Number Inlet dia. Exhaust dia. Nominal cc Standard fitment
2A628 1.0625"/26.99 1.00"/25.4 24.5 850, 998 Mini & 948 Sprite
2A629 1.0625"/26.99 1.00"/25.4 24.5 948, A35 & Morris Minor
12A1456 1.0625"/26.99 1.00"/25.4 24.5 850, 998 Mini & 948 Sprite
12G202 1.156"/29.36 1.00"/25.4 26.1 997 Cooper, Austin 1100 Mk
CAM4180 1.0625"/26.99 1.00"/25.4 25.5 998 Mini - A+ (1980-on) and lead free
12G206 1.218"/30.93 1.00"/25.4 28.3 Early 998 Cooper, MG1100
12G295 1.218"/30.93 1.00"/25.4 28.3 998 Cooper, MG1100
12A185 1.401"/35.6 1.22"/31.0 21.4 First Mk1 Cooper S
AEG163 1.401"/35.6 1.22"/31.0 21.4 Later Mk1 & Mk2 Cooper S
12G940* 1.312"/33.33 1.15"/29.2 21.4 1275GT, Austin 1300, all late A+ models inc. Turbo but not MG Metro
12G940 1.401"/35.6 1.15"/29.2 21.4

Stamped '12G1805' on flat area by thermostat - MG1300, 1300GT & Mk3  Cooper S

12G940* 1.401"/35.6 1.15"/29.2 21.4 As above plus MG Metro

*Note:

A+ head castings - despite having the same 12G940 casting number - are considerably different in appearance, loosing the flat area behind the thermostat housing and around the rest of the rocker/head stud areas to a sculptured look - where these areas are replaced by a 'sunken' cast finish.
The A+ castings were introduced with the appearance of the Metro in 1980, although some late 1275GTs (1979-on) had them fitted. Generally they are easily recognised by their colour - a putrid yellow on the small-bore castings and bright red on the large-bore ones, a paint finish that is VERY difficult to remove, even in a chemical tank.

The MG Metro Turbo shares the same valve sizes with the other non-'big valve' 12G940 variants but has 8mm, sodium-filled exhaust valves.

A+ heads generally have three collet grooves in the valves, collets are 14-degree cone taper instead of the 10-degree on single groove collets (so parts are not interchangeable between single and triple groove types) and the top-caps have a raised ridge around the collet hole.