TR4/4A Converting TR4 gear box to O/D

[Adrio]

I have a question for group. I have a spare TR3 O/D transmission and for my TR4A have two non O/D tranmissions. I was thinking of moving the O/D unit from the spare TR3 box to one of the TR4 transmission. Is this worth doing or should I try and sell the TR3 O/D box and buy a TR4 O/D box.

I understand that to add O/D i would need a new main shaft if I were to convert the unit. Are these main shafts available, and if so does anyone know where from?

I guess what I am asking is, given what I have, which way should I go about getting O/D for my TR4.

 

[71tr]

I am in the process of adding a j-type OD unit to my non-OD TR6 transmission. As mentioned you will need a new main shaft plus a few other parts; eccentric cam drive, cam key, interruptor switch, wiring harness, speedo angle drive etc. All of these parts are available from multiple sources but a good place to start is www.quantumechanics.com.

I am not familiar with the differences between TR3 & TR4 gearboxes. Would it be possible to simply put the TR3 OD tranny in your TR4?

 

[Geo Hahn]

[ QUOTE ]
Would it be possible to simply put the TR3 OD tranny in your TR4?

[/ QUOTE ]

Depends on whether the bell housing on the TR3A gearbox is for a pre-50000 or post-50000 car. The former lacks the bulge required for the starter used on the late TR3A and all TR4s. This can be overcome by punching a hole in the bell housing or converting the starter & flywheel to the pre-50000 starter. If the TR3A gearbox is a post-50000 then it is a straight-forward replacement -- though either way you lose the 1st gear synchro.

IMO, trading 1st gear synchro for an OD is a no-brainer.

 

[Adrio]

The TR3 gearbox is an early type. So I can't use it with my current starter and flyweel. That was my original intention until I realized the starter problem (I was willing to live without syncro in 1st). I have a lot of parts from my TR3 parts car but the starter and flywheel are parts I only have one each of.

After driving the TR3 for 20 years with O/D I am finding that I want the O/D in my "mew" TR4. If I did not have the two to compare I may have been not be so keen.

 

[Alan_Myers]

Hi Adrio,

There are more issues than have been mentioned.

In addition to the synchro first gear, somewhat later gearboxes also have a heavier and stronger flange on the bellhousing. By later, I think post-TR4A, but I'm not sure exactly when the heavier flange was implemented. Depending upon your use of the car, that might be a consideration. On the other hand, if originality is important, you'll likely want the thinner flanged gearbox.

More importantly, 4A and later use a different input shaft. The "nose" that fits into the pilot bearing at the rear of the crankshaft is longer on gearboxes up through TR4, than it is on TR4A and later. You can mate up the later gearbox to the earlier cars, but will find it impossible to fit the TR4 and earlier gearboxes to a TR4A or later car - that uses the smaller, diaphragm clutch - without more modifications.

In other words, if you stand the TR4A-TR6 gearbox on end, on its bellhousing, it will sit level because the input shaft is short, roughly flush with the bellhousing. The TR4 or earlier gearbox won't sit level if stood on end, because the input shaft is longer, protrudes past the bellhousing. Also, the clutch splines don't match, and I think the throwout bearing carrier (aka "front cover") is also a different diameter.

I doubt the "guts" of a TR3 gearbox could be used in a TR4 or TR4A box, anyway. The two are slightly different overall length and a lot of the internals differ, particularly around the 1st gear cluster.

You might want to find a buyer for the spare TR3 box/OD and use the funds for a TR4A box/OD, or to build up and convert the non-OD TR4 gearbox you've already got. But, even with the latter you are going to need more than just the mainshaft to get it to fit up to a TR4A.

Alternatively, one of the vendors might work an exchange with you. Some only offer A-types and OD gearboxes in exchange for like-units, but would probably be happy to get hold of an early TR3 gearbox/OD, if you really want to let it go.

I was going to recommend Quantum Mechanics, too. I'll be sending them my A-type OD for a rebuild. I think The Roadster Factory does some gearbox/OD work, too (or farms it out to someone). Quantum's name is the one I hear mentioned most often when it comes to OD units.

Quantum does sell the special main shaft, the adapter plate, inhibitor switches, etc. to convert a TR4 non-OD gearbox to OD. You could then use the A-type you've already got.

If you end up building up a gearbox, it's a good opportunity to upgrade a few things. For example, the input shaft seems to benefit from extra roller bearings. A-type ODs can be upgraded, too.

If you exchange a gearbox and/or OD unit, be sure you know what you are getting. The A-type has early and late versions. The later one is a little softer kicking in, due to a smaller accumulator piston. There are a lot of non-TR A-types around, although I don't think most knowledgeable vendors will try to sell you one, since the rear mount would probably not match up.

It's a different matter with gearboxes, though. There are many sedan and other gearboxes around being rebuilt to TR specification. That might be fine, functionally, but might have a few details that are unoriginal.

If having one rebuilt, have both rebuilt. Since they share oil, there is too much likelihood of cross-contanimation if one or the other is old, while the other is freshly rebuilt.

Let us know what you end up doing!

Cheers!

Alan Myers
San Jose, Calif.
'62 TR4 CT17602L