precup/head issues and questions

[neo]

In another thread, I describe the unfortunate discovery of a cylinder with low compression. I'll update that thread, but what I found was cylinder seven leaking slightly through the gasket to the coolant pass through. I took the head to a local machine shop.
THe machine shop cleaned up the head, checked it for cracks and ended up having to take .012" off it to clean it up. Apparently, the machine shop that originally did the work on the mating surface, gouged it is several spots and .012" cut got most of it clean. I believe it will seal well now however, here is my questions:
1. Will the .012" head difference (I measured the recess in the head and it was a little less than that to a stock head, .008 to .010) make the head mis-matched and cause a ballance issue given the other head has not had that much cut off? THis is better than cuttting the deck right?
2. THis head (and the one on the vehicle) have the 2 dot precups, can I use one of my other heads with the single dot (I also have a set of three dot) and be alright or does that too make an imballance?
I know the "right thing" to do is probably replace both heads, and I have considerable $$ in the block and rotating parts and really don't want to screw them up, but just don't have the $$ right now to do a whole bunch. ANy guidance is appreciated.
Neo

[More Power]

Having everything identical on both heads is recommended (decking and precups). That is the ideal. Decking heads changes the geometry of the valve train. There are a couple of solutions.

1- Shim the rocker arm shafts an equal amount to what was taken off the deck.
2- Trim the ends of the valve stems to maintain static clearance.

Shimming is easier, and you won't risk taking off a hardened surface layer on the end of the valve stems (if, in fact they are hardened).

That said, the heads used in our Power Project 6.5 were milled .005". The engine shop buzzed an equal amount off the valve stems, and that engine ran for hundreds of thousands of miles afterward without a problem.

Jim

[neo]

Thanks for the info Jim. Is there a referce to the differences in the different precups and if the can be mixed? Thanks again!
I have a set of heads from a mil engine i dropped at the shop today to have the valves ground and cleaned up.
Ill likely use these, but it will require changing both i think since these are three dot cups.

[More Power]

We did a story some time ago about precups, which is stlll available in the subscriber section.
http://www.thedieselpage.com/members/features/65fueleconomyc.htm

Mixing & matching cups? In general, cups come with one of two different size ports. These ports are what the gases and fuel pass through onto the tops of the pistons. The 6.5TD ports are nearly twice as large as those used on the 6.2 and non-turbo 6.5s.

There are minor differences between the various large port cups and between the various small port cups seen through the years. In an ideal world, you'd always want a matched set. Though you should never mix a large and small port cup set on the same engine no matter what. The relatively minor differences between cups having the same approximate port size could be overlooked in a budget rebuild when there is no other option (no money or simply can't find the parts).

Jim

[neo]

Thanks Jim. SIncerely appreciate the guidance. While I was traveling I had a chance to read the article. Good stuff, as always!! So, not to try to pin you, or anyone else with a perspective to the wall, but I got my set of heads back from the shop today. They are ready for install. THese heads have the exact same casting number as the original heads, of which there is still one (pass side) installed and performing very well.
The heads that were originally on the vehicle were the military/HD 6.2L heads contianing the ricardo comet "two-dot" cups. THe new ones have "three dots" In pursuit of conservation of time and money, I am inclined to simply replace the one now (drivers side) and come back later (perhaps) and replace the passenger side. I measured the cups and the 3 dot are no more than about .050 larger than the 2 dot.
Will that small of a hole really make a percievable/detectable difference? Can that throw off th eengines ballance? I have invested considerable (to me) $$ into the engine, and really don't want to mess it up but at the same time, the pass side is very hard to get to and would really prefer to replace the bad side and be able to run it again. Your advice is greatly appreciated.
Neo

[More Power]

Given the many-many different variables involved that work together (or against) a smooth running engine, I doubt an experienced person could tell by listening to the engine run or while driving with two "slightly" different sets of precups.

I'd be inclined to do what you're proposing - install one head, then do the other on down the road.

Incidentally, having the same head casting numbers may not always indicate that the precups are the same. I'm ordering a new set of heads for our latest project. I can't get a set of new AM General heads with small port precups, so Peninsular is knocking the large port precups out and installing a set of small port cups.

Also, Peninsular tells me that the newest genuine AMG heads are now equipped with replaceable valve seats. Not sure what this might mean in terms of longevity or durability, but I'll find out why AMG did it, and report on what I learn in a future update.

Jim