mk1-forum.net

after a rebore on my 998 cooper engine,im gonna be running plus 20 std type hepolite 998 pistons instead of "D tops"

this is gonna lower the c.r like crazy

how much to shave off the head to bring the c.r back to a proper level

im running a 286 cam so maybe a little compression raise would be useful as these suffer low speed torque???

any advice will be well recieved
cheers

You really need to measure everything up and do the sums, to be certain. I did it several years ago so I've forgotten the calculation, but its in Vizard's yellow book. From memory you need the combustion chamber volume and the volume of the dish on top of the piston. You also need to add an allowance for the head gasket, and any space above the pistons if they aren't coming right to the top of the cylinder. Once you know what volume you need to remove from the combustion chamber, you need to measure its cross sectional area so you can to calculate the height to plane off.

For my 1100, I removed 0.050" which was as much as I was brave enough to remove. You may find that you can't quite get enough off a 295 head without breaking through to the oil drilling, and if its a well used one, they can also corrode inside so that the water jacket is closer to the face than it should be.

Tim

I made a nice little excel program for calculating this type stuff. The program has a couple small spelling mistakes, but is easy enough to use. Download it from my link below. (the blue button for download)
Then the only question is; how much compression do you want?

If you re-bore an engine, then you will raise the compression ratio (as long as nothing else changes).
-Bigger cylinder, draws in more fuel/air.

So, if you were happy with the way the engine was running before (no pinking etc) then you need to work out the compression ratio from original, and then adjust for the bigger bore and pistons without 'D'.

You'll probably find that the change is not that significant.

Remember to skim the block as well...

As a GOOD rule of thumb:

12 thou off the head face = 1cc off the chamber capacity.

60 thou then?

The D alone is worth 2cc, an overbore will increase the CR marginally. So to get back to square one you should look to take off about 20 - 24 thou.

60 thou will reduce the volume by 5cc & will give a fairly large increase in CR.

I don't have my CR calculator & figures handy, but I am sure someone will do the sums.

M.

good coz compression ratios duz my skull in and im terrible at maths.

im lead to believe the scatter 286 cam likes increased c.r so maybe the 60 thou skim may help??

brook shaw wrote:
im lead to believe the scatter 286 cam likes increased c.r

hows that work then?

dunno-
just folklore

Longer duration cams close the exhaust valve later, allowing less trapped air to be compressed. Higher compression ratio compensates for this.

"good coz compression ratios duz my skull in and im terrible at maths."
That's why I made the CR program. It really does work miracles

9.75 to 1 would do me

The raised D has a volume of 4.5cc (according to Vizard) not that it matters if you are using flat top pistons.

12 thou=1cc rule is for a 1275 head, a 12G295 head is probably 15 thou=1cc.

998 + 0.20" will give a cylinder volume of 253cc, so if you have 29cc (chamber volume + gasket + volume above piston + ring land volume) this will give a CR of 9.7:1, if you reduce the volume to 28cc you should get a CR of 10.0:1

If you run the pistons flush with the top of the block then ring land volume + gasket volume should be about 3.5cc so you will need the chambers to be at 25.5cc (0.040" skim is about right).

With the pistons 0.025" down the bores you will get an extra 2.1cc plus the 3.5cc will give 5.6cc so you will need need 23.4cc chambers. If you are using a standard 295 head with 28cc chambers you will need to reduce this by 4.6cc (0.070" skim) to achieve a 9.7:1 CR.

A good standard 295 head should be ok with a skim up to 0.100"

Always best to measure everything very accurately before skimming.

Vizard covers calculating CR in one of his chapters on cylinder heads.

12G940 and 12G295 have the same chamber shape and size, 29.2 cm2. skimming 0.010inch of either one reduces chamber volume by 0.74cc

Sorry, corrected, to inch

my pistons stop 25 thou short of block face and have the std 998 cc piston dish...

hate this confusing stuff,,id much rather restore bodywork

I've actually tried to measured the D, and found it to be approx 2,5-3,5 cc. It's NOT 4,5.

Skim the block 10 thou then, and aim for 10.5:1 with a 286.

Fly over here and do some bodywork for me. I'll build your engines

Frogeye61 wrote:12G940 and 12G295 have the same chamber shape and size, 29.2 cm2. skimming 0.1mm of either one reduces chamber volume by 0.74cc

So this means 1mm (39.4 thou) = 7.4cc or a 40 thou skim reduces chamber volume of a 1275 head to 13.9cc

Sorry, I meant 0.010 inch, not 0.1mm
Don't you think it's odd that the measurements are all mixed up like that.

A 0.010 skim reduces a 1275 head from 21.4 to 20.66cc. 0.020 reduces it to 19.92cc etc
But what's more relevant is the 12G295, the first 0.010" skim reduces the chamber volume from 28.3 to 27.56.

A similar discussion is found here
http://www.spritespot.com/phpBB3/viewto ... =10&t=3610

I figured I should post the best estimates I have for all this stuff

Chamber volume cc
inch skim___std.__0.010__0.020__0.030__0.040__0.050__0.060
12A1456___24.5__23.93__23.36__22.79__22.21
12G202____26.1__25.46__24.82__24.19__23.55__22.91__22.27
12G295____28.3__27.56__26.82__26.07__25.33__24.59__23.85
12G940____21.4__20.66__19.92__19.17__18.43

This is without any further chamber reworking or valve sinking

Original head thickness is 2.75 inch except the 12G940 which was 2.725 inch
In this way one can measure their head and probably see if the head had already been skimmed and by how much

These values are only close estimates since all heads are slightly different

For the total clearance volume one must add a couple cc for head gasket, 1/3 cc for piston clearance, any pocketing and any piston deck clearance volume.

The static compression ratio is (swept volume + clearance volume) / clearance volume