GT6 GT6 Gearbox Rebuild Parts (and Options) in 2026

[R2daICKY]

Hi all,

As I start to set up the "priority list" of repairs on my new-to-me 1971 GT6 MkIII, the transmission is pretty quickly rising to the top. It's a standard 4-speed unit (no O/D). It drives/shifts fine currently, but it has some fairly significant gear whine and an intermittent, rotational grumbling noise at idle (sometimes). I suspect this means the cage bearing between the input shaft and mainshaft is on the way out, and the layshaft and needle bearings aren't far behind. I've rebuