mismatching 12a housings?
 

I have a 1980 12a i am rebuilding. The front housing has a hairline crack between the coolant passage and combustion chamber so it is junk. The rear is ok minus some chrome flake. I have a pair of housings from an old 83 engine of mine. They are both in decent shape except one has a very tiny scrape to the left of the exhaust port about 1 1\2 inches long. Im wondering what if anything i can do.

Should i source better parts? Can i mismatch the 80 and 83 housings? Should i just use the 83 housings and assume the affect of the scrape is minimal? Im not building a horsepower monster, just something reliable that lasts a few more years.

Btw i have no emissions and am running a full racing beat exhaust.

I think you can mix-match housings, you know - 12A and 12A of course. IIRC, the only difference in 12A housings is that 79/80 housings have the "square block" casting on the top near the two bolt holes that are used to hold the rats nest in place, and the 81-85 have the "H" casting on the top. I think there are also some slight differences in the coolant passages, but nothing that would stop you for mixing them. I believe that the passages align ok, it's just that one has more baffles than the other which, lacking the baffles, makes the passage more "open", if that makes sense. I'm basing this on the differences I've seen in the different motors I've disassembled.

Maybe one of the rebuild experts will comment here, but it would be easier to weigh in on the use of the one with the scratch if there were a pic to look at, but having said that, I know what the manual says about tolerances and go/no go checks, and I know that some people live with and rebuild with housings that are somewhat out of spec. I would think that it may only lead to more oil consumption and a shorter life of the housing, but perhaps someone with some actual rebuild experience can tell you for sure.

The main differences are that exhaust ports are different, and the coolant passageways are different.

How different are the coolant passageways? Show us a picture.

In my opinion the "water jackets" are the same or similar enough that they will work on all 79-83 (and still compatible with 84-85 with their extra bracing). Same goes for the exhaust ports; some differences, but similar enough. Port timing should be the same etc. You will need a header to block off the thermal reactor port on the SA housing, or fill it in with something.

My apologies to the OP, I should have qualified my answer with "one of the differences" vice "the only difference" because obviously I assumed that timing would be the same, which may or may not be the case depending on the housings, and I had not thought of the exhaust ports. Sorry! :blush:

There are quite a few engine building details, photos and comments from the experts in this thread, if you have the patience for a lot of reading:

https://www.rx7club.com/1st-generati...-shop-1018461/

Here's an example of the content, from page 4 of the thread:

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And from page 2:

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