In the excellent book the Story of the BMC 1100 Steve Lee writes the following in the chapter about 1275 Engines fitted in the ADO16:-
“In early Mk2 cars from June 1967 a 1275cc engine was offered as an option to the 1098cc engine. This engine, suffixed by engine number 12G, is an extremely rare unit. These engines have double valve springs on the cylinder head and have a smaller 1.625 inch connecting rod crankshaft diameter also used in the Mini Cooper S. Tappet chest covers were still present on this engine and it is possible that the crankshaft was made from EN40B steel as used in the Cooper S engine. The introduction of the 1275cc engine utilised a completely different crankshaft, connecting rods, camshaft and cylinder head compared to the 1098cc unit”
What Steve has written above is factual but why did he start with the Mk2 cars that had the 1275cc engine installed?
ADO16 cars were fitted with S type engines in early 1967 until the 12H 1275cc engine became available later in that year. In order for BMC to provide a 1275 cc ADO16 from early 67 they used the 1275 cc S type engine to provide the option of 1098 cc or 1275 cc engines. The first 1275 cc engines were suffixed by engine number 12G and indeed are extremely rare. These engines all had a 9 stud cylinder head, AEG510 cam, forged rocker assembly and the EN40B crank with smaller 1.5/8 inch journals, connecting rods, crankshaft diameter being the same size as the Cooper S engines. The cranks were all marked EN40B and dated. They also all used a single 1½ inch carburettor. This engine was only fitted to the very late Mk1 MG, Wolseley, Riley Kestrel and Vanden Plas which were called the 1275 model. These cars were built in late 1966 and early 1967 before becoming available in early March of that year. As Steve states in his book very early Mk2 cars did have the 12G engine.
To further complicate matters, in July 1967 the solid wall block appeared with engine number 12G with EN16T (12G1288) crank that had 1.5/8 inch journals. These engines were only used in the Riley and MG 1300. It is important to remember that the EN16T crankshaft out of this solid wall engine has a different centre main bearing width to the Cooper S engine.
It is hard to date when the 12H solid wall block lost the 12G1288 crank. This engine now gained the 12G1505 crank that became the normal run of the mill BMC 1300 engine used in all ADO16 1300 cars and 1275 GT Mini from 1969.
The 12H 70 BHP engine is a sister of the above and had the 12G1683 EN16T crank with 1.3/4 inch journals, stronger connecting rods, high compression pistons and large valve 11 stud cylinder head. These engines date from about September 1967 and were only used in the Riley 1300, MG 1300 and Austin / Morris 1300 GT. I have one of these engines dated October 1967 and the MG 1300 it came out of was first registered in September 1968. It is worth saying that much as the 70 BHP cars arrived in the showrooms in September 1968 these cars were first built in early 1968 and production of 70 BHP engines must have started in the last quarter of 1967.
Some believe overseas cars had these upgrades earlier than the home market which may be correct from what we have seen over here
Mk1 1275 models were built in very small numbers and are very rare. A family friend has a peat brown 1967 Vanden Plas 1275 from new with 24,000 miles on the clock. The original bill of sale is dated very early April 1967 so the car must have been built in the first quarter of 1967. The danger with this car is some head case will buy this beautiful rare low mileage car for the engine to clone another fake Mini Cooper S