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MG MGA - cylinder head types

A cylinder head query: I have two spare 12H 2709 heads and was measuring the inlet valves of each. One has the 1.562" inlet valve and the other the 1.630" valve. Was the head with the larger valve original or has it been machined out with new inserts at some time in the past? If I put this head on a 1622 block are eyebrows required?
J H Cole

No eyebrows needed unless you run a high lift cam, I believe, but do check clearances - that is a mid 70s MGB head. If you aren't getting at least .050" between valve and block at full open, better fly-cut the block.
Bill Spohn

It may not be that simple. 12H2709 is the casting number for the European spec MGB head for 1972-1974.5

The North American spec head for the same years is 12H2923. The N/A head is referred to as the "L head, having a large "L" cast on top at the back along with "18". This is the head with shallower combustion chamber (38cc vs 43cc) and one size larger intake valve (1.625 vs 1.5625). The shallow combustion chamber does require cutting relief eyebrows in the block.

Starting 1975 the N/A engines went back to 43cc chamber volume heads, but I believe the European engines continued to use the smaller chamber for higher compression, which is why all 18V engines from 1974 through end of production in 1980 have eyebrows from the factory.

I do not know for certain the original spec for valve size in the 12H2709 head. I thought it was the same for European spec MGB as for N/A spec MGB. Perhaps the same casting number was machined for the smaller valves for some other engine application.
Barney Gaylord

Thanks Barney, I think I can manage the eyebrow test and cutting as per your web site but what concerns me is that my existing early mgb head has about a 40ish cc combustion chamber and the 12H12709 around 35 cc. I estimate that if I swop heads the compression ratio will rise from my existing ratio of around 8.2 to 1 to 9.5 to one. This is a big increase and will there be any detrimental effects either long or short term?
J H Cole

This thread was discussed between 01/07/2007 and 02/07/2007

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