"Hey man, my IDI's chipped!!!"
Seriously, IIRC some cracking on the precups is normal, and there is a tolerance limit...per Gary in this thread, as long as the cracks aren't crossing the fire ring, you should be okay. WIth that said, if you can practically find a replacement for the one with the chips missing, I think I would...
I've never actually done anything with a precup, but I think that those who are looking to lower the IDI's compression ratio (to stuff more boost into the engine) would do well to look at taking material out of the inside of the precup instead of shaving piston tops. You have to be careful with airflow and fuel spraypatterns (certainly, that's beyond me), but I contend that, if you can use the precups to get the compression ratio down to 18:1 or so, you'll get the same end goal as shaving piston tops, but you'll have more air to be heated by the glow plugs, which SHOULD lead to easier cold-starting, which in turn will make a more streetable truck (most lowered-compression IDI's take entire cans of ether and a wing and a prayer to get lit off; note this .wav file). With that having been said, unless you want to go that route, I don't think there's much that needs to be done to optimize the prcups any better...Has anyone ever played with the precups to get them more durable (ceramic coated), or altered the openings? I mean are they at there optimal design? If im going to replace them I might be just ballzy enough to try something different.