Reference · A and A+ SeriesDraftv0.3

Engine Identification

Which engine is this? The engine number, the casting marks on block and head, and the external details that tell the A and A+ Series units apart.

Compiled from the published references listed under Sources, not from factory records. Where those references disagree the disagreement is noted. No single mark is proof: after sixty years most engines are a mix of parts, so check several marks against each other.

What to check, in order

1. Engine number. If the plate is still there, its prefix names the capacity and the original application. Type it into the plate above, or see Engine numbers.
2. Tappet chest covers. Two pressed covers on the back of the block mean a small-bore engine (848 to 1098 cc) or a Cooper S. A solid back means a 1275 that is not a Cooper S. See Block.
3. Block casting number. Narrows a small-bore block to its capacity and a 1275 block to its generation.
4. Cylinder head casting number. Identifies the head only. Heads are swapped more often than anything else, so it says little about the block underneath. See Cylinder head.
5. A or A+. Ribs, flange, distributor clamp and dipstick date the block to before or after 1980. See A or A+.
6. Measure. With no plate and an ambiguous casting, the stroke measured through a plug hole settles most cases. See Measuring.

Displacements

Seven capacities were fitted to the Mini. They fall into two families: the small-bore engines up to 1098 cc, and the large-bore Cooper S and 1275 engines, which share one bore and differ only in stroke.

Figures are rounded to one decimal. The 1071 cc engine is also written 1070, and published strokes for the 1098 vary between 83.7 and 83.8 mm.

Engine numbers

The engine number is stamped into a small metal plate riveted to a ledge at the top front of the block, just below the thermostat housing and above the dynamo or alternator mount. The plate is easily lost and blank replacements are sold, so a missing or freshly stamped plate is common and proves nothing either way.

Three numbering schemes were used. All of them start with the capacity.

To about 1970: make letters

A number such as 8AM/U/H101 reads: capacity, make, gearbox group, compression, then the serial number.

The Cooper S breaks the capacity rule: all three sizes carry a 9F prefix, including the 1275, and are told apart by the last letter.

The last letter separates the Cooper S sizes: X is the 970, H the 1071 and Y the 1275. The 997 Cooper and the 1071 Cooper S share 9F/Sa/H and 9FD/Sa/H is shared with the 998 Cooper, so on those two the serial number decides. Only the 9F Cooper S engines had the forged, nitrided EN40B crankshaft.

About 1970 to 1990: capacity and H

The make letters were replaced by a two-digit capacity (85, 99, 10, 12) and a letter for the installation: H for a transverse, front-wheel-drive engine and V for an in-line one. A 12V engine is therefore a 1275 from a rear-wheel-drive car, not from a Mini. Three characters after the letter identify the exact build, and with it the car the engine was made for. That matters because the same prefixes were used for every transverse A Series car: 99H 997 is a Mini engine, 99H 907 a Metro one. A trailing letter or two often follows the build code; none of the sources consulted explains it.

Every 12H is a plain 1275 except 12H397 and 12H398, which are the Mk3 Cooper S. For 12H706 Calver lists a rod-change gearbox and the Guess-Works decoder a remote-change one. The Guess-Works decoder covers several hundred build codes and is the place to look up one that is not listed here.

1990 on: 12A

The last 1275 engines carry a 12A prefix, a two-character code for the fuel system and a three-character build code, as in 12A2LK70.

Block

Look at the back of the block, the side facing the bulkhead. Small-bore engines have two pressed-steel tappet chest covers there. The 1275 block is solid in that area for stiffness. The Cooper S is the one exception: it is a large-bore engine that kept the tappet covers.

A Cooper S also has an extra head stud and bolt, visible outside the rocker cover, which has two small cut-outs to clear them. Some other 1275 engines have the eleven-stud head too, the Innocenti among them, so this supports an identification but does not make one.

The casting number then narrows it down. On small-bore blocks it is usually below the rear tappet cover; A+ blocks carry theirs on the front or the back.

Because 12A497 covers both the 998 and the 1098, the casting alone does not separate them; the tag or the stroke does. For the 970 Cooper S, Mini Spares gives AEG131 in its catalogue and AEG151 in its blog.

Cylinder head

Most casting numbers are on top of the head, read with the spark plugs towards you; a few are upside down or sideways. Heads made from the very late 1970s can have the number behind the thermostat, and the last injection-era heads have it on the end of the head.

Two details separate small-bore from large-bore heads without reading any number:

Notes on this table: Mini Spares gives the 12G206 inlet valve as 29.4 mm, Calver as 30.9 mm. Calver prints the 998 A+ head as CAM4180 where Mini Spares has CAM4810. 12A185 is also the casting of an ordinary 997 Cooper head with small valves, so on that number the valve sizes decide. A 12G940 with 35.6 mm inlets is the MG Metro version.

A or A+

The A+ arrived with the Metro in 1980, and Mini engine numbers list A+ blocks from that year. The parts did not all change at once: some engines left the factory with an A+ block and the earlier clutch, others with an A block and A+ ancillaries, so count several signs before deciding.

Metro engines also carried an "A+" sticker on the rocker cover. Some late 1275GT engines from 1979 already had the A+ style head.

Measuring

This section is worked out from the dimensions above; it is not taken from the sources. With no plate and an ambiguous casting, the stroke can be measured without dismantling: remove a spark plug, rest a thin rod on the piston crown, and mark the rod at top and bottom dead centre. The plug hole is angled, so the reading is good to a few millimetres at best. That is enough to separate 848, 998 and 1098, which are more than 7 mm apart, but not 81.3 from 83.7 mm.

The two rows with a Cooper S in them are the reason a suspected S needs more than a stroke measurement: the bore (62.9 against 70.6 mm at 68.3 mm stroke, 62.4 against 70.6 mm at 81.3 mm) is only visible with the head off, so rely on the AEG casting number first.