Deck height

[SJ Lambert]

I'm at 10 thou below the block deck with my pistons. I've spoken to Dale who can offer gaskets at 1.1mm and 1.5mm thick and increasing thicker combinations of those two base thicknesses.
Though he recomends that for reliability that either of the two base thicknesses be used in situations where the gasket will be "tested".

I'm going to go with a 1.5 mm one which'll give me plenty of clearance and the ability to go thinner at a later stage if necessary.

[promotor]

That'll give you approx 0.070" squish (if those 1.5mm and 1.1mm figures are compressed gasket thickness) which is an awful lot for a performance engine - I think stock figure is 0.060"-0.065".

I would have thought that will not help with efficient burning in the combustion chamber. A 1.1m gasket with pistons 0.010" below deck will still give a fairly high clearance and which could still be improved on while remaining on the conservative side for later adjustment.

[rgh0]

The gaskets compress about 30% of their thickness. So the 1.1 mm gasket is about 0.7 to 0.8 mm thick installed
This gives a static clearance of around 40 thou or 1 mm when combine with your 10 thou piston deck to top of block. That is about the minimum I would aim for given the deflections you get at 8000 rpm in rods and crank.

I know people who push it and go for less clearance than this and I also know people who have the pistons kiss the head.

If you go for the thicker 1.5mm gasket it will be about 1mm to 1.1 mm thick installed, so you will have about 1.3 mm or 50 thou clearance when assembled.

cheers
Rohan

[SJ Lambert]

Dale reckoned gaskets would only compress about 7 thou - I must admit I suspected that even the 1.5mm one might thin down a bit more than that.

My sums have using the 1.1 mm gasket giving 13.2 compression - I figured that using the thicker gasket and not adjusting the intruders will allow me to run it at "wide spec" vis-?-vis keeping the pistons away from the head & valves - see how it goes and then make adjustments if necessary. ( and I do have delusions of grandeur over achieving a lot of RPMs in this motor!!)

Shaving stuff now makes it harder/impossible to go the other way.............

[rgh0]

I think Dale is a little off in how much his head gaskets compress in a twin cam based on my measurements.

I normally base my engines on a bit more clearance from piston deck to top of block ( say 20 thou) and then use the thinner gasket and if you fit a "thin" head has had a lot of machining you can use the thicker gasket to adjust the compression ratio. without needing to pull the pistons for more machining.

cheers
Rohan

[SJ Lambert]

Rohan, are you using Dale's normal composite gaskets (that he says one can use hylomar with to ensure a good seal) or his "enhanced race" ones with extra steel on their faces (for which he recomends a 3 bond sealant)?

Cheers

James

[rgh0]

Hi James

I use his "normal" composite gasket with the "silver paint" surface. I don't use any additional sealant and have never had any leakage problems using either conventional head bolts or ARP studs. A friend of mine building similar high compression race engines did have some problems with external water leakage at the rear of the head at 9000+ rpm but these were fixed when he changed to ARP head studs. A couple of pictures below of what I use.

The twin cam appears not like to stiff a head gasket and people I know have struggled to get the Cometic multi layer steel gaskets to work. I would be worried you would have the same problem with a steel faced enhanced race gasket from Dale although I have never tried them.

cheers
Rohan

[SJ Lambert]

Thanks Rohan, I ordered a silver one too, the mention of steel enhancement did sound a bit disconcerting.