My 3.8 engine head is off at the moment and so at one point I will have to put it back, and so I was wondering what was the current consensus about the head gasket to order?
As far as I know there are 3 available (sng barratt et al):
~50€ stamped metallic one (similar to the one which was installed in the engine)
~110€ composite one (Payen)
~200€ composite one (Cometic)
I guess a composite one with the viton seals is probably a good idea so I would tend to rule out the metallic one (?), but is there an obvious difference between the Payen and the Cometic one? (other than the price)
FYI my head and engine block (water passages etc) look pretty good to me at first sight. I would need to post a few photos.
On a side note, I don’t know if the head have already been skimmed in the past. How do are we supposed to check? Is the original height documented somewhere? (I think there are thicker gaskets for skimmed heads, which is good news in case I need one!)
Finally, the workshop manual says “Check the bottom face of the cylinder head for truth.” which I am sure sounds nice in a Buddhism lecture but in engineering terms is not so very precise, so, again what is a consensus about the flatness tolerance? Any data published by the gaskets suppliers for instance? I reckon they know better than anybody else what their gasket can cope with?
Sure but what is the tolerance?
On another car I have for instance (4 cylinder) it’s should be max 0.25 of a millimetre between cylinder 2-3 and about 0.12 between 1&2 or 3&4, and 0.06 in the transverse measurement.
But as far as I know it’s not specified anywhere for the XK? (maybe in the late manuals for the 80s cars?)
I used the Cometic because you can get them in different thicknesses to make up for shaving during rebuild. If your head and block have never been decked then a standard gasket should suffice.
The tin one will work, but you should really give it some help. The spray on Permatex copper gasket sealant will help it seal. As mention, everything must be perfect, especially around water jackets. Otherwise just go for the thicker stock composite. It will lower your compression a hair but not enough to worry about.
0.001 to 0.0025? Isn’t it a bit overkill? I have no experience in that regard but as I said I have my workshop manual from another car (mid 90s Lotus Esprit, alloy block and head) and the tolerance to not machine it is much more than that (composite gasket too). The way I understand it is it should be as flat as 0.015" maximum after machining it but it can cope with more than that before having to machine it?
I thought the Payen was available in different thicknesses too but indeed it makes a point for the Cometic if it’s the only one.
Of course I don’t know if the head has already been machined since I am not the original owner; what is the original height of the head?
I agree the tin gasket is not a bad item, or at least the original one made in England as the one I had in the engine was there for at least 30 years and has no apparent fault (except that it was leaking coolant for some reason, might be a torque issue after installing a hook similar to the Churchill J8?)
As for the compression I have 1:8 pistons and it had about 10.6 - 10.8 bars on all cylinders so it seems pretty healthy. I hope it wouldn’t lower it too much! I also read some people had trouble fitting the chains because of a thicker gasket, so it would be better to check the head height anyway.
It’s established by measuring the volume of a cylinder dome. You bolt a piece of Lexan over a hemi. The Lexan is sealed lightly around the edge with grease or whatever, and it will have a small hole to drain a viscous fluid like ATF into it from an accurate burette. Once it is completely full you determine how much you added and plug it into a spreadsheet and arrive at your optimal gasket thickness. It is the best, most scientific approach, and can be fairly essential given the tolerances of the original casting/machining, and the fact that they are 50+ years old. My head has been rebuilt 3X that I know of. Without this step, it’s just a guessing game. It’ll run no matter what you use. It just might not run as well as it could.
As we know there is stuff in the archives about this and the potential to get over 10:1 compression with the wrong gasket selection. This can result in “pinging” at standard ignition timing - even with better fuels.