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It seems I can't get a straight answer on what the little H cast into the top of some 12A housings means. I'm not talking about the big H where folks compare it to the block cast into the very top of the housing, rather this smaller H is between the top of the housing and the intake manifold (photos show it below).
From the various threads I've read on the matter I've seen people suggest that it's a 12AT housing, a 6-port 12A housing, or an S3 12A housing. None of these seem right though; the port timing matches a normal 12A so it's not a 12AT, there's no extra exhaust ports like the 6-port had, and lastly I have two other S3 12As that do not have this H cast on the housings. My thought is that it's a late production S3 12A thing, since I know a couple guys with 85s that have it on their engines. Then the question becomes, what does it mean? Supposedly it has better chrome, but that was in reference to the 12AT and 6-port housings so...? These housings have zero chrome flake so it's perhaps believable, but I don't know for certain. Or maybe it's none of the things I mentioned.
I suppose I might also ask if anyone knows what the "K" that's stamped next to the "big H" means as well (also visible in the below photos).
Curious if anyone knows exactly what the deal is here.
Last edited by Benjamin4456; Jun 27, 2022 at 03:44 PM.
It seems I can't get a straight answer on what the little H cast into the top of some 12A housings means. I'm not talking about the big H where folks compare it to the block cast into the very top of the housing, rather this smaller H is between the top of the housing and the intake manifold (photos show it below).
From the various threads I've read on the matter I've seen people suggest that it's a 12AT housing, a 6-port 12A housing, or an S3 12A housing. None of these seem right though; the port timing matches a normal 12A so it's not a 12AT, there's no extra exhaust ports like the 6-port had, and lastly I have two other S3 12As that do not have this H cast on the housings. My thought is that it's a late production S3 12A thing, since I know a couple guys with 85s that have it on their engines. Then the question becomes, what does it mean? Supposedly it has better chrome, but that was in reference to the 12AT and 6-port housings so...? These housings have zero chrome flake so it's perhaps believable, but I don't know for certain. Or maybe it's none of the things I mentioned.
I suppose I might also ask if anyone knows what the "K" that's stamped next to the "big H" means as well (also visible in the below photos).
Curious if anyone knows exactly what the deal is here.
For it to be a 79 12A and have this H it would have to be a different casting than I have here - my casting does not have the provisions for the thermal reactor passages which were used from 79-80. Perhaps the H is common and means little, although I feel like that would not be the case. Curious for other folks to chime in.
I wonder if it means that it was manufactured in the Hiroshima plant. I honestly never thought about it until now. I'll be curious to hear what others have to say.
After reading this I looked at some of our cars, my 84' doesn't have it. But the 85' GSL made in 4/85 does have this H. The engine I know to be original (I have the original window sticker that has the serial#).
While going around and looking at all of our 12a cars I have found that two of them have the small "H". One I have already mentioned but the other was made 3/85 and clearly has the "H". I do wonder what this means, none of our 84 cars/engines have this H. I am interested in others input on this.
Sorry for the crappy pic, lighting in the barn isn't the greatest.
there are a bunch of different rotor housing castings, and the big H means its got a catalyst, instead of a reactor
here is a pic of an earlier engine (79-80 13B)
the 12A turbo is similar to yours, except while yours has the casting boss for the metering pump nozzles, the 12AT is machined out, and no water O Rings
there are a bunch of different rotor housing castings, and the big H means its got a catalyst, instead of a reactor
here is a pic of an earlier engine (79-80 13B)
the 12A turbo is similar to yours, except while yours has the casting boss for the metering pump nozzles, the 12AT is machined out, and no water O Rings
That's quite a cool document you've got there; I ought to read more of the files on foxed.ca sometime.
Based on that doc, it appears that my casting is from 84-85 as it has the additional ribs near the top. It didn't reference the small H, although I'm thinking that's just how the 84-85 housings where marked. My other housings without the small H do not have the extra ribs.
It's also cool to see that it has the better chrome, and now I'm curious if it was changed again (or perhaps multiple times) for the later 13Bs. I've always understood that the early engines have worse chrome, so do the 84-85 engines have some "middle of the pack" chrome in comparison to the rest of the engines, or is it the same stuff that was used from then on out in the 13Bs? I'm sure it's mentioned in another document somewhere. Anyway, interesting stuff.
It's also cool to see that it has the better chrome, and now I'm curious if it was changed again (or perhaps multiple times) for the later 13Bs. I've always understood that the early engines have worse chrome, so do the 84-85 engines have some "middle of the pack" chrome in comparison to the rest of the engines, or is it the same stuff that was used from then on out in the 13Bs? I'm sure it's mentioned in another document somewhere. Anyway, interesting stuff.
the FD gets a Teflon coating (or is it graphite?), and then the Rx8 gets something else. i'm unclear if the replacement parts get the newer coating, or they got the old ones. it seems silly to me to make the FC rotor housing and the FD rotor housing and the Rx8 housings, and not put the same coatings on all of them, but maybe they do
the H's are there because they have a little strap thing that holds the engine together while they are assembling it, but also since they are/were building many different specs of engine at the same time, it helps the assembler see that they have the correct parts. remember Santa is a professional, he checks twice, once at the beginning and once at the end. i suspect Mazda checks a third time, but more than that and its not fast enough, and only once is too sloppy...
