The 1600-MK-II engine (1622cc) is substantially different from the prior 1500/1600 engines. The crankshaft is a bit stronger with thicker webs and slightly narrower main bearings. The cylinder head has one size larger valves, a little better porting and larger volume combustion chamber (43cc vs. 38cc in the "15" head). The head is marked "16" on top at the back. These engines and heads are a bit rare, having been used for only 14 months of production in the MGA.

Some early 1622 engines (about 300 units) were apparently fitted with heads marked "15", but having the combustion chambers machined out to the new "16" specifications with larger valves. Speculation is that the new "16" head castings were not ready in time for the intended production date of the new 1600-MK-II model car, so a few hundred of the "15" heads were machined to meet the "16" head specifications.

Photos here came from an early 1622 engine with "15" head machined to "16" spec's. The engine number is 16GC-U-H-490. Valves are certainly the larger dimensions. Combustion chamber has been milled to larger size, but no way to tell if this is original or has been done more recently. Combustion chambers are 3.27" at widest valve to valve. Head thickness is original 3.187". The combustion chamber has been milled, but not milled all the way around. It's still rough(ish) casting where it's siamesed to #2, and some other areas. When compared to an as-cast original "16" head the dimensions of this machined out "15" head are identical.

Machined "15 head" - - - - - As-cast "16" head