+1 to Jem compression ratio is not that crucial in terms of power. I think old schoolers played with comp ratios cause of the bad fuel quality they had back.

We used a thick gasket to make up for a block that had been repeatedly decked.  We had some sort of Formula Jr. historic engine that went together with .080 of piston poking OUT the block. The owner never indicated it had been hacked this much, so much machining $$ had already gone into it.  We got a Cometic made up to 'simulate' a zero deck and 'normal' head gasket thickness, so about .110" if I remember.  The engine worked great for many years in Vintage racing.

What can be more important than static compression ratio for the way the engine works is the "effective" ratio, once cam shaft overlap and timing are considered.  "race" engines running 14 to 1 have huge overlap with BOTH valves open as the piston rises in the bore. This causes some of the mixture to be pushed back out the intake AND out the exhaust valves such that your 'effective' compression ratio is more like 9 or 10 to 1.  Anyone who has installed a "race cam" in a stock engine has learned that generally results in a dog of an engine! With 'normal' compression, lots of overlap results in a VERY low effective compression ratio and disappointing performance.  Cam timing can be tweaked to close the intake valve a little earlier (relative to the piston rising on it's compression stroke) as a way of fine tuning the effective compression ratio.  To me fractions of a point of 'mechanical' compression are just not worth chasing after.  There are easier ways.  I favor as much compression as possible.  It's one of those rare things that gives you power AND fuel economy!  The downside CAN be engine destruction however, so an attentive driver is mandatory!

It's not BAD per se to have pistons down the bore, but it is robbing power. and it's the same whether the space is in a bore or if it is in a gasket (extension of the bore, essentially) Aaron Anderson said he experimented with this in some LeMons build motors (zero decking block and machining pistons and carving out heads) and he said he was amazed at the difference it made.

Standard bore Rover 1275 for Japan shows 11 cc above the new std. Rover piston rings to deck, with Payen gasket at 0.40" compressed and 21.4 cc in chamber works out to CR of 9.72:1.

On my block (the old race build), I'm thinking the Burette will show 6.7 cc above the rings & dish...which will be the key number before wrestling over digging into the head or going Cometic.  With the same Payen gasket, the indicated CR is 10.65:1.

Let the measuring begin....

Engine building gets some people thinking about all kinds of things. My block was or is a old 1275 Cooper S one that had been run at 13.30 to 1 for most of it's life. I have always though that zero decking reduced preignition. Next thing I know I'm looking at a piston crown way down in the gasket. May have had something to do with my picking 9.5 to 1. That and Alex will be driving soon and mid grade gas is cheaper than hi test. 

I'm building my next road burner power unit a 1310 Metro with Mega pistons, SW5-07 cam, alloy 5 port head, 1.5 roller rockers, LCB, MSC intake and HIF44. CR is a almost perfect 10.25 to 1 and it's atop a wide ratio double groove gear set and two O/D drops and a 2.76 CWP. I'm looking forward to testing this one 3000 rpm at 70 mph and 4000 rpm at 96 mph. Steve (CTR)

.140" is the thickest they list, and that is $228, so .180" could easily be $250. 

Makes me wonder though- why is it so bad to have the pistons down the bore but okay to use a thick gasket?

my gramps 850 came factory copper, lasted good 35yrs till I disassembled the head

Some years ago an engineering shop here in Oz was water jet cutting solid copper gaskets. For race cars of various type, including A series.

I don't know how successful annealed copper gaskets were on an A series, my only experience with these was on a 4 cylinder Mercruiser stern drive years ago. They worked OK there.

I had a custom one made some .180 thick. I'm sure I can find the paper work some place. Steve (CTR)

Haven't ordered anything Alex...doing research against what I think we are likely to find.  If much closer to 10:1 than 11:1, we might get out the engineering blue and grinders.

The MLS items are pricey and then the shipping delay but if we near or over 11:1 when measurements are all in, the MLS may be the best way to go. 

The list price on the Cometic website for a 1300 Mini MLS gasket up to .051" thick is $98.24 - from .055" to .075" is still only $125.99...

Which begs the question - was yours shipped in a gold plated box and hand delivered by the MD ??

