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MG MGA - Early B Head on late 1500 A

My car is a 1958 1500 'leaded' A.

I also have a 3 bearing 1800 B engine that I will rebuild sometime in the future. If I want to go 'unleaded' now it would seem a better investment to get the B head converted to 'unleaded' as I will do the rest of engine in the future.

The question is what will be the impact be of fitting the B head to the 1500 engine? I understand that it has better porting/shape but it would lower the compression ratio (I believe it is 43cc rather that 39cc as on the 1500).

Thoughts would be appreciated.

Paul Dean

Solution to the compression ratio issue is to shave the head about 0.047" to reduce chamber volume from 43cc to 38cc. The head might have been shaved previously, so do check the overall thickness. Original thickness was 3.187", and you will be shooting for final thickness around 3.140".

When finished, do check valve travel and over travel to be sure you still have adequate valve head clearance at full lift. You may need to add clearance eyebrows in the block. The 1500 engine (small bore) in particular with the 18 head (larger valves) will have both intake and exhaust valves overlapping over the block.
Barney Gaylord

This thread was discussed on 10/05/2013

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