Cometic Head Gasket Recommendations?

(Christopher Potempa) #1

Guys I’m sorry if I’m beating a dead horse here but in my search of the archives I’m not finding an exact answer:

For an XK120, I’m considering fitting the motor with a Cometic head gasket. In researching which one to buy I’m a little bewildered by the choices. Perhaps you have some ideas of which would be the best candidates? I’m partial to their multi-layered-steel types. My engine is a 3.4L, 8:1 compression model with new Mahle pistons of 0.020" oversize. My machinist took 0.014" off the bottom of the cylinder head and at most, 0.002-0.003" off the top of the block to clean things up. Is that enough info to start with? Thanks.

(Paul Wigton) #2

First: did the machinist bolt the timing chain cover on, before decking the block?

Second: @Ray_Livingston has a great write-up, about deterring changes to compression ratios.

(Christopher Potempa) #3

Yes, he did Paul. I was adamant on that one.

I will look for the @Ray_Livingston write-up, thanks.

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(Lee140FHC) #4

When rebuilding my 3.4, I also had the head cut about 15 thou…but this was after doing MANY volume measurements of combustion chambers and pistons, using on-line calculators to help determine compression ratio. As it turns out, my original engine came out to about 7.5 to 1, not 8 to 1. I ended up using a Cometic MLS gasket, which, IIRC, was only about 17 or 20 thou thick…you can get them in various thicknesses.

(Christopher Potempa) #5

Something that might make things a little easier is that the block, pistons and crank were all still at their factory specs. So I’m not left wondering how many times the block has been decked and such. With the non-original (but still XK120 numbered) head, I’m not so sure. But it having been successfully mated for years with the block that still had its original pistons, I’m thinking I should be able to buy a standard thickness gasket plus 0.017" or so. Paul has referred me to a wealth of info so I’m still researching this.

(Lee140FHC) #6

Another note on the Cometic: The one I received was not quite “right” with respect to the stud holes…I had to slightly relieve(elongate) a few of the stud holes to get the alignment correct so the studs would pass thru into their bores and get the gasket to fit over the (4) alignment dowels I used. Also, CC your combustion chambers to get them even and to calculate CR…IIRC, mine ended up at 104 CCs.

(wardell) #7

Funnily enough I have 2 heads in my workshop right now. One I have had since 1977 and never been skimmed to my knowledge.
I’ll measure the distance from the face to the very top of the combustion chamber and report back.

(Christopher Potempa) #8

I’ll be curious to know what yours measures out to. I’ll do the same with mine. I have to cc it and also figure the cc of the domed area on my new Mahle 0.020" OS pistons. There was no paperwork that came with them and nothing on the side of the box either.

(wardell) #9

The Stype head, as far as I can tell has never been skimmed and has done about 75,000 miles. The head off my MK9 has been off twice, and I assumed if may have been skimmed. However both heads have a measurement of 1 5/16 inches 33.375mm from the top of the combustion chamber to the face of the head.
The MK9 head has chamferd combustion chambers the Stypea hasn’t.
I could measure the diameter of the combustion chamber where it meets the head if you wish. This diameter will reduce if the head has been skimmed.
Does your head have chamferd combustion chambers?

(wardell) #10

I have just realised that my engine is 3.8 whereas yours is 3.4. This might be relevant, not sure?