EDIT: If you search the Cometic site for an MG Midget 1275 gasket instead of an Austin Mini 1300, it returns a bore size of 74mm instead of 73mm, which should cover all but the never-done-anymore 74.7mm overbore.  Prices are identical to the 73mm Mini gasket.

When I was supporting several vintage cars I was spending a whole lot more. Then gas and everything went up. I do mostly street builds now. Steve (CTR)

if I did this in my previous job(parts salesman) my boss would kick me out.

Very nearly the same issue building my 1360 S engine. The proper Cooper S block had been with me since 1971. Having powered a SCCA Solo 2 national car  the block was bored .020/ .040/ .060 line bored squared and zero decked before getting 1360 pistons and becoming a street unit. I have made a draw bar to pull pistons back against a face plate on my lathe to allow me to shorten the crowns. I recently took part in a build requiring me to grind a full CC out of all chambers on a 940 head. My Cometic was a little over $250. with shipping. I was a little unhappy with the boys. I called on a friday asking about a price and thickness for a custom gasket. Quoted over phone $150. plus shipping. I thanked the guy wrote his name down and promised to place the order monday after going over my numbers once more. When I called monday the price was $250. with shipping. When I ask about the price increase I was told he checked my records and I was not buying enough to continue getting a discount. I refigured my numbers and decided to order a gasket which would give me 9.5 to 1 rather than 10.2. I can run mid grade fuel and there is plenty power to get anything I want to do done. I have also had gaskets made for vintage racers needing a CR reduction. 

Working on a 1310  this week and the alloy head has 24 cc chambers. Looks like the numbers work out at 10.25 going with it. Steve (CTR)

I do have NEW ARP studs, NEW 1.3 roller tip rockers...guides were reamed/honed to suit the valves being used (not all valve stems are the same).

Will keep up the info & photos on the 1293 build thread. 

Thanks Chuck!!

Another Cometic user. Used on the 1360cc, supercharged Moke to drop compression to 8.3:1. Also used on a stroked 1360 (1406cc) to get compression down to 10.1:1. No problems with either and saved having to dig out heads.

I had a problem in the very beginning using a Cometic gasket, but have had no problems since that initial issue (that was as much my fault as it was the gasket's).

They are indeed reusable. Be careful that the block and the head are completely flat and of the correct finish. When I've reused them, I've used a thin coating of the copper spray adhesive. Oh, and ARP studs.

I ordered my last round off of ebay. Whomever you get them from, they are drop-shipped straight from Cometic. If you email them directly, they will help you select the thickness that you need to achieve the compression ratio you want.

I'm looking at th 0.063" and 0.070" inch Cometic gasket because the block I'm using was built more than 20 years ago to go racing...they never got the engine to run and the owner died.

On pulling the head I was looking at clean piston crowns, fresh honing in the bores, clean combustion chambers...I set aside (and later sold) the 10 30 crank, to use a NEW A+ crank, with NEW A+ rods and tossed the head for an unmolested 12G940, which has been rebuilt with new guides, std. valves, springs etc.

When testing crank & rod bearing clearances, the 20 over Cooper S 19320 pistons were 13 thou down the bore suggesting that with std. 12G940 and hardened exhaust seats we might be looking at closer to 11:1 versus 10:1...for a road engine with SW5 cam, MG Metro intake, Maniflow Cooper Freeflow header and Manifliow twin box exhaust.

I like the idea of the Cometic because I want no more than a 10:1 compression ratio.

We will measure the volume below the deck of the block (est. at 6.7cc - 6cc dish & 0.7cc above the top ring) and the chambers with new hardened exhaust seats in the 12G940 head (21.4cc??) to see exactly where we are.

Bottomline, it seems that the Cometic (reuseable) gasket can offer a solution versus taking meat out of the std. head.

PHOTO:  the old Austin America block, crack tested, hot tanked, prepped for engine build. 

I had Cometic make two copper gaskets for my twin cam A-series. They were great to work with And resulting dead soft copper gaskets dialed in the CR at 10.2:1. That was as close to 10:1 as I could get. Quick turn around and resonably priced. I believe they have patterns for stock A and +A heads.

I wanted to have the gasket in their multi-layer steel material, the water hole spacing was too close to the cylinders to allow their hole edge treatment to provide proper sealing when installed.