Cylinder head blemish?

[bobswei]

My friend is rebuilding a '71 T engine and all the machine work is done but one question area. One of the six heads has a small blemish line where the head gasket sits. Is this a concern? The machine shop guy says, "Maybe ok but if it were mine, machine them all and use thicker head gaskets".
Opinions? Sealing agent?
Thanks

[dannobee]

Machine all six heads and use thicker base gaskets to make up the difference. I wouldn't run it like that either.
No sealing agent can take the abuse of a head gasket.

[r lane]

I assume your machinest is familiar with the 911 motor. The cut has to be the same on all heads, at least on each side. It is about the cam housing laying flat. Common knowledge, but just in case. And yes, that needs repair. On that subject, is it a flat cut or does the surface have a slight angle.
I think I have seen or read that somewhere. Bob

[targa72e]

The heads need to be machined. Here is information on performing this procedure from the tech booklet. While it says "head gasket" it should really say base gasket as others have mentioned. Also you can take off less than the .25 if less is all that is needed to clean up the mating surface.




Most machine shops that work on Porsche engines will have the appropriate tools to do this procedure.
I was able to make a fixture from a old cam tower and do in on my home lathe. Picture of my setup below.



john

[bobswei]

"Most machine shops that work on Porsche engines will have the appropriate tools to do this procedure.
I was able to make a fixture from a old cam tower and do in on my home lathe. Picture of my setup below.

OK, it looks unanimous. I'll pass the word on. I know he's already done every little extra, (line bore, decking, studs, etc...) so one more thing. The machine shop was supposed to work up a price by today, I believe. I trust the machine shop since some local Porsche guys go to him. And one of the mechanics at Porsche Marin, that's rebuilding a 3.0L, is using him.
Not sure if there is a slight angle but John's post shows "bevel edges slightly" in the tech book.
So, no such thing as a budget rebuild. Thanks for all the responses.
Bob

[PeteKz]

"budget" and "Porsche" don't work in the same sentence.

[Henry Schmidt]

Originally Posted by bobswei

My friend is rebuilding a '71 T engine and all the machine work is done but one question area. One of the six heads has a small blemish line where the head gasket sits. Is this a concern? The machine shop guy says, "Maybe ok but if it were mine, machine them all and use thicker head gaskets".

Hi Bob
Email response sent.
There are some high temp ceramic products that can be used to rehabilitate that mirror groove [we use it to repair 964 exhaust inserts] but the best repair is to simply machine the heads. The book says "one .25mm cut and you're done" but with the value and increasing rarity of these engine parts, we machine a minimum amount from the worst head. Then match the remaining 5 heads to that specification. After a minimum cut, a slight telltale impression is a non-issue.

[Henry Schmidt]

Originally Posted by PeteKz

"budget" and "Porsche" don't work in the same sentence.

Well, that attitude can produce some pretty outrageous expenditures.
Even build should/must have a budget and great result can happen with a reasoned approach to engine/component rebuilds. The "best" is quite often overkill and why waste money.
The best example I see often is the Carrillo rod bolt choice.
The Carrillo rods come with a standard WMC bolt but for an extra $ 259+ you can get Carr bolts.
The WMC bolts are rated at 1100 hp and can be reused up to 8 times. The Carr bolts are Inconel with a rating of 1500 hp and can be used until they exceed their stretch limit [basically for ever].
Carr bolts are cool but what 911 engine will benefit at all with that extra expenditure...
Wait for it... "what about a race engine?". Well if you're building a race engine, there are better places to put your money.

[PeteKz]

Henry, don't be so literal--I'm joking, in the context of this thread. But only a little bit.

[PeteKz]

And let me suggest another observation: If the OP is going to use the same cylinders with the head rings, then it's important to get those sealing surfaces flat. However, if he's planning to use new cylinders that don't use those rings, then a small imperfection where those rings eroded can be better tolerated, because the new cylinders have that slight bevel at the top edge that r_lane asked about, and the sealing surface becomes the inner part of the cylinder/head joint.

You pros know this, but for others watching this thread: When the head sealing surface is cut down, that reduces the clearance between the piston and head, and between the piston and valves. Anyone doing this must add shims under the cylinder barrels (not to the head), or test assemble and measure the clearances to make sure they don't get too tight. 0.25mm will make virtually no difference so far as the timing chain is concerned.

[r_towle]

Not sure if what I am seeing is reality, but I have heard of using piano wire to increase the seal at the head to cylinder when using higher CR or forced induction.

Might be a used head from a different engine??