COMPRESSION RATIO - PISTON HEIGHT

[terry h]

Hi Folks , I am rebuilding a 1098 engine, the block is standard bore and pistons and has been skimmed by 30 thou, the crank /flywheel/damper/clutch are NOS BMC/BL and have been balanced,cylinder head is a recon 12g295 with a combustion chamber of 23.5cc .
The top of piston to the block face is only 9 thou, is this enough ? and will the compression ratio be to high ? the engine has small bore 3 branch manifold and exhaust, hs4 carb on water heated manifod also what distributor should i use? Thanks Terry

[BobHood]

The head gasket will add some clearance to the piston height, however, you have to make sure that the valves don't touch the piston at TDC, and you can only do this once the head is on and properly torqued down. So be careful turning it over when you first bolt it down.

[oliver90owner]

Piston cross sectional area and ~0.2mm will change the effective combustion chamber volume by about 0.7cm^3 (easily calculated?). Different suppliers might have differences of compressed gasket thickness of that order? Check out yours? Combustion chamber size in the head is not a particularly practical measurement if you wish to calculate compression ratios accurately.

[philthehill]

If your 12G295 head does actually have a combustion chamber of 23.5 cc it has been skimmed and/or has modified combustion chambers..
A 12G295 head normally has a combustion chamber of 28.3 cc.
It is not unusual to have the top of the pistons just below the top of the block.
My own 1380cc engine has had the block height reduced so that the pistons are just below the top of the block.

What I would do and always do to check the piston to valve clearance is to fit a used head gasket, put a thin strip of plasticine on top of the pistons, fit the head and valve gear, torque everything up and gently turn over the engine through at least five complete revolutions. If any resistance to turning is felt - stop and find out why.
Strip the head from the block and see if there is any indentation in the plasticine. The thickness of the plasticine below any indentation will give you the working clearance.

Please let us know what the clearance is so that any further advice if required can be accurately given.

A used head gasket after compression is 1mm thickness.

[terry h]

Hi ,Thanks for your replies, in one book i have it says that a combustion chamber volume of 23.2cc used on a standard 1098 block would give a compression ratio of 10.5 to 1, i was thinking that having 30 thou skimmed of the block would make the compression to high for a every day car. The piston height, i realize that any head gasket will give clearance between the piston and the head i have checked a standard 1098 engine and that has a clearance of 20thou so i was thinking that i might have to remove some metal from the top of the pistons, sorry if i did not make this clear before, Thanks Terry

[terry h]

Hi , Philthehill i will see if i can find a used gasket and check it out like you say will let you know when i have done it my second post should have been before yours . Thanks Terry

[terry h]

Hi, there was no indention in the plasticine so everything seems to be ok ,there was no resistance felt when turning the engine over, sorry it has taken so long to reply i had to take the cylinder head back to have the exhaust valves replaced. Terry