Head Gasket

mike-b · Post by **mike-b** » Sun Feb 05, 2012 10:55 pm

Hi all, Sooner or later I will have to remove the `heads on my 3.9.
I would prefer to fit the composite gaskets having used composite inlet ones and found them good.
Obviously a composite head gasket in place of a tin will lower the C.R.
On an engine that has gone no further than a standard 3.9 with performer inlet and eddy 500 - and not likely to go beyond, will a composite head gasket make that much difference?
If I have to skim the heads to make the C.R. back to normal how much will I need removing?
Thats it - thanks guys.

ChrisJC · Post by **ChrisJC** » Mon Feb 06, 2012 8:49 am

According to Rimmers, the tins are 0.5mm thick, the composite 1.2mm thick, so you need to remove 0.7mm from your heads to use the composite gaskets.

In my experience, composite are vastly superior, although don't forget to use symmetrical clamping by omitting the outer row of four head bolts (to leave 10)

Chris.

mike-b · Post by **mike-b** » Mon Feb 06, 2012 2:26 pm

Chris, Many thanks for that - I was hoping to "get-away" with a head skim however I could then be sure they were flat!
Noted the outer four bolts - no problem the block hasn`t got the holes for them - thanks for pointing it out though.

sidecar · Post by **sidecar** » Mon Feb 06, 2012 5:44 pm

This all depends on how fussy you want to be over all of this. The reason is that the head gasket is a full complete circle, the combustion chamber is not.

Assuming that you are talking about 94mm diameter gaskets then the volume that they would add is 4.85 cm3. (This is for a gasket thickness increase of 0.7mm). In order to remove the same volume from the heads you need to remove more than 0.7mm. You can use the fact that every 8 thou off the heads is worth 1cm3 from the chambers. (I've measured this on several heads and the formula does work).

4.85 x 8 = 38.8 thou.

That works out to be 0.97 mm off the heads. I'm pretty sure that Real Steel do 1mm thick gaskets, V8 Dev sell 1.2 and 1mm gaskets. (Their 1mm gaskets are in fact Real Steel ones and they charge you more for them!)

You can also try to work out how much the comp gaskets will compress....I'll get my hat!

mike-b · Post by **mike-b** » Mon Feb 06, 2012 9:07 pm

Chris & sidecar, many thanks to you both. I feel well in myself but my mind has just been wacked.
Chris mentions a .7mm over thick for a 1.2mm composite. Ok fair enough.
I worked it out as only 28thou would need removing from the head.
This is how I done it, 1mm = 40 thou, therefore 0.5mm = 20 thou and 0.2mm = 8thou so chop 28 thou of the heads = same C.R as tin gasket.
I have no intention to argue or disagree with either of you - I`m learning still. The basic problem here is I`m electronic & electrical trade wise, mechanics I can do - however - mechanical aclculating is most definately NOT my forte.
Please sidecar, if you have time - explain your calculation - here I am learning - most definately NOT taking the p**s.

ChrisJC · Post by **ChrisJC** » Mon Feb 06, 2012 9:24 pm

You end up with two possibly conflicting constraints here:
1. Keeping the tappet preload the same, so the distance between the rockers and the camshaft is kept the same.
2. Keeping the same combustion chamber volume.

I always go for the first one.

Chris.

sidecar · Post by **sidecar** » Mon Feb 06, 2012 9:42 pm

The problem is that the head gasket has cylindrical holes, one for each cylinder, these add volume to the total volume above the piston at TDC. Now to remove some of this volume the heads are skimmed but the combustion chambers do not go right out to the edges of the cylinders, they are basically not 'complete circles'. This means that more skimming is required in order to remove the 'excess' volume if you want the CR to remain at 9.75:1 (or whatever it happens to be). If you skimmed the block then you would just remove the 0.7mm that the thick gaskets have added.

Chris is right in that you have two choices but of course you can always have both if you shim up the rocker shafts so that the pre-load of the lifters remains 'in spec'. Really I'm not a fan of pedestal shims, I'd rather use adjustable push rods but they do cost a lot more than shims.

Your engine is displaying the classic case of changing one thing which then causes a change in a load of other things! (All part of the fun!)

mike-b · Post by **mike-b** » Mon Feb 06, 2012 9:57 pm

Gentlemen, I thank you both for your time & trouble.
Today I have learned something new - that is never a bad thing.
I now see the calc, in one form correct - in another wrong - mine.
Pedastel shims! No thanks - I use adjustable rods.
Enough said - thanks guys.

cammmy · Post by **cammmy** » Mon Mar 12, 2012 7:44 pm

Sorry to drag this one back up but I'm curious. Could you not have the block decked and liners machined to suit? More expensive but easier to work out how much to take off and also gives better squish for a happier engine? Would still need adjustable pushrods though.

If you are going to the trouble of taking material off to increase the CR, could you not go a wee bit higher than standard for a bit of extra oomph? I've never tried this myself but I've heard that decking the block as opposed to skimming the head can allow you to run a bit more comp due to the better squish.

Post by **katanaman** » Thu Mar 15, 2012 12:01 am

You can deck the block and that's what I did with my engine. You could then use Chris's calculation and get the right cr and the preloads will be correct. The downside is that its a much bigger job with an engine out and a complete strip down. You also have to be careful not to take too much off or the pistons will hit. No problem in this case but something to keep in mind if your really want to up the CR.

kiwicar · Post by **kiwicar** » Thu Mar 15, 2012 11:22 am

I think this is the best way to do the job as it restores the squish clearance, but as said above you can go too far and hit the heads with the pistons.
Best regards
Mike

mike-b · Post by **mike-b** » Sun Mar 18, 2012 10:56 pm

Gentlemen please, slow down. I thank you all for yor comments and advice you have all been very helpfull and yes I have learn a lot simply by posing one question and true to form advice has poured in from all directions - can`t fault it. However .....
Here I have a simple 3.9 with the 500 and performer fitted and a BW T5 WC on the back. This all fits in my 1984 Granada. The engine change is due to the fact I happen to think the Ford 2.8 V6 is bloody rubbish so it got recycled along with its Ford C3 slush box.
The RV8 is a lot better fuel wise and I keep the system as simple as possible but at the end of the day I am not looking for masses of power or kicking ass on the queens highway either, well not untill a "Shopping Trolley with a drainpipe exhaust appears then its a different ball game.
I just like keeping it all simple .... I follow the "KISS" system throughout my work on the car. It may well not be the best way of doing a job but it is very reliable.
So I thank you all for the info and it has answered my original question.