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mastaphixa
11-04-2017, 04:49 PM
I have heard of Tr4 transmissions being swapped into a Tr3, but is it practical to swap a Tr4 overdrive transmission into a Tr3A non-overdrive car. What are the mechanical issues and what are the electrical issues?

TR3driver
11-04-2017, 06:13 PM
Sure, no problem. In fact, I have a TR4 overdrive in my originally non-overdrive TR3 (along with a TR6 main gearbox housing).

Mechanical issues are the same as the non-OD case. For a TR4 box, I believe all that is needed is to slot the holes in the rear crossmember, so the rear mount can be moved back by about 3/8". On my 3A, there was a bent lip on the crossmember that had to be bent down a bit as well. But since the TR4 still had the thin flange on the bellhousing, you won't need to worry about longer bolts & studs or clutch slave mounting.

If you have an early 3A (before TS50001), you'll need to modify the trans tunnel a bit for more clearance around the starter bulge. The early starter will still work, but the later gearbox has a bulge there that the early tunnel rubs on.

Oh yeah, you'll need the longer speedo cable (96" IIRC). It gets routed across the body on the ledge in front of the battery, then curves down the firewall and runs back along the RH frame rail. There's a gap someplace so it can get past the brace. Should be a clamp on the ledge, and ISTR a clip into the frame rail. Then a broad curve under the RH floor and up through the hole in the floor behind the handbrake lever. Mine was rubbing on the floor, so I added a slit length of old fuel line to protect it.
https://i.imgur.com/h96qFYU.jpg (https://imgur.com/h96qFYU)

Electrically, you'll need to add the relay and operating switch; and the wires that run to them. Fairly simple, just 6 wires. See the workshop manual for instructions. It only shows one isolator switch, so you'll also need jumpers to wire the second switch in parallel with the first.

I've found it advantageous to add two things not originally in the circuit : An in-line fuse to limit current to the relay and solenoid; plus a diode across the relay contacts to absorb the kickback from the solenoid. The fuse will hopefully keep the solenoid from burning up if it gets stuck somehow, the diode makes the relay contacts last much longer (and allows the use of a cheap modern relay).

PS, if you do have an early 3A, you might want to consider modifying the top cover so you can keep the dipstick functional. I rebuilt my early top cover with the shift rods and forks from a TR6, but I hear you can just drill the boss for the dipstick.

PPS, there's something to be said for adding your commission number to your signature. Makes it easier to answer your question :)

mastaphixa
11-04-2017, 08:39 PM
Commission number added to my sig. Thanks for the great reply!

Marvin Gruber
11-04-2017, 10:47 PM
I have a couple of working TR3 overdrive trans if interested.
Marv

mastaphixa
11-05-2017, 02:32 PM
So looking at the parts break down, it looks like the tail housing and the overdrive just swap out. Does it make sense to do it that way?

malbaby
11-05-2017, 06:08 PM
You can also fit an OD box out of a later model 6 cyl Triumph Sedan

Marvin Gruber
11-05-2017, 06:54 PM
Not that easy, mainshafts are different lenghts. You could use the gears from Tr3 trans to another TR3 trans. TR4 gears are diffrent from TR3. Only way its going to work out is if you had a TR3 overdrive with bad gear, you could use gears from a good TR3 nonoverdrive trans.
Marv

TR3driver
11-05-2017, 11:26 PM
You can't R&R the OD without pulling the gearbox as well. Changing the mainshaft is a PITA, requires basically a full teardown of the gearbox. And of course you'd have to have the OD mainshaft to match the gearbox you're adding it to (mainshaft is different as well). Don't forget, the top cover assembly is different too, so you'd have to swap the TR3 shift rods & forks into your TR4 cover, or else drill and tap the TR3 cover for the lockout switches.

If you've already got a working TR4 gearbox & OD; I'd say it's a lot simpler to just use that. Synchromesh on 1st gear is an added bonus.