is it possible to ****** a known good cylinder head and have the machine work done on that one for you? i realize that it'll be a little more $$$, but you'll have all the parts on hand, and all you gotta do is ****** the head off, prep block, put head gasket on and new fresh cylinder head, and you're ready to rock and roll....

 

How does Cummins do the heat soak to do re torque on the heads when new?? The time and run stands would be a major PITA for them.

 

With a 1990, you probably still have the old style head gasket, which is good to about 250K and no more than ~30psi boost. Exceed either of those variables, and you are running on borrowed time, IMO.

The head on my 91 (nonIC) was within .002" of flat end to end. I cleaned it up and put it back on. Ran another 70K before selling it.

My 92 needed milling. They took off .008" and I put a .020 over gasket on. I read somewhere that you need a super smooth, almost polished surface on the head and block to maximize sealing. My head didnt get that, so I may get another head later on....

DP

 

The thing I'm referring to with the valves is that apparently, when machining the head, the valves protrude just that much more into the combustion chamber (for what it is).

It's a Valve/Piston kissing thing.

One has to be kinda funny about grinding the valves as was said earlier, the integral/OEM valve seats are induction hardened. --> The valve seats and the head are one and the same metal. A high electric current is passed through the metal of the immediate valve seat area there by hardening it. But only so deep.

As such, the OEM valve seat can only be machined once. After that, they must be cut out and replaced with a service valve seat (a ring of new metal).

Further, with the nature of the relatively small combustion chamber, recessing the valve too much in the head will drop the overall compression ratio. The FSM makes it sound as though valve depth is critical.


Read it for yourself ~


Machining the head:




Valve Seats (General) Note the reference to machining the head:





Valve Seats (Integral):





Valve Seats (Service/Replacement):




Sorry about the big images, I just wanted folks to be able to read them.

Hope this helps.

 

From experience, don't deck it unless you have money to burn, and time to spare. Reseting the vales with be fun enough with out that. have it checked, clean it, get a good gasket, slap her together.

 

^^ Quite possibly the worst advice I've ever heard.

Do you want the job done...for now (could be a week, could be a year)?
or,
Do you want the job done...PERIOD?

Deck the head, then you'll only have to buy one new gasket!

 

I know of many successful "just swap the gasket" head gasket changes on our engines. On a high mileage engine, head gasket failure is usually nothing more than a tired head gasket. As long as the head and block are straight and smooth things will be fine for another 300k. But it is risky to just throw a gasket in there without checking things over thoroughly, so you need make sure the head and block deck are in spec.

 

I have taken .023 off a head (combined) without messing with the valves and without valve/piston contact.

I also ran valves set @ .006 intake and .008 exhaust after all was said and done.

 

Thats exactly what I meant, take your time, and check things properly, if its at or close to spec, mill it, if its under by a few thousandths you'll be ok, just don't pull the head off, pull off the old gasket, and throw the new one on, you'll be sorry you did most likely.