Thanks guys, Yes bones will give them a call. Atleast i see that its been tried and tested. Will defnately give the copper gasket a try.

Head is skimmed 2 much, but still its clean, havnt used it since, Block i will check soon.

I've tried hylomar and vht copper spray with good results. Got even better with a version of 'three-bond' (can't remember which off the top of my head, but the tube is likely still in the shed since around 2001/2 when I actually used it). I think that 'hondabond' might be repackaged threebond, it anecdotally works as well and might be easier to find in some places but likely not cheap.

Within reason, it's not a great way to reduce the comp ratio, because it increases the quench gap to a such a distance that it no longer 'works' at preventing detonation. Don't get me wrong, if there's no other way, then it's 'the go' but using a really thick head gasket to drop the comp ratio - if you could hypothetically run tight quench clearances and use a dished piston (dish to mirror the chamber shape) you might be safe with a cr of 9.5:1 and a given fuel and boost, but with a thick head gasket and no quench, you might have to go down to 8.5:1 to be safe - quench/flame propogation is a big deal to say the least.

Having said that, if it's detonating or pre-igniting, running the thick gasket and lower than 'optimal' comp ratio is still going to work miles better than destroying and engine!

So you are saying, i wont be making my setup work with n 2mm copper gasket and comp-r of 11:1 ? :( :(

Hey Clive,

I am in the same situation as you. My GX head has been milled so much [to run methanol] that I now need to drop the compression ratio.

For me the best, cheapest option is a thick head gasket, the other option is if I run a different head with a different manifold with different carbs.....this all costs extra money though.

Just out of curiosity, what is your compression ratio at the moment? Also, what is the volume of the combustion chamber [cc]. Reason I ask is; you may want to consider 'opening' up the combustion chambers to a larger capacity, that is what I am considering, instead of the thick gasket [and all the sealing issues].

Other option is using a high octane fuel. Do you guys have ethanol blend fuels in SAfrica?

Cheers,
Chris

You can get custom one off cometic head gaskets in mormal thickness but Im sure they come in other thickness options. Supposedly much better than copper.

okay lets try : my gx head is currently : 21 cc on in the combustion chamber.

bore will be 60thou so 77.5mm
Stroke 82 mm A15 crank.
(77.5 * 77.5 * 82 * 3.14(pi))/1000 = 1546cc

so 1546/4 to get to a single cylinder would be 386.622 cc

Im running flat-tops so there is no gain except for the small pockets for the valves.

Headgasket if its 2mm would be (if i work on 78mm bore)
78 * 78 * 2mm * 3.14 = 38.207 devided by 4 = 9.55cc

So a single cylinder = 386.622cc
Head gasket = 9.55cc
head = 21cc

Compresion ratio = 386.622 + (9.55 + 21) / (9.55 + 21)
= 13.6 : 1 about.

Is this right ?

So if it is im considering to have the head metal-sprayed back to std spec...

I did the math too, and that seems right. What about deck clearance? Do the pistons come up flush to the deck?

I wouldnt touch the head. what is the crown thickness on the pistons? you maybe able to machine a slight dish in them. Its not a very big job in the right hands.

What crown thickness is acceptable?

Sorry Clive, i cant remember. Its around .200" from memory. That could have been for turbo applications tho. The minimum depends on piston material and bore size also.
A question for an engine builder..

It would certainly be the best way to drop the comp. Leave a squish band around the edge of the piston. you wouldnt have to take much off.

If you could take 2mm off the top, leaving an 8mm squish band would drop your comp ratio to 11.6:1

so you would be removing 61.5mm diam x 2mm deep material.


Edit. this is presuming pistons are flush with with the deck. You might be able to get a little bit out of the combustion chamber also.

I will find out 2nite i hope, see im running A14 pistons wich im going 2 have machined down to flat tops. Because of the 82mm stroke of the A15 crank. Will take measurements.