According to details I have:-
1962 to 1970, MGB cylinderheads were casting number 48G318, with a
1.562" dia inlet valve, ( 18G through to 18GG) double valve springs.
1970 to 1975 they were 12H2708, ( 18V) inlet valve 1.625" dia, single
valve springs.
Oct 1975 onwards, 12H4735, 18V MGB & Marina head with offest oil feed to
the rockers, requiring a different pedestal , single valve springs,
inlet valves 1.562" dia, and for the USA leadfree exhaust valve seats.
48G318 is prone to cracking between the ports.
Neil
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