are the original so important? Why can't you replace them, or why is it not ideal? Is it cause it wouldn't be worn to match the block side? Or something else?

The main caps are put on the engine and then align bored. If you replace a cap it's almost certain it won't be in line with the other caps causing the bearing to be either to tight or to lose. Which will lead to bearing failure, low oil pressure and then boom!

They are not exact when they bore for the crank side to side when the block is new/cast, so the caps are exclusive to that block. When it is bored/honed the caps then fit perfectly ( or so they should) When you install  different used caps it's a pretty good bet they will be off side to side, and they might not clean up.  New caps should have extra material to compensate for this.
In addition the dimension between the crank and cam is important. Not much wiggle room there or the timing chain will be loose. They do make chains to compensate for this.