The V8 Owners Forum -

I am replacing the tin gasket with a thick gasket. How much should I get skimmed off to keep the compression standard (9.25:1) ?

The engine is a 3.9 l Range Rover (1990) vintage.

Thanks.

sam russell wrote:I am replacing the tin gasket with a thick gasket. How much should I get skimmed off to keep the compression standard (9.25:1) ?

The engine is a 3.9 l Range Rover (1990) vintage.

Thanks.

It depends on which thick head gaskets you have gone for, some are slightly thicker than others.

20-30 thou would be OK and you should not have to mess about with lifter pre-load or a inlet manifold skim as the heads will be in the same place.

Also I'd ditch the stretch bolts and go for normal SD1 jobbies. (you can't re-use stretch bolts anyway. There are some cheap dodgy ones on ebay)

Pete

I was going with 20 thou, would 30 thou increase the compression ratio much?

sam russell wrote:I was going with 20 thou, would 30 thou increase the compression ratio much?

It would not increase it much by going to 30 thou, it might not increase it at all if the new gaskets are quite thick. At the end of the day 9.25:1 is pretty low anyway, if it ended a fraction higher is would not make any odds to the power or the fuel that you would have to use.

My heads (36cc chambers) have had 70 thou taken off them to get the CR to 10:1 on my 4.6 lump.

If you really, really wanted to keep it EXACTLY the same you would have to "mic" up the gaskets and somehow account for the fact that the new gaskets will compress, you then have to work it out using maths.

Pete

OK, I think I will go with 30 thou, as a small increase in compression will not hurt, and will give a better performance - even if slight.

The reason for going with composite is that there are clear signs of head gasket leak out at the ends of both heads. Thanks for the replies.

When LR did the interim engine and used they comp gasket on the older heads they skimmed an extra 40 thou off to keep the compression the same. When they went to do the full cross bolt engines they redid the casting which was again 40 thou less. I guess this is the result of the of the chamber being a bowl so as you skim you get diminishing returns.

Real Steel told me to skim 20 thou to retain the same comp ratio. I went for 25thou, and all seems fine.

Pete

Thanks for all the answers, it appears a bit more flexible than I thought. Anything from 20 to 40 thou. So 30 thou it is. I'll let you know when I have it up and running.

Sam,

With Real Steel gaskets for the 3.9 the tin is 18 thou. compressed and the composite is 46 thou. so a difference of 28thou, a 30 thou skim will be fine and maintain lifter preload approx as before.

Kevin.