Gearbox Identification - Help

[TR6BobNF]

A couple of years ago I picked up a spare gearbox from a guy thinking it was a TR6 box. I had already bought a spare TR6 engine, diff and misc other parts from this guy who had scrapped 5 or 6 TR6's. All were good deals with no issues. This summer, when I cleaned/degreased the spare gearbox I found it did not have a TR6 commission number. The number is CT78323, which I am told is a TR4A box but I cannot confirm this. The guy had moved away so no chance at returning it but I managed to contact him by phone and he assured me he removed that box, CT78323 from a TR6 he scrapped and at no time did he ever have or scrap a TR4. He's a good guy and I believe what he old me; that the box was in a TR6 at some time during its life. The box is in good shape and I had bought new gaskets, seals, dowel pins, bearings and clutch shaft for my current gearbox a couple years ago and used those parts to partially rebuild CT78323 with no problems fitting parts. I was good to go with what I believe to be a good reconditioned box but a TR6 friend now warns about possible gear ratio issues with fitting what appears to be a TR4A box to a TR6. Can any of you TR4/TR6 guys out there offer any advice on this issue? Can I pull the covers off both boxes and count teeth in the gears? Is there a definitive source to accurately identify the origin of CT78323? The TR6 box I just pulled definitely has wear issues and high mileage and will require a more extensive rebuild while CT78323 is most assuredly low mileage as TR's generally rusted out around here before racking up 60,000 miles. It is tighter than the TR6 box, has no visible wear on the gears or synchros and no debris in the gear oil. The only problem I found with it was a broken clutch fork pin. I'm hoping just to use it with my new engine but would like to know more about what I have before final installation. Any advice would be really appreciated.
Thanks,

 

[TR3driver]

That number would make it a late TR4A box. The TR6 gearboxes were beefed up a bit, which at some point included a slightly higher first gear ratio (which gives a bit less stress on the countershaft). At least some of the changes were so the late TR6 could share gearbox parts with the V8 Stag. I'm guessing that if you compare them side-by-side, you can see some of the changes in the casting. The mounting flange got thicker, and the stiffening ribs more pronounced.

However, if you are not given to racing or trying to do burnouts; the TR4 box should work just fine in a stock TR6.

One place where you might have a problem is the speedometer drive ratio. TR2-early TR6 always had a 2.5:1 ratio (speedo cable turns once for 2.5 turns of driveshaft) and the speedometer head was changed to suit the final drive ratio, tire sizes, etc. But in 1972 (same time the J-type OD was introduced), they went to having all the speedometer heads be 1000 turns per mile and the drive gears in the gearbox (or OD) were changed to suit the final drive. As a result, using an early gearbox (even a TR6 unit) with a later speedometer will result in the speedo reading about 11% low.

I don't know if it is possible to change the driven gear in the early gearboxes to suit the later speedo or not. But one option is to have a speedometer shop make up a "ratio adapter" (which can also be adjusted to cover any changes in tire size, etc.) Some years ago I was quoted $100 for the initial box (and $40 to change the ratio); but they are probably more expensive today.

 

[TR4nut]

While I don't think it will be an issue, on a TR4 box the input shaft sticks out past the bellhousing (you can't sit the gearbox upright without hitting the shaft). Not sure if the 4A is the same, but I think it is - you may want to check that the extra length is not an issue when you mate it to the TR6 engine.

 

[TR6BobNF]

Randall/Randy:
Thanks for the advice. I did not bother with questioning this before as I always thought the TR4 box fit the TR6. In fact, I sold a couple of TR4 boxes to TR6 owners back in the '70's and nobody ever came looking for their money back. While I'm not too worried about the possible speedo issue I am concerned about the input shaft length as Randy points out and had hoped to get more comments on this thread. If I don't get a definitive answer on the input shaft issue I will just reattach the clutch and pressure plate to the pulled TR6 engine and try to mate the TR4 box to it. If it fits up I will move on to other work and install the pair when the detailing is complete in the engine bay. Plan B will then be to check speedo calibration and get a rebuild if required as you suggest Randall. Any and all advice still greatly appreciated. Thanks again and Happy New Year to All!
Cheers,

 

[TR3driver]

The difference in length seems to be only the length of the spigot that protrudes into the pilot bearing. So if the extra length did prove to be a problem, I think you could just carefully trim the excess length (making sure to keep the shaft cool to avoid ruining the temper and/or damaging the seal).

I've never heard of it being a problem; but personally I've only gone the other way (my TR3 has a later TR6 gearbox in it).

 

[TR6BobNF]

Thanks again Randall. I will try to fit it first and then cut the input shaft if it's too long.
Cheers,