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MG MGA - change cylinderhead 1500/1800

Will it be worthwhile to change the head of my 1500 engine by the head of a 1800 ?

gva guido

Only worth doing if there is an issue with the current head. It may bring a small improvement in performance, depending on the head you fit - there are many different B heads, and some require block modifications to fit on a 1500 block. See here:
dominic clancy

The MGB head had larger valves which will likely hit the top of the cylinder block when they open.
The solution is to mill 'eyebrows' into the top of the block, fore and aft of each cylinder to allow working room for the valves.
See Barney Gaylord's site for more information.
Fraser Cooper

It is not necessary to mill eyebrows for an early B head. All the BMC B series heads (MGA and MGB) have valves which overlap the bores a bit, but the valve travel is not so great as to have the valves touch, at least for the heads mentioned. Exceptions if the head has been planed a lot or if high lift cams or rockers are used.
Art Pearse

Thanks for the answers on my request.
this remains an incredible source of information for our beloved MGA's !

gva guido

An 1800 head on a 1500 would give a much lower compression ratio than normal. You would then have to plane the head which could cause the valves to hit the top of the engine block.

M F Anderson

15 head for 1500/1600 has 38cc chamber.
16 head for 1622 has 43cc chamber (flat top pistons).
18 head foe 1800 has 43cc chamber (dished pistons), except "L" head which is 38cc.
All heads (except "L" head) start at 3.187" thick with .495" deep chamber (+/-.015" depending on how valves are machined). Head gasket is about .020" thick compressed. Stock valve lift is .355" (with zero rocker clearance). Full lift leaves valves more than .125 from top of block.

Solution for 18 head on 1500/1600 engine is to mill the bottom surface about .047" from 3.187" to 3.140" thick to reduce chamber from 43cc to 38cc, restoring original compression ratio. Valves still have adequate clearance from block (but not a lot to spare).

If you shave the head farther or use high lift cam or high ratio rocker arms, then you need eyebrows in the block. 1500 needs eyebrows for both intake and exhaust valves. Larger bores need eyebrows for exhaust valves only.

The "L"head was used 1972-1974 in North American market, and later in European markets. The "L" head has shallow chamber for higher compression (and larger intake valves). All 18V engines from 1972 on have exhaust eyebrows as original factory issue.

I hope that covers it (and hope all my numbers are correct).
Barney Gaylord

This thread was discussed between 15/11/2009 and 16/11/2009

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This thread is from the archive. The Live MG MGA BBS is active now.