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TR6 TR6 OD gearbox installed in TR3 ?

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Brinkerhoff

Brinkerhoff

Jedi Knight
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Can I install a TR6 later J series overdrive gearbox into my late TR3A ? I see the rear mount is different but what am I getting myself into? I'd prefer an earlier box but this one was given to me. thx. Kevin
 
I have not tried it, but I believe you are going to get into modifying the frame to clear the OD (which is larger and sits farther back than the A-type). For sure you'll have to cut away the tabs where the original cross member went, and I suspect you'll need to trim the rails some too. Then since you're weakening a structure that was none too strong to begin with, you'll want to add some reinforcements on the outside.

But I could be mistaken. There is a nice article at
https://www.buckeyetriumphs.org/technical/jod/AdaptingJOD/AdaptingJOD.htm
showing how to install a J-type into an early TR6 with just cutting away the tabs. I'm basing the rest on my belief that the early IRS frame had more room in that area than the TR3 frame.

I have J-types on both Stags, and an A-type with TR6 gearbox on my TR3. Even if it was a bolt-in proposition, I'd sell the J-type to someone else and find an A-type. Although no doubt the J-type is more rugged and reliable; the A-type is much sportier and better suited to the character of the TR3 IMO. I regret not hunting down a Stag A-type as well.
 
Brinkerhoff said:
Can I install a TR6 later J series overdrive gearbox into my late TR3A ? I see the rear mount is different but what am I getting myself into? I'd prefer an earlier box but this one was given to me. thx. Kevin
Click to expand...

I'd expect you could sell/buy and end up about even...
 
Thanks. The gearbox I have may have an internal issue and I am hesitant to pass it on as a "good" piece. On the other hand it may not have an issue in which case its worth a few bucks right? I think I'll take it down and see. I could rebuild it and see if it fits in the car , I'll let you know. It should bolt on to the TR3 engine though, correct? What clutch release system would I use? Kevin
 
It will bolt to the TR3 engine, but you'll need to make some changes. It may depend on the year/series of the gearbox, but I had to install longer studs for the 3 studs at the top as my TR6 box has a flange that is twice the thickness (at least, maybe more) of the TR3 box. Longer bolts all round, too.

You can keep the TR3 clutch, but you'll want to use the TR3 release bearing and bearing carrier with it.

There are several ways to go with the hydraulics, I chose to keep the TR3 arrangement with the slave mounted to the back of the gearbox flange, which required shortening the pushrod a bit. (I actually made up a new pushrod to avoid cutting the original, it's just a length of mild steel rod with threads on one end and the other end rounded off.) When I did it originally, I also made a little tab for the return spring (so the installed length was as before) but I didn't move the tab to the TR3 and it seems to be doing fine.

Another option would be to move to the TR6 style mount, with the bracket on the front of the gearbox flange and the slave on the rear of the bracket. But that will require either leaving off the flywheel cover or beating the edge of the cover flat.
 
That will work out . I'll be able to use the TR3 parts then. I'll watch out for the clutch rod also. If it doesn't install in the chassis without cutting , then I'd be more inclined to keep it. I'd rather hold out for an earlier gearbox if I have to cut anything, or of course , I'd trade it. Thanks again, Kevin
 
I helped do this install a couple of years ago. No frame mods were needed but we did have to make an adapter plate for the rear mount to bolt to the frame. The biggest headache I recall was that because the TR6 box is a bit longer the exhaust pipe contacted the trans and it needed to be relocated and dimpled slightly to provide enough clearance. The conversion was carried out in several hours in a well equipped shop.
 
There is no good place to run the exhaust except under the frame. This is IMHO not good on a tr3/4 it is too low.Sell your tr6 box to a tr6 guy and get a supra tranny and eagle gate kit to install it.You will be much happier.
MD(Mad dog)
 
I have fitted a4 speed TR6 gearbox into my TR2 while the OD was being rebuilt. Since the TR2 uses a bullet nose style starter I had to change the flywheel, clutch & starter to the TR4 set up. The rest was just bolt in.
 
trgriff said:
I have fitted a4 speed TR6 gearbox into my TR2 while the OD was being rebuilt. Since the TR2 uses a bullet nose style starter I had to change the flywheel, clutch & starter to the TR4 set up. The rest was just bolt in.
Click to expand...
I must be missing something. The early starter will work just fine with the later gearbox housing; it's the other combination that doesn't work (late starter with early gearbox).
 
I've got the two gearboxes next to each other on the bench , the std. TR3A and the later TR6 with the J type overdrive. It looks to me like the rear mount will be a problem . There looks to be a lot of overdrive body landing right where the chassis cross member for the std. mount is. Randall , are you thinking the speedometer gearing will need to be changed also? Kevin
 
Brinkerhoff said:
Randall , are you thinking the speedometer gearing will need to be changed also? Kevin
Click to expand...
Yes, almost certainly your speedo will read about 10% low if you don't do something.

Unfortunately, although the J-type can be set up to match the TR3 speedo, I believe it requires tearing the OD completely apart to change the speedo drive gear mounted on the annulus shaft. I don't have my notes handy though, it is possible that you can get close by just changing the driven gear (which is much easier to access).

Another solution would be to have any speedo shop make up a "ratio adapter" for you. When I inquired a few years ago, the local shop said they could do the ratio adapter and modified cables all for about $100. Of course that price will probably have gone up by now, but maybe not too much.

Another option is to modify your speedometer calibration. The speedo movement itself is fairly easy to change (just remagnetize the flying magnet to be a bit stronger), but it's more complicated to modify the odometer and tripmeter to reflect the new ratio.
 
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