94 6.5 turbo pre-cup

[twilly]

1994 6.5 turbo diesel K 3500 pre-cups seem more than just stressed
cracked. They look burnt and are developing gaps in the cracks. Heads
were from Promar in New Jersey, after cracking old heads.
Also did not know of the cooling upgrades and overheated engine
after turning up with EZ tork chip and
Cracked #2 piston after a couple of hard pulls. Had installed Promar
heads 20k miles before toasting piston. Have pre-cups out of my old
cracked heads but they are .020 taller (.095") , they have smaller
holes, but after 100K miles they are not cracked or burnt. They appear
to be harder material. Both machine shops in my area do not want to mess
with. One says he had the cups come loose after surfacing, the other
says the cracked pre-cups are fine to re-use. Engine code F .Old good
pre-cups with smaller hole appear to have circle and newer deteriorated
pre-cups have a dot. Could a shop grind .020 of pre-cups then put in
head? Would the smaller holes affect power output? Thanks for any
assistance.

[john8662]

Sounds like the replacement heads were the aftermarket heads, usually made off-shore, and with inferior components (precups are one).

You can purchase new precups from GM. For power, I'd recommend the most commonly available "diamond" precups, which have a diamond symbol on the cup. These can be had from GM or one of our vendors (like Peninsular Diesel).

The new precups will be taller, just like your original GM ones you pulled out of your old cylinder heads. The reason for this is head surfacing, the aftermarket heads are usually all resurfaced, they aren't even produced in the proper way (maching, then setting cups of proper size in correct depth hole).

The proper way to resurface these heads is to remove the precups from the head, then resurface/reface. Then you'd install the precups after boring the depth of the hole that the precup sits in deeper, then reinstall the precup. There is a special took made that is a cutter that cuts the head to install the new cups. You'll need to find a Diesel shop that's aware of this procedure (and owns the tool), perhaps one our vendors on the site can help you reman your heads. One of my local machine shops here locally in my town is getting one to do my heads with.

TDP Vendors (http://www.thedieselpage.com/vendors/main.htm)