It is becoming more popular to install TR4 overdrive transmissions into TR3s. This is because the TR4 transmission has syncro between first and second gears, parts are still readily available to rebuild the TR4 transmission and the TR4 transmission is almost a bolt-in swap. The TR4 transmission is externally very similar to the TR3 transmission.

Follow your workshop manual's instructions for removing and reinstalling the transmission . The TR4 Transmission will bolt up to your TR3 bell housing . You will need the next length longer bolts at the bottom of the bell housing because the lip on the TR4 transmission is wider then on the TR3 transmission. This will also require you to readjust the clutch slave cylinder rod.

The transmission mounting holes will be slightly to the rear of the cross member mounting holes. This can be corrected by elongating the mounting holes on the bracket where it mounts to the transmission and where the bracket mounts to the frame.

You can use your TR3 drive shaft without modifications. By the way, the transmission goes in easier if the drive shaft is completely removed. Otherwise it gets in the way of lining up the transmission for installation. Since the TR4 shift lever is different, use the TR3 shift lever. It is a direct fit.

When wiring up the overdrive, do not use the wiring schematic provided in the red factory TR2 manual or the Haynes manual. This schematic is for the TR2 and early TR3 transmission which has only one overdrive cutout switch on the top of the transmission. Use the schematic that comes in the TR3A and TR4 owner's manuals or in the Haynes manual for the TR4.

The relay, when fitted by the factory, was placed on the lower rear of the battery box towards the right side. The overdrive switch is located on the instrument panel along side the heater switch.

If your old transmission was not fitted with an overdrive unit, you will need to get a new longer speedometer cable. The speedometer cables for overdrive transmissions are readily available. However they are routed differently. The cable is draped over the bell housing, goes under the floor and attaches to the overdrive unit by the hand brake (now you know why the rubber boot for the hand brake is shaped so funny).

IMPORTANT Before you replace the transmission cover or start the car, test the new wiring for the overdrive. There is a lever attached to the bottom of the solenoid. When the solenoid is activated you will hear a click and the lever will move. If there is rust or grime built up inside the solenoid you may have to remove it and clean out hole and the piston.

Turn the ignition on, turn the over drive switch on, then shift into each of the gears, especially reverse. The solenoid should click on in every gear except first and reverse. If it engages in first or reverse check the wiring and the switch adjustment on the transmission. If the overdrive engages in reverse while the car is moving, it will fail in a very expensive way.

IMPORTANT After you fill the transmission with oil, run the engine for a few minutes and refill the transmission. The overdrive unit has a internal oil reservoir that gets pumped full when the engine is running.  I like Redline MTL transmission fluid.  It works quite well with Triumph overdrive gearboxes.

It is possible to bolt a TR6 gearbox to a TR3/4 engine.  And if you are going racing or auto crossing the TR6 box is a good idea because it has the strongest housing and is less apt to break under repeated stress. The gearbox mounts are farther to the rear than the TR4's mounts so you will need to elongate the mounting holes. You would need to bend down the flange on the TR3's cross member and drill new holes. The clutch slave cylinder push rod may need to be shortened, and you may need slightly longer bolts to fix the gearbox to the engine.  Again you can swap in the TR3 shift leaver. The prop shaft will bolt up.  The splines are long enough to compress that far. But if you are going to exercise the rear suspension it might be a good idea to take the shaft to a specialist and have them shorten it the difference between the TR3 box and the TR6 box.  I have not done the conversion but I was able to look at one closely.

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