double head gasket
Turbo Mini Forums

Hi all

is it possible to use 2 x head gaskets to lower compression ratio?

I have a 1380 block
omega 73.5 pistons (measured dish with a syringe filled with water and a tad of washing up liquid and seemed to take 7.5ml)
gap to top of block from piston...dont know...but carried on filling the dish in the piston until it overflowed and filled the rest of the area until flush with top of block. This measured 11ml

I did the same for the head chamber volume and got 26ml (head has had work)

so...tell me if i am right here.... 1380CC divided by 4 (number of pistons) = 345
head chamber of 26 + bore space 11 = 37
+ turbo head gasket of 3.8
total = 40.8

40.8 + 345 = 385.8

385.8 divided 40.8 gives a compression ratio of 9.456

if i could add a second head gasket of 3.8 i would get (385.8 + 3.8 = 389.6) divided by (40.8 + 3.8 = 44.6)

389.6 divided by 44.6 would be a compression ratio of 8.735

this is closer to where i want to be in order to run more boost.

is this feasible or just plain stupid?

it can be done, but I'd only recomend it as a temporary measure,

for the price of getting it done its worth looking at getting the chamber taken out bigger if it is possible.

dont really want to mess around with the head anymore....it already has 37mm inlet and 29mm exhaust along with porting , polishing and 26cc already seems pretty large....would hate anything to happen to it while getting work done :o

well, as long as its not been skimmed 28cc is a common chamber size,

the other posibility is getting the piston dishes taken out larger

so does 11cc seem about right for the bore chamber?

You'd defintely be better off machining a couple of cc's out of the head and a couple out of the pistons.

Fitting 2 head gaskets may work for a while, but if you're planning on running any kind of boost through it I can't imagine them lasting overly long.

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1/4mile in 13.2sec @ 111 terminal on 15psi

Its feasible but insane!!!
do what the lads recommed above

Scrap that, just re-read it.

If your deck height is about .035" or about a millimetre then the 11cc may be correct.

Edited by Alex on 7th Nov, 2012.

Metric is for people who can't do fractions.

hmmm....so where to get the extra 3.5 - 4cc from?
if i hollowed out the the top point of the heart shape on the head (where the inlet and exhaust valves sit..opposite the spark plug) would this be the best place?...the head already seems pretty optimised and reading DV book (how to modify your mini) he rekons 25-26cc is about it

Remember to check you rocker geo after as your changing the height of the head.... Only buy a small a mount but still worth checking.

When I checked mine they needed a 3mm shim under them to correct. The other option might be a single headgasket of a different thickness, I think faraday do special stuff and also iirc cometic do a thicker one... But I might be dreaming..

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

hi gr4h4m
dont know what you mean?
got 1.5 rockers...and not really changing height of head...ah yes i am if i use bigger head gasket...but surely the valves open the same amount so why is ther a difference?

gr4h4m see what you mean
extra height means the push rods will be short.
cant i sort this by doing the rocker tappetts?

I wouldnt use two head gaskets, especially on an fi engine, its asking for trouble. Probably the best way as said is to take a few ccs out of the chambers

You can adjust them but thee might foul the holes in the head, or run out of adjustment, the advice I have had is when closed you want the rocker parallel with the surface of the head, with push rod height being wrong they will be nose up or down!

My head had had a lot chopped off so I have to run a 3mm shim to get them back to square, so much so that when I ran 1.5 I ran out of adjustment on the nut.

Just some thing to think about, a better solution is to size the piston and head. Next down from than might be a faraday gasket, however they recommend having everything perfectly flat (head and block).

Two gaskets is going to be a bodge and will be less reliable than doing it properly.

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

This makes no sense, how can altering the thickness of the head affect the rocker alignment in relation to the valve?

It does alter it in relation to the pushrod, i.e. you have to adjust the adjuster. The further out the adjuster the less lift at the valve you get. But the thickness of a head gasket is neither here nor there, assuming it's ok at the moment.

The only proper way to compensate for the head being skimmed or raised (in this case) is different length pushrods.

Spacers under the rockers are not to compensate for head thiskness or block decking. Rather they are to fix issues of rocker geometry where you have sunken valves, or long or short valve stems.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

First question should be what level of boost did you intend to run.

9.5:1 would be fine up to 10psi with a good intercooler.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Having used a de-compression (steel sheet) plate with 2 head gaskets I can recommend not going this way. As a stop gap to buying pistons I had a copper plate machined as a one piece gasket/de-compression plate. 146nannas and holding strong.

Decompression plates only work of they are made out of cast iron - the differing expansion rates of the head, block and (cheap) steel plate tears the gaskets.

Metric is for people who can't do fractions.

Remember that it is the temperature of the air entering the combustion chamber that is the enemy, not the boost.

So a better matched turbo compressor and an efficient intercooler could do a better job than lowering the CR.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

so what would you recommend then Paul?

Depends on your hp target.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

lol....lots

In that case i'd be investing in some decent pistons with a larger dish.

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1/4mile in 13.2sec @ 111 terminal on 15psi

You know what your correct. It was a different issue that caused the reason for the shimming.

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!