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FITTING A Triumph 2500 GEARBOX & OVERDRIVE to a TR
By Brian Richards
This article is provided for your personal use - Copyright remains with Brian Richards.
NOT to be reproduced or copied without the express permission of the Author
(also seen in "Sidescreen" & reproduced courtesy of Brian and Editor Bob Slender)
The TR gearbox is basically a good strong unit that gives long, reliable service and given the synchro technology is some fifty odd years old, a nice one to use. This is especially so when mated to an overdrive. The only real problem is that with no synchro on first, a marked or chipped first gear is not uncommon.

However it is now at least forty-five years old and most of them have had a hard life in one way or the other due to the very nature of the TR…made fast to go fast. On top of that, it is getting harder to find good, reliable, local and competitively priced repairers who are prepared to take on a troubled TR box.

With any type of failure you are going to be looking at over a $1000.00 to overhaul it. One alternative is to source and fit a Toyota Supra box with the commercial conversion kits that are available. If like me, you want to stay with a box that is still Triumph and has the same feeling as the TR, then a Triumph 2500 box is the way to go. If you look around they are still available. I purchased one, overhauled it including a new OD clutch, oil pump, and uprated lay shaft for under $1000.00.

The main advantages of the 2500 box are:

  • Basically the same size, design, and feel as the TR one.
  • Same ratios. The OD ratio is a little different but you will not notice it.
  • Synchro on first gear.
  • Stronger 2 nd and 3 rd gear bearings.
  • J type overdrive that is less prone to oil leaks than the A type fitted to the TRs.
  • They are at least 15 years younger and normally have had an easier life.

The main disadvantages are,

  • The lay shaft bearings are more susceptible to failure if worked hard.
  • Only OD on 3 rd and 4 th gear although this can be overcome if desired.
  • Incorrect speedo ratio.
  • It is not a direct replacement and some modification is required.

The good news is that all the disadvantages can be easily overcome and the results are a very good, strong, and reliable box that is more pleasant to drive (synchro on first) and leave less oil on the garage floor.

The problem with the lay shaft bearings is mainly experienced by people who drive their car hard. It can destroy the cluster gear and feed contaminated oil through the OD. The fix is to fit an uprated cluster gear with two bearings at the rear (the end that fails). These can be made locally by machining your old gear or can be purchased from various sources in the U.K. and the USA . I had mine done locally but it worked out to be about the same price as importing one because my cluster gear was unusable due to a failed bearing.

This problem can be experienced on the TR box but it is not nearly as common.

Areas that require modification are,

Clutch plate. The input shaft of the 2500 has a finer spline than the TR so a clutch plate change is required. You can fit a 2500 plate which is 8.5”diam or one from an Isuzu something or other that is 9.0” diam, the same as the TR. I fitted the Isuzu option from Daikin Clutches P/N R712-531 / DHC532

Clutch throw out bearing, guide and operating fork. The ones on the 2500 are smaller than the TR so you need to swap them over. They are a direct swap, no mods.

Output drive flange. The bolt pattern is slightly different than the TR so you need to re-drill the four holes at 45º to the old ones. The flange will then have eight holes but apart from having a few more options when bolting up the tail shaft, it is not a problem.

Gear stick. The one on the 2500 is longer and bent. There are two options. One is to cut and shut the stick to the same length as the TR one. With this option you can fit and use the 2500 gear knob that has the OD switch incorporated in it. I took this option.

The other option is to change over the stick from the TR.

Speedo drive . The attachment of the outer cable is by way of a flange whereas the TR uses a screwed fitting. Again, there are two options. The simplest is to have a flange fitting fitted to your current cable, or buy a new one. The cable length required is the same as the original.

The other option is to machine up an adaptor to fit to the flange fitting on the box and then use the original cable.

Exhaust routing. The J type overdrive is smaller than the A type and has a small sump plate to give access to the magnet / filter and other hydraulic bits and pieces. This will require the exhaust to be modified to provide the required clearance for servicing. It cost me $90.00 to have a new length made and that included having a flange fitted to the junction of the exhaust extractors and the next length of pipe. This flange makes it so much easier to remove the extractors if needed, and is gas tight.

Exhaust support. Just how your exhaust is currently supported at the rear of the gearbox will determine what you need to do. If your exhaust is mounted off the gearbox or the re-routing of the exhaust makes it impossible to use the current method, then the following is a good alternative. Fabricate a ‘T' piece that bolts between the two rear mounting pads at the very rear of the overdrive, with the leg of the T pointing down alongside the exhaust. Then fit a ‘P' clamp to the exhaust and bolt it the leg of the ‘T' bracket. This should be done once the box is fitted and exhaust modified. See diagram.

Electrical . This is the easiest to fix. Apart from the fact that the 2500 only uses one isolation switch, the wiring is the same but you will need to fit an earth wire to the solenoid. The A type solenoid only has one wire going to it and utilises its body as an earth. The J type has two connections, one, the main operating one and the other an earth. They both look the same and it does not matter which way they are connected.

If you use the 2500 gear knob and switch, then you will need to make some more extensive wiring changes.

Speedo ratio. Two options again. First is to do nothing. The ratios work out in such a manner that you can take the reading on the speedo, double it and call it Kph. So if it reads 30MPH then you are doing 60KPH. This is correct to within 11/5KPH. I find this is not a problem and in fact find I do not need to do the normal MPH to KPH conversions we do when changing speed zones on the road. The other option is to get the speedo recalibrated, about $80.00.

Rear gearbox mount. As the J type overdrive mount is significantly different from the A type, a new rear mount will be required. Again there are two options. The easiest is to purchase a conversion kit from Moss in the U.K. , P/N 211361X. I have not seen one, only a drawing from their catalogue so I do not know how it will affect the exhaust or speedo drive. It consists of a replacement mounting plate and new mount. It looks a good option but I do not know its cost.

The other option is to fabricate your own. There have been a number of designs by various members but this is the one I used and utilises a 2500 rear mount. This mount consist of a central plate that bolts to the OD with two plates, one fore and one aft of the central plate and are bonded to this central plate by rubber blocks. Two steel tubes attach these two plates to each other and form the mounts by which it is attached to the chassis. On the 2500 there is a steel plate that is used to attach the mount to the OD. This is not required for the conversion and will require the two mounting holes of the central mounting plate to be elongated slightly to fit over the OD mounting bolts. The forward of the two plates will need to be relieved at its central upper position to allow sufficient clearance from the OD housing. Remember this forward plate is attached to the chassis and must be clear of the OD.

Left and right mounting brackets are then fabricated. These mounts attach to the original chassis mounts. The left bracket attaches to the rear of the mount to give clearance for the solenoid and the right one to the front of the mount to provide clearance for the speedo cable.

Fitting.

Fitting of the box is the same as for the original TR box except for the speedo drive. It will not travel through the cabin as the TR one does, it stays under the floor. You will find a hole in the chassis in just the right position to pass the cable through and line up with the OD speedo attachment. Why these holes are there I do not know but they are there on the TR2, 3, and 3A, very convenient. It is suggested that a short length of hose be fitted over the cable where is passes through the chassis to protect it from chaffing.

Click Pics to View Larger Image
Overdrive mount (not used)
Rear Gearbox Mount

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