Since these same transmission interchange questions seem to come up on a regular basis and no one wants to search for a thread that we could add information to. I will do my best to cover it here then archive it. :-)

I will list the most common references.
All 79-85 12A engines will accept any 79-91 transmissions.
All 84-91 13B engines will accept any 79-91 transmissions

There are modifications needed for certain year and type of transmissions however to fit into your particular application.
I am going to stick to the N/A transmissions since there are threads in the archives about usage of the TII transmission.
To put a 81-85 transmission into a Sa it requires modification to the tunnel area of the car to be modified. Cutting the tunnel approx 2 inches towards the rear of the car.
On the later 80 models you are able to remove the shifter housing and the shift forks to fit the 81-85 transmisssion (there is a write up somewhere on this). Therefore not having to cut the tunnel.

You use your SA driveshaft when using any 81-85 N/A transmission in your SA.

To install the 86-88 N/A in the SA I don't know if the shifter forks and housing will interchange. If not you will need to use a 81-85 tail shaft housing and cut the tunnel.
You use your existing drive shaft with the 86-88 N/A transmission

All transmissions from the 81-85 12A and 13B applications will interchange with no modification. Being that a 81 12a trans will fit into a 13b SE and vice versa.

86-88 N/A transmissions will fit the 81-85 models with a minor modification.
You only need to swap out the tail shaft housing.
Meaning you remove the housing from your 81-85 and install it on your 86-88 N/A transmission.
The N/A driveshaft from your vehicle will work with the 86-88 N/A application.

On the S5 N/A transmission in a 81-85 model there are modifications needed.


You remove the rear counterweight or harmonic balancer as they call it removed from the S5 transmission then swap the tail shaft housings over.
You would use your appropriate drive shaft from your 81-85 model with the S5 transmission.



Also if you have information that would enhance this thread then post up and I will amend as needed.