3.8 Composite Head Gasket Bore Size?

[scarr27]

Hi All,
I have purchased a composite head gasket from a jag suppler (not one of the usual).
What is confusing me is that the bore sizes on the gasket are 84mm but a 3.8 cylinder bore size is 87mm.
This started me thinking and after some research I have found that you can get a single gaskets that covers both the 3.8 and 3.4 jag engine. (e.g. Cometic C4303). This would explain the 84mm bore which must suite the 3.4 engine.
So..... If used on a 3.8 engine, the metal lip around the gasket bore must intrude into the combustion chamber area by a small amount (1.5mm). The piston does not quite reach the top of the cylinder at full stroke so I guess this isn't a problem but 2 things are bothering me:

1. There would be less of the metal 'ringed' part of the gasket getting squashed by the compression of
torqueing down the head. (remember it's a composite)
2. Would this not affect compression ratio slightly? (The gasket is 1.5mm thick.)

I'm not sure about using it but is there any head gasket experts out there that could put my mind at rest??

Any reassurance appreciated.
Thanks
Sean

[abowie]

The top of the piston should sit below the deck of the block at TDC, so your piston will not hit the gasket. The piston dome is small enough to go in the hole.

Personally though I'd feel happier if the gasket was the right size. A few thou overlap on an overbored cylinder is one thing but yours is sticking out a lot more than that. The crush area won't work as well and who knows what could happen if the protruding edge breaks off..

WRT the thickness, 1.5mm or 60 thou is fine. In fact given that your block and head will have been in all probability skimmed more than once over the years a slightly thicker gasket will be good, especially if you have used 9:1 pistons in your rebuild.

[christopher storey]

I have to say that I would be very unhappy with composite in the combustion space, and 1.5mm is a not inconsiderable protrusion . I would be very surprised if it did not burn or at the very least become incandescent. If the latter occurs then you are likely to get quite pronounced pinking , and probably running on, both of which can be very damaging if they are prolonged

[scarr27]

Thanks for your replies guys.
You've both backed-up my concerns so think I will see if I can get a gasket with the correct bore.
Still leaves the question though of why SNG, Cometic etc are selling a gasket that covers both 3.4 & 3.8 engines?

Cheers
Sean

[mgcjag]

Hi...have a look here ...they list a whole range of cometics...and what models they are suitable for https://www.burtonpower.com/cometic-hea ... 5-059.html

Burton power list a C4303 but in its spec it dosnt include E type. .......do all 3.8 xk engins have the same bore size?....Steve

[christopher storey]

Yes , all 3.8 are 87 X 106mm