5 replies to this topic

[sansamn]

I recently bought a cylinder head from a member on this forum. It’s an early MG Metro head that has had a 10 thou skim. It will be going on my pre-84 1275 metro engine that has had no changes to the pistons. I know that the early MG Metros had the highest compression ratio used in a standard A-Series engine (10.5:1) and that skimming the head will higher the compression ratio but will a 10 thou skim be enough to make much of a difference?

 

 

[Craig89]

Yes it will make a difference, your best bet will be to measure the cc of the chambers in the head then the compression ratio will be able to be calculated fairly easily.

Search the forum for more info on both

[DomCr250]

It will make a difference, but ten thou is just a quarter of a millimeter, so not that much difference.

 

As an example taking 2mm off the head reduces the camber volume by approximately 6cc - so your head's reduced by 0.75cc.

 

You have a 1275, so the head casting is the same as the MG one, (12G940) - the MG head did not change that much in combustion chamber from earlier 12G940's.

 

When you have both heads off you will see what I mean.

 

The MG compression ratio comes from the piston dish volume and height in the bore, not the head.

[Dan]

From the BL era onwards (and to a certain extent throughout production) all factory compression ratio fiddling was done at the piston with crown height and dish changes. The chambers of the heads you are swapping (assuming you are swapping) as said above will be near identical, in fact the only difference is a slightly larger inlet valve on the MG head. So the compression ratio will be near as dammit exactly the same as the engine was born with, the skim won't cause you problems.

[racingenglishcars]

IF your old compression ratio was 10.5:1, the new would be around 10.7:1

[sansamn]

Thank you all for your replies, it was the news I was hoping for.