the 12A evolves a bit over the years, sometimes in totally unimportant ways.
74-75 12A's have the big port, intake closes @50 instead of 40, exhaust closes i think @48 instead of 38 degrees. the rotor housings have the big font MAZDA on them. irons are non nitrided and painted black. rotors are MDR like a 13B
76-78 12A. these get the smaller port, which the 12A keeps in the us, until 85. rotor housings are the "mazda automobiles" ones. irons are still non nitrided, and black. rotors become LDR. the 76-78 parts basically superseded to the 79 spec.
79-80 12A. basically the same as the 76-78, except the irons get nitrided. in 1980 they went from 18 to 17 tension bolts.
81-82 12A. new rotor housings for the catalyst exhaust. lighter flywheel, its like 21# instead of 30#.
83-85 12A. the rear iron gets a T heater hose connection for the beehive. the rotors are lighter. oil pump is the bigger one.
crib notes.
front, middle iron, eshaft and stat gears interchange from 76-85. rear iron is either 83-85 or 76-82. rotor housings are 76-80 or 81-85, reactor or cat. rotors are 76-82 (these have a F or R stamp), 83-85 (NF or NR stamp). flywheel is 76-80, 81-82 (same balance but lighter) and 83-85.
this is ONLY for the US, the rest of the world is slightly different after like 79-80. some markets keep the reactor, some markets got bigger ports, etc etc
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