Head to Cylinder Seating

[WERK I]

I'm doing some pre-assembly work on my 930 engine. I recently had the heads worked on by Kelly-Moss (valve guides, exhaust valves, head sealing surface touchup). I just measured the gap between the two pads of the cylinder and head and came up with a gap of approx. .006" (.152mm). Does this seem excessive?

[cgarr]

Normally when cut you would also take off the same amount on that surface too. As long as there is clearance,

The depth of the sealing surface to the pad on the head should be around .100

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[WERK I]

Craig,
Thanks for your input, much appreciated. I had to remeasure the above dimension since I found a little valve lapping compound hiding on the head surface. The dimension is more like .005" after the cleanup. I was asking about this dimension to see if the head sealing surface would distort once the heads are torqued down.

The pads on the heads were touched at the same time the sealing surfaces were touched up.

Out of curiosity, is this process a one-step procedure when your cutting the heads? In other words does the machining cut both areas at the same with a special bit? Do you have a pic?

[cgarr]

I make 2 passes, sealing surface then the boss area with a .100 step.




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[WERK I]

Beautiful! Is that a special treatment you've applied to the combustion chamber?

Last question, promise. If I wanted to use the C2 Turbo Head Gasket (.025"), should I machine the sealing area of the head the same amount?

[DRACO A5OG]

Master CGarr, will better answer this but from my understanding is to use the Flame Ring (964) gasket you need to accommodate cut the head to keep it in place.

[WERK I]

The 964 Turbo Gasket (pictured below) is just that. A metal gasket that sits between the head and the cylinder. It's approx. .025" thick and sits nicely in the cylinder head recess. I don't know whether the cylinder head has to be machined an additional .025" to compensate.
A flame ring such as the Ni-Resist ring below requires machined grooves in both the head and cylinder.

[WERK I]

Still no answer to this question about the C2 Turbo Head Gaskets. Maybe some background on this would help. This engine I'm rebuilding is an 930 3.3l that originally had the half-finned cylinders. The previous owner fried a piston AND cylinder so the Porsche shop put in a new set of C2 Turbo pistons and cylinders. The original 930 heads were retained. They did not mount the C2 turbo head gaskets.
I know there are far fewer C2 Turbo's out there compared to the total of 930 3.3 turbo's, so the knowledge pool on these heads might be more limited. It would great to know if the C2 turbo heads are different to compensate for the .025" of gasket material. Otherwise the compression ratio will probably drop a quarter point (.25). 7:1 compression is bad enough as it is.

[cgarr]

If you measure the sealing surface to the cam carrier surface it should be 84.5mm if the C2 heads are .25mm less that would be to adjust for the ring I would guess.


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[WERK I]

Thanks for replying, Craig. I'll measure the distance between the two surfaces. The only reason Porsche added the head gasket, from what I've read, was to address cylinder-to-head oil weepage on the C2 turbo's. I may forego the head gaskets if the heads are at 930 3.3l specs.

I'm using 993 TT head studs with 964 head nuts to help insure equal clamping across the cylinder under operating conditions.

[dt930]

Hi Dave, You need to have the heads cut to use the 964 head ring. Been there done that on the blue beast. Worth the trouble.

[WERK I]

Dave,
Thanks for the advice. What hot projects you working on? Watched a couple of YouTube videos of you and 993 FabCar at RA. Great driving that beast!
Going to be in the shop Monday? Give you a call?

[dt930]

Yes, Give me a call.