GT6 TR6 Transmission w/OD in a GT6?

[Jhnybravo]

Has anyone seen or have any direct knowledge of the implications of installing a TR6 ('71-'74?) transmission (think it's an "A" type) into a 1972 GT6 MK3? I recently purchased a "low mileage" GT6 MK3 and have found it to have a TR6 transmission. The tunnel cover required slight mods but it's liveable. Is there any implications to weight, performance, clutch life, ,etc, etc? In other words, is this a bonus, or something I should remove and go back original.
Transmissions AREN'T my speciality....by no means!
Any info is appreciated!

Thanks,
Jhnybravo

 

[Alan_Myers]

Hi,

I'm not terribly knowledgeable about GT6, but have been looking into getting one (eventually) and have done a little research.

One comment I've often found is that the GT6 gearbox is perhaps the car's weakest point (some models also have axle issues). I've also seen some reference to installing a TR6 gearbox as a stronger replacement unit. So, if strict originality isn't a concern of yours, you might have a really great setup.

Personally, I'd keep the TR6 gearbox. It's a strong, proven gearbox that reached the peak of it's design in the TR6 (AFAIK) and might be easier to repair or rpelace, if ever needed.

There are also quite a few options for TR4A/TR250/TR5/TR6 clutches, since there were so many of these cars built originally and still on the road. By, comparison, far fewer GT6 were built, so unless it shares its clutch with other cars, might be harder to find clutch parts for, or your choices might be more limited.

There have been some issues with TR6 clutches. But many of these now have well-documented, developed and tested over the years as need arose. I don't know if the GT6 clutch was comparitively more or less reliable or problematic.

I don't know the weight of a GT6 gearbox, for comparison. But, the TR6 gearbox is about 75-80 lbs. The A-type OD, if that's what it is, adds another 30 or so lbs for a total of 120 lbs. approx.

You can tell if the OD unit is an A-type for certain by looking on top for the Laycock serial number plate. It will specify. It will also show the amount of overdrive reduction, the first two numbers of the serial number would be "25xxxxx" if it's 25%, etc., unless it's been modified.

On the other hand, getting to that serial number plate that means removing the gearbox cover, at least. SO, some other A-type clues include: A vertical solenoid on the LH side (most A-types, I think there were a few models with a horizontal one, but all J-type had horizontal solenoids AFAIK). External operating linkage at the solenoid (usually, some early TR2 or TR3 had an enclosed box, but those are rare). A large, brass drain plug in the bottom (bigger on the earlier A-types, still pretty large on later).

An A-type OD would be a bonus, IMHO. Roger William's in his various "Restoring Triumph..." and "Improving Triumph..." books really likes the J-type and recommends it. However, John Esposito at Quantum Mechanics, who rebuilds lots of the things professionally, feels the A-type is a more robust unit and recommends it over the J-type. Early A-type really pop into OD quickly and are prefered by anyone racing with one. Later A-type - which is what yours would be if it's early TR6 - are a bit softer and slightly slower shifting. J-type are a little softer/slower still. All of them can be modified for faster/snappier shifting, but that makes the rest of the drive train work harder.

 

[Jhnybravo]

I believe the number on the side of the overdrive is:
Laycock & Normanville Overdrive - Serial Number 22/61753/013352 A

So this is a 22% reduction? I think the tranmission came out of a '71-'74 TR6, but can't confirm it.

Thanks for your insight!

 

[Alan_Myers]

[ QUOTE ]
I believe the number on the side of the overdrive is:
Laycock & Normanville Overdrive - Serial Number 22/61753/013352 A

So this is a 22% reduction?

[/ QUOTE ]

Yep, that's an A-type and yes, that would be 22% reduction *unless* someone had it rebuilt to a different amount of reduction at some time in the past. I thought TR6 usually had a 25% reduction installed, whether early A-type or later J-type. However, I might be wrong or it could be a TR3/4/4A/250/5 A-type matched up to a TR6 gearbox. I can't tell from the serial number, maybe someone else can.

Some folks looking for lower rpms on the highway prefer 25% or even 28% reduction, both of which are possible modifications, if you wish.

22% reduction seems better for "in-between gears", IMHO, if you choose to use the OD for "spirited driving". I.e., if you treat the gearbox/OD like a 6-speed or 7-speed, depending upon whether it's set up to engage on 2nd gear.

Most TR6 ODs only engage on 3rd & 4th (so are similar to a 6-speed), but can be modified to work on 2nd gear. That mod would require adding another isolator switch to the g'box top cover and using a TR3/4 wiring sub-harness.

