Which cyl. head is this?

[Andy W]

I have this spare head...I think it says 12G940 on it.

Is this head
1/ worth renovating?
2/ would it give a power improvement on my 1275 Marina engine?

Thanks.

Photo 1[frame][/frame]

[Andy W]

Photo 2[frame][/frame]

[Andy W]

Photo 3[frame][/frame]

[philthehill]

To be honest - the head pictured will not give any noticeable improvement over the 940 head already fitted to the 1275cc Marina (group 'D' in the list below).
Only if you are prepared to spend some time cleaning up the ports will you see some improvement in power output - but there again a lot of work for little gain.[frame][/frame]

[IslipMinor]

Phil,
With the 'sculptured' bit around the thermostat area, could it be an unleaded head? I think the unleaded versions were flat in that area?

[philthehill]

Richard
I am not sure as to whether the head in question is unleaded or not.
The unleaded element will not impactor on or effect the performance/use of the head only its longevity.
The red paint would indicate that it possibly came off a MG 1275cc of some description - possibly Metro.
Phil

[Mark Wilson]

I've a red Midget 1275 12G940 which is flat adjacent to the thermostat, without the casting ID, which may support Phil's MG Metro theory.

[Andy W]

So do I take it that one 1275 head is no better than another?

[IslipMinor]

Andy,

No, there are some differences, mainly in the inlet valve sizes.

Apart from the early Cooper 'S', all the 12G940 heads had the same exhaust valve size - 1 5/32" diameter (1.156" or ~29.4mm). The early Cooper 'S' had 1 7/32" exhaust valves (1.219" or ~31.0mm), but they were prone to cracking between the valve seats, so the exhaust valve was reduced to the same as the other 'big valve' heads, 1 5/32").

The 'big valve' 12G940 heads were fitted to Mini Cooper 'S', later MG 1300, 1300GT and non-unleaded MG Metro and had 1.4" inlet valves (~35.6mm), but the later unleaded MG Metro reverted to the smaller inlet valves (1 5/32") I believe?

[philthehill]

Richard
To add to your comment above:-
The early Cooper S head 12G185 had the valve guide centres closer together (which did not help as regards the head cracking) and the combustion chamber is more like an enlarged Minor combustion chamber and is 11mm deep against the more shallower 940 combustion chamber. The head was fitted with nimonic valves which were shorter in overall length.
The assorted 940 heads had minor variances but overall the performance generated was much the same.
Unless the rest of the engine is breathed upon i.e. improved camshaft and carburation etc. fitting the larger valve 940 head in standard condition will be of little benefit over the small valve 940 head.
Comparison photo of the 12G185 and 940 head (top (& fitted with larger valves)) combustion chambers.
Phil[frame][/frame]