View Full Version : TR4/4A Adding overdrive to tr4

06-05-2005, 11:24 PM
What is needed to add an overdrive unit to a tr4 with non overdrive transmission? Does it just add on in place of the extension? Can you use the same drive shaft?
Also, the manual says to drain oil from transmission before removing. I just put new oil in. Will it spill out somewhere, or what?
Thanks, Phil Brooks

06-05-2005, 11:39 PM
You need the O.D. unit and the adapter flange. I believe the driveshafts are the same. You'll also need the switch for the steering column, the switches for the gear lever,and the wiring.
When you take the tailshaft housing off the tranny (that's the piece the O.D. unit replaces), all the fluid will run out. The only way around losing the fluid would be to catch it in a clean container to use it over. Keep in mind you will need extra fluid upon reassembly as the O.D. requires a little more fluid capacity than a non O.D. trans.

06-05-2005, 11:49 PM
Hi Phil,

First, the gearbox needs to be rebuilt around a shorter mainshaft. Then an adapter plate is installed on the rear of the gearbox in place of the extension, to which the OD unit attaches. With A-type or J-type ODs for TRs, the drive shaft is the same. In a TR4, a TR A-type will fit the rear motor mount To use a TR J-type in a TR4 will require some modification of the mount.

(NOTE: there are non-TR A-type and J-type ODs, used in Triumph saloons, Volvos and some other cars, which will not be as easily mounted, but can be adapted.)

If you are re-using the top gearbox cover, it will need to be drilled and tapped for two inhibitor switches to be installed. You'll need a different speedometer cable, a wiring harness, an OD switch on the column (or elsewhere, on my TR4 it's inset in the shift knob), a relay, and a solenoid to operate the OD.

An angle drive on the speedometer cable is optional (was used with a shorter cable on later cars, but not the TR4). There are some interesting modern options, too. For example, Revington TR has a "black box" that makes the OD operate more like a 7-speed transmission, or a 6-speed, depending on if it's set up on 2nd gear or not... some "hot" modified engines should avoid using OD on 2nd gear.

The gearbox and the OD "share" the same oil. Most often I've seen 20W50 Valvoline Racing Oil (non-detergent) recommended. But, because the gearbox and OD share oil, it's wise to have both rebuilt at the same time, so that gunk and shavings from one don't contaminate the other.

Other key things you need to know include how to adjust the solenoid to properly engage the A-type OD. Incorrectly set it will quickly burn itself up. There is an a hole in a lever on the opposite side of the OD, that is aligned with a hole in the case when adjustment is correct. Still, I think that's a bit rough and prefer to use a dial test indicator on the release valve, to set it more accurately.

Hope this helps!


06-06-2005, 08:41 AM
I just changed out the transmission in my TR3 and went to an overdrive tranny. Cost $2200 with a new clutch, u-joints and diff. seals. Worth every cent!! However unless you are going for originalty I would think about going to a 5 speed conversion for about the same cost.

06-06-2005, 02:11 PM
Thanks Alan, Banjo and Bill for your help. There is currently an overdrive on E-Bay #7979143614 that claims to be for a tr4 but does not look like what I have seem before nor what is in my Bently book. If any one has time to check it I sure would appreciate that. What might it be worth?? I just realized I should be able to attach a link to the e-bay site. I'll try. Thanks to all Phil
Just type in 7979143164 on e-bay home page.

06-06-2005, 05:12 PM
This looks like a regular A-type which will fit the TR4. Note that the mainshaft is for a TR6. I believe the early TR6 overdrive mainshafts work in the TR4 but later shafts are different. Something to do with the spigot bearing being pressed on rather than slid on. You can purchase virtually any parts you need from Quantum Mechanics in CT. Also, the ebay text is not clear but my take is that this unit has not been rebuilt? If so the selling price should be considerably less, say less than $100.

06-06-2005, 05:49 PM
Hi again Phil,

What's it worth? A fully rebuilt and functional TR A-type installed on the proper gearbox box sells upwards of $2000. So Bill's cost is right in there.

The OD in auction 7979143164 *is not* a TR A-type. The horzontal solenoid was only used on A-types in larger Triumphs such as Stag or 2500. The seller is incorrect refering to this as the "improved variety". It is the "sedan variety" setup.

It's still possible it could be made to work, but the rear mount will likely need some sort of adapter and there might be a problem with speedometer cable attachment because the boss on the tailpiece is horizontal (should angle down about 30 degrees for TR applications).

Good things about this auction are that it includes the adapter plate (a $100+ item) and the gearbox mainshaft (a $200 item), a working solenoid and the oil pump cam.

One question you might ask is the serial number of the OD. The first two digits will indicate the amount of reduction: 22, 25 or 28%. TR A-type are 22%. Most TR J-type are 25%. But, any of them can be rebuilt to greater or lesser reduction specification (at a cost, of course!)

It so happens there is also a J-type offered on eBay, 4554467536. This might also be used.

Note also that there are two different types of A-types, call them "early" and "late". The early version has a larger accumulator piston and really jumps into OD quickly. The later A-type and all J-type are all softer shifting into OD, although there are some tweaks that can be done to improve the snappiness of the shifting. I really can't tell if the A-type in the auction is early or late version... don't know if the sedan A-types came in the same versions as the TR A-types.


06-06-2005, 06:40 PM
Oops.... a couple more things...

Since that A-type in the auction is out of a larger Triumph (or maybe even another manaufacturer entirely), unless it's been switched, the rear flange might not bolt up to your TR4's drive shaft. Perhaps it's already been switched for a TR flange, or if it hasn't it could be re-drilled to fit.

Bill mentioned a 5-speed conversion as a possiblity, certainly an interesting option if originality isn't a concern and your goal is only top gear overdrive capability. The Toyota Supra gearbox is one popular conversion and thanks to it's alloy case is actually about 50 lbs lighter than the TR gearbox/A-type combination (which is about 125-130 lbs total). The Toyota Celica 5-speed is also an option, but doesn't save as much weight since it uses a steel casing. There are other 5-speeds, but these are probably the two most commonly used for conversion.

However, one advantage of the A-type over a 5-speed gearbox is that because OD works on 2nd (optionally), 3rd and 4th gears, this allows it to act in effect as a 6- or 7-speed, close-ratio gearbox. This is particularly useful with a 2.2 liter, 4-cylinder engine. The "black box" controller I mentioned in my first post makes the OD "feel" even more like a 6- or 7-speed gearbox.