However, caution is recommended if using 2nd gear OD on any TR. A performance-modified/higher HP engine setup and/or hard use of 2nd gear OD can actually do damage and make the OD pretty short-lived.

Overall, it sounds like a good setup to use in a GT6.

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[Jhnybravo]

I bought the car for highway driving. I work about 30 miles away and 90% of the trip is interstate with a 70 mph "suggested" speed limit....so I run between 70 and 85 on the way to work in my F-250 diesel 4 door, with it sucking fuel the whole way. :-O
That's where the GT-6 comes in. How difficult is it to up the ratio to 25 or 28% reduction? That would probably pay for itself in time to come if its not that expensive.
Say, I've got another question for you..how and where does the switch for the overdrive go? I bought an overdrive switch and knob for a GT-6 but the threads are way too large for the shifter lever. Any ideas?

Thanks!!

Tony

 

[Alan_Myers]

Hi Tony,

[ QUOTE ]
How difficult is it to up the ratio to 25 or 28% reduction? That would probably pay for itself in time to come if its not that expensive.

[/ QUOTE ]

It's a lot of parts needing swapped and set up, best done during a regular rebuild, which ain't cheap (gearbox probably should be rebuilt at the same time, don't want one contaminating the other, since they share oil). You might ask John Esposito at Quantum Mechanics www.quantumechanics.com , but be sitting down when the quote comes in.

Even at today's prices, I'm afraid you'd need to drive the car for many, many years to get gas savings alone to pay for the conversion to a bigger OD reduction! Try it with the 22% reduction and see if you like it, would be my advice. Tire size, differential ratio and general engine tune will be more easily changed and affordable factors in terms of fuel economy. If you ever find an OD rebuild necessary, *then* inquire about increasing the amount of reduction.

[ QUOTE ]
Say, I've got another question for you..how and where does the switch for the overdrive go? I bought an overdrive switch and knob for a GT-6 but the threads are way too large for the shifter lever. Any ideas?

[/ QUOTE ]

I don't really know for sure about the GT6 setup. On my TR4 the OD switch would normally be a lever on the RH side of the steering column. However, I have a custom made switch in the shift knob that just seems a more natural location to me. On GT6, I think it might have also been a button on the shift knob, at least in some cases. I say this because I've had recommendations to use a GT6 shift lever (or Stag) because it's hollow to accommodate OD wiring. A TR2/3/4/4A/etc. shift lever is solid, so wiring must be run externally with a setup like mine. See if you can find an adapter sleeve to thread into the knob.

Depending upon what size threads are needed inside and out, you might find an adapter at a hardware store, or be able to use something like a Timesert, which is used to repair stripped threads. Another possibility would be a photo store that sells professional equipment. Some cameras and lighting equipment use a 3/8" or larger mounting hole, while most tripods and many light stands use a 1/4" coarse thread. So, thread adapters, often brass, are a common item. If one or the other thread/hole size isn't quite right, it might be possible to drill and tap new threads, or even have a machine shop make something up from scratch.

Hope this helps.

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[vagt6]

Johnny, if the TR gearbox in your GT6 works well, leave it. It's a better box than the GT6 one. The Laycock A-type is a great OD, I've never seen one in a GT6. Must have been a real project to put the TR6 gearbox and Atype in there.

The switch for MKIII GT6 goes in the gearshift knob itself. They're a bit hard to find. The wiring to the switch is easy. Some folks use both the escutcheon OD switch and the gear shift knob switch. A bit redundant, but fun.

If you can't find a gearshift know switch, let us know. I'm sure someone can point you in the right direction.

Good luck, sounds like a good setup you have there.

 

[Jhnybravo]

Mark,
I followed you until you said "Some folks use both the escutcheon OD switch and the gear shift knob switch. A bit redundant, but fun." What is an escutcheon OD Switch?
I bought a used shifter knob (needs new chrome plating) that was 'supposed' to fit a GT6 but the threads are way TOO large (7/16" fine) and the shifter (5/16" coarse) for the transmission isn't hollow. I assumed the wires should go through the shifter lever arm, but I may be wrong. I like the idea of keeping the performance features of this transmission but I want the shifter to look original. I'm not thrilled with a toggle switch tie-wrapped to the shifter arm! hehehe That's too redneck even for me !!

Thanks !!!

 

[RHWins]

Check out that sexy GT 6 Mk I in the classifieds!(it has O.D. too!)

 

[billspit]

Both OD switches are available from VB or TRF. The OD knob is hard to find. The shifter is only hollow for a short distance, then the wire comes out. It is hidden by the shifter boot.