Engine # and Chassis #
 

Is there any number on the block that I could compare to a number on the chassis. Just want to know if I should keep my other engine. thanks

Nope, they're all marked on the rotor housing either 12a or 13b.

The chassis will be your VIN.

So any 12a block is just a 12a block, not orignally belonging to any specific car?

Exactly.

Mazda didn't mark the engine to go with a specific car like the american companies do. I don't find any use in that "matching numbers" thing anyway.

The engine number and chassis numbers are completely different, but there are records of which engine originally came in a particular car.

All the 12A engine versions came with with the same 12A cast mark, from the RX-2 in 1973 onwards to the RX-7 GT-X Turbo in 1985. The engine numbers appear to be in different batches, for example my 12A 1985 is 249 0184 but my 12A efi turbo engine from the same year is 716 712.

I also know of some 1.3 litre engines with 12A cast marks.

As a matter of interest it is 12A not 12a, arising out of Japanese not having a lower case. They generally used either Arabic numbers or English capitals letters as there is no alpabetical sequence in a character based language.

my 12a has an engine number in the front iron...

Yes that is where the Engine number is stamped. The Rotor housings only marked 12A or 13B Mazda, not the actual Number.