What are the difference by year in 12A's
 

I am thinking about picking up a 12A that I found with good compression tohave as a spare for my RX3 racecar. The guy says that it's out of an RX2, but how can I tell what year exactly the motor is? I have an 83 right now, and I know that the twin dizzy motors have different intake and exhaust bolt patterns and different cylinder linings.

Without looking at the flywheel how can I tell what year it is and does it matter?

cant tell u exactly what u want but what i noticed with my rx2 73 12a was the starter is on top so i believe u cant mate a later model tranny. like u said rotor housings are little different etc main thing is the tension bolt pattern is slighty different and one bolt less so u cant interchange parts, hope this helped most likey would be better off finding a 79 and up 12a for your spare.not to mention the coolent seals failed alot on the twin dizzy .

Have you seen the motor yet? If not do you know if it is a Twin, or a single that was later swapped into the car? That being said if It is a single dizzie keep in mind that people do swap the distributor covers to make timing easy so it could be a twin dizzie motor with a single dizzie cover. Anyways look at the housings, they'll give you a general idea of the span of years when the motor was made. If the housings have a little plaque with a 1 on it then its early pre 74. If the housing has MAZDA written in the 70's font its 74-78, if the Housing says MAZDA in the same font as the one on your 83, well thats self explanitory. Also One of the 12As has a thicker middle plate then the early ones, I think its the 80's 12As but I'm not sure.

I haven't seen it yet. Checking it out this weekend. He says it came from an RX2, but since it isn't a twin, that can't be right.....right?

The first thing I planned on checking was the "mazda 12A" casting in the top like you said. Then I'll go from there.

I believe 74 RX2 came single dizzy..... Last time I saw one....

Yeah 74 is the first year of the single Dizzie motors, first single dizzie and I believe first year where they changed how they made the chrome in the housings, so it's also the first motor with the newer style 3mm Apex seals that aren't carbon like the original seals.

So....
What happened?
Did you pick up the engine? What markings did it have?

...in suspense...