Sentra Building Story No.1: The Transmission – 1 of 2

Backyard tranny transplant. Can it be done?

For those who have the luxury of working on cars from the ‘80s, they will understand how easy it is to swap to a different transmission. In today’s standard, however, everything in the powertrain is connected to the ECM, swapping out the 5-spd MT  to a 6 would most likely require some ECM reprogramming since each gear could be running on a different fuel map. As sophisticated as the ECM might be, the added 6th gear may actually confuse the ECM to go haywire like doing a kidney transplant without telling your brains its IP address first.

For our LeMon fighter, how much does it take to do a tranny transplant in our own backyard with limited surgical instruments only find in a machine shop?

RN4F31A to RS5F31A Swapability

Let’s decode RN4F31A and RS5F31A. The RN4F31A is transaxle code for the stock 4-speed transmission in our ’89 Sentra. The four forward gears hence the 4F in the code. From this logic you can guess the RS5F31A is the five-speed unit as the factory optional upgrade. The F31A suggests that the clutch housing, driveaxle and assembly mounting points are the same for both 4- and 5-spd units. The only visual difference is the longer transaxle housing in the 5-spd unit due to the extra gear it contains.

The both transaxles are made by Aichikikai in Japan, the same manufacturer that makes very sophisticated transmission for the current Nissan GT-R.

If I were the car manufacturer it will be wise to utilize the same transmission mounts in the engine bay for both transaxles. Knowing Nissan, they have got this all sorted out in the Sentra. So at the most, maybe the mounting bracket, driveaxles are slightly different between the two.

One quick way to check is to compare part numbers between the 4-spd, and 5-spd units.

So the swapability is as simple as replacing a clutch. All the existing hardware from the 4-spd unit will work on the 5-spd. However, it all looks good on paper. Did we do the swap? Nope. Here is why. End.

Next: Sentra Building Story No.2: The Transmission – 2 of 2