Transmissions like LEGO, What can I build?
 

What I have:
1978 production four speed transmission. Out of original car but car is in our ownership
1980 five speed. currently in 78 build with a 84 12a. ( the 78 engine needs rebuilt and the plan was to reinstall the stock 12a and original 4 speed) The fifth gear is nice to have.
1985 GSLSE (vented bell housing, closer ratio OD)
1984 12a transmission (from that 12a mentioned earlier) .75 OD. good for cruising.

I have been reading the partsfische from foxed.ca, and what I am looking for clarification on is this. I would like to use the vented SE housing with the 84 gears and OD with the 4 speed tail housing from the 78. As I recall the earlier SA models were swappable with the older but the newer 1980 linkage was not compatible.

Also are any specialty tools I am not thinking of if this can be done?

Just recently I swapped my original '80 tail housing onto an '82 body and bell housing with no major issues. You do need to change the shift fork "fingers" (for lack of the proper term). The SA's use a different shifter arm and linkage that in turn requires slightly different interfaces to the fork rods. The "fingers" can be swapped over by using a punch or nail to drive out the retaining pins and then installing them on the other main body.

Here's a photo I took of the rods with the 1/2 and 3/4 "fingers" removed:
https://cimg3.ibsrv.net/gimg/www.rx7...548343ee32.jpg

The SA's also had a sensor called the "top switch" mounted on the gear casing. It's somehow related to emissions and as I don't run the rats nest anymore, it wasn't an issue, although your mileage may vary. The SA tail housing uses a different reverse / shifter centering mechanism than later years, and also do not have a removable "shifter housing" as it is cast into the tail housing. I believe there is something wonky about swapping '79 tail housings, but I can't remember what off the top of my head. As far as I know, all of the first gen tranny's have the same casing dimensions with the addition of some extra's (such as the vents on the GSLSE trans). The four speed '78 may be different in terms of length, although I have never seen one in person so...

Another note is that the shifter position moved backward in the FB's, causing the hole in the chassis to not line up with the FB trans in the SA without the tail housing swap - although I assume you already knew this based on your question. Drive shaft length remains the same across all of the 5-speeds as far as I know.

I would say your best option is the SE housing with the '84 gears like you said, but with the '80 tail housing rather than the '78. Like I said, I'm not positive, but I seem to remember something being different about the '78 / '79 and 4-speed tail housings.

Best of luck on your build! :)

turns out the existing SA 5 speed is a 79. If I understand there are 3 shift fingers that the shift lever manipulates to select a chosen gear. On the earlier models it is bolted on, later models are attached with roll pins...

One unlikely option is to get the fingers with the rolled pins that work with the SA housing. Was this used on any other Mazda ? B series, etc? Hell, can this be tack welded on?

PeeJay and i looked into this a really long time ago, if you have an SA trans with the roll pin shift fingers, it is FB compatible. if you have the bolted on fingers, it is not.

it goes like this SA trans from the beginning up to some point, then SA case has FB guts. then the case changes to the FB case, so you have an FB case and FB guts. if that makes sense

change occurs are SA22C-583527 or 10/1/1979

In case you have not come across these in your searches. Posts 9 and 10 in this link are good reading.

https://www.rx7club.com/1st-generati...nchro-1128951/