Re-installing OM 603Cylinder Head
 

I had to pull the head on my 1991 350SD (200k mi.) due to a broken exhaust manifold stud. I cleaned the head and block with an abrasive pad and some fine emery paper. What is the recommended surface finish on the head and cylinder block? I have the MB gasket set and new head bolts. Can anyone offer some guidelines on the head re-installation?

Should I replace the valve guide seals? Lap the valves? Check for valve guide wear? What's the best way to measure the piston heights (for bent rods) without getting the IP out of time?

Any advise is greatly appreciated.

Greg

Since you have the head off, you "might as well" send it to a good automotive machine shop and have them go over it. They'll clean it, tell you if it needs to be resurfaced (they may just want to take a skim cut anyway, which I think is fine if it hasn't been done before), they'll check the valves, guides, and springs; change the stem seals, etc. Shouldn't cost more than $300. Money better spent than screwing around with it yourself, imo.

Before you install the head, clean every head bolt hole in the block. Not a bad idea to run a tap through the threads to remove any gunk, at least use a clean old bolt to make sure the threads run free. Make sure there is no debris or fluid in the hole before installing the head. Follow the instructions about lubricating the threads and underside of the bolt head. Don't overdo it on the thread lube... a drop or two is enough, starting at the bottom thread.

I recently had skim done. Would not recommend it, unless needed. After just .004" surface removal a thicker, much more expensive head gasket is required plus prechamber measuring and shiming.

I just did the head on my SDL and am not aware of a more expensive head gasket. You can not re-use the old head gasket.

why didnt you try the extract the broken stud and repair the theads with helicoil??

At 200K, and with the head already off, its defo a good idea to check the valves and guides. They are prolly just fine, but it would be a good idea to replace the stem-seals anyway:)

You should have the head measured by a machine shop for flatness and square.
The head can be out of spec after you pull it, mine was off by 0.007" , so the cut was 0.010" or 0.254 mm.
If it is off more than spec, have it milled, buy the new head gasket , pre-chamber shims, valve stem seals, pressure test etc....
2 of the pre-chambers on my head were very close to minimum spec. on pulling the head.
I re-shimmed all of them at $6.00 per shim. They come in 0.3, 0.6 and 1.0 mm sizes. Be sure to have the machine shop measure the pre-chambers before they cut the head.
Valve stem seals are cheap , be sure to do this.
Additionally you will get a better finish on the head and IMHO , a better surface seal after machining. Also , the new head gasket has some improvements to it compared to the original.
The block surface spec for roughness is really large number , something like 128 or 256 RA ( I think that was the conversion I came up with).
Any minor cleaning you do to prep the block for mating to the new head gasket will probably have little to no impact on the surface finish.

Some believe that the pre-chambers should be replaced at 200k miles, I did not since they were in good shape. The FSM gives some vague info regarding what would be cause for replacing the pre-chambers.

Also, some measure and re-use head bolts, I decided not to because of the probability of measurement error. A new set isn't that expensive.

Now while you have the head off , you can do all of the other wonderful things like the upper timing chain guide and so many other fun things.

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Mercedes head gasket info: P/N A6030163220, 1.74 thick (std), $107, P/N 6030163320, 1.97 thick, $272. Roy and Phil recommended I use the 1.97 gasket. Noel of Noel's Engines in Fla also recommended the 1.97 and using the 617 seal rings (shims) on the pre chambers, no measuring required. I believe they are 2 mm thick.

The factory spec for the pre-chamber protrusion distance beyond the mating surface of the cylinder head is 7.6 - 8.1 mm. I don't believe you can use the 2.00 mm 617 shims / washers, believe the diameter is smaller on the 617 seals.
If the pre-chamber is at 8.1 mm protrusion prior to milling and you mill 0.254 mm (0.010")= 7.864 mm + 2.00 mm 617 shim == 9.864 mm.
If the pre-chamber is at 7.6 mm protrusion prior to milling and you mill 0.254 mm(0.010")= 7.346 mm + 2.00 mm 617 shim == 9.346 mm.
In a case where you mill 0.127 mm (0.005") , the error is larger.
For the 2.00 mm shims to work you would need to mill more than the FSM maximum amount of 0.500 mm.

As far as the head gasket no. 3320, the additional thickness of 0.23 mm seems to me to have no affect on valve clearance or whatever the purpose of the 0.23 mm is.
Correct me if I'm wrong.

6030163320 1.97 mm Thick Head Gasket Purpose
 

Now I'm curious as to why Mercedes makes a 1.74 mm and a special order 1.97 mm thick head gasket.
Anyone want to explain the purpose of the 1.97 mm head gasket compared to the standard 1.74 mm head gasket listed in this thread?

Thicker gasket makes up for head material removed. I am advised it is better to be too thick than too thin.
Regarding the prechamber measurement, 7.85 is mid point of 7.6 - 8.1 range. Mercedes recommends seal ring (shim) .1 greater than needed and not more than .3 greater than needed. If prechamber starts at 8.1 and .127mm (.005") is removed, measurement is now 8.227. A .6 shim would be my choice.