Cometic head gasket question

I I have decided to replace my head gasket since it appears to be leaking. I wanted to use a cometic Gasket and was wondering what people’s opinions were. It also comes in 2 sizes .043 and .058. I I am not planning on skimming the head so would assume that the .043 would be the correct gasket but just wanted to confirm

I’m about to install .043 in mine. I’m not sure how that compares to a stock composite 4.2 gasket. It is quite a bit thicker than stock 3.8 though since that is flat tin. If it is thicker than stock, then only CC’ing the head can tell you if it’s correct.

Has it or the block ever been skimmed before? Are you getting any detonation under load? Is this S3 car? That is what is listed in your profile.

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Sorry, 67 series 1 4.2. Dont think it has been skimmed. No detonation under load, but car isnt driveable.

I’ll hazard a guess the the stock tin is ~0.025"

How critical is the thickness?

It would simply lower your compression ratio slightly, assuming it’s actually thicker. I’m looking at the one I received and to my eyes it doesn’t look much thicker than the stock 4.2. Maybe not at all. It’s quite a bit thinner than the MLS Cometic I had on it. I hope it is sufficient. It does not have additional sealing properties for the water jackets. It does have metal crush rings for the cylinders which I think the stock Payen gasket doesn’t have…not sure though.

Have you pulled the head yet? I’m wondering why it’s leaking. An obvious cause would be corrosion that might indicate a skim is advisable, although it’s possible to use JB if it’s not severe and you clean it well. It’s also possible that it wasn’t torqued properly though.

do use the search magnify at the top rt. ,type in cometic gasket, and search, later use head gasket…and search…,click “more” at the bottom of the first list, You will find many archive results that will be of interest, Nick

If you suspect the head’s not been off in quite some time, It’s highly likely that the water passages are corroded, and will need welding and then planing. At the time, a complete head job would be recommended.

There rarely is a cheap way out of doing a reseal on an XK head.

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The leak was very slight but I am changing my valve stem seals so figured it would make sense to put a new head gasket. I was also wondering if when I order it I use the fact that my engine block was changed to a 1970 block and the head is the original from a 67.

I have a couple of head gaskets for a Ser II block. They are approximately .056 thick. The 1970 block is different from the 67 block in a number of significant ways, studs go through the deck into main bearing casting, and the block is longer with extra cooling holes, in particular at the rear. Presumably these have been blocked off, as they will leak with a short head from a '67. I’d use a Ser II head gasket as it will work with a '67 head though a bit of the gasket will extend past the head at the rear.

PS Unless there is something wrong with the mating surfaces you don’t need a Cometic gasket. The stock Ser II gaskets will seal just about anything.

As said above, the thicker the gasket the lower your cylinder compression pressure would be.

40 thou of gasket thickness contains 6cc of air per cylinder on a 4.2.

If you are concerned the easiest thing to do is CC your head. Then you will know exactly what your compression ratio is and you can pick a gasket to give you whatever you want, within reason.

Years ago I made a mistake on my first 4.2 rebuild and ended up with about 195 psi compression which led to that engine pinging despite everything I could think of, including fitting an EDIS system.

I ended up taking off the head, CCing it and fitting an 80 odd thou gasket to replace the stock composite one, which brought the compression down to about 170 psi and fixed the problem.

I know I am ignorant but what is CC ing and how is it done?

Search under “Ray Livingston cc’ing a head”.

Odd how some engines handle that compression fine. I think Drew said his '67 is near 200 psi but no pinks. Mine pinked badly with the stock 3.8 tin gasket. Stepping down a bit now with the .043 after having a thicker MLS in there for years. Hopefully it’s sufficient.

Yes, I have pretty high compression with some cylinders being up around 205. I did have pinking at times early on until I found two cylinders were lean and adjusted a carb. I also run a 123 distributor that CJ adjusted to fit the engine. I’ve put over 20k miles on the engine since the rebuild at CJ and it runs beautifully these days (knock on wood, fingers crossed, etc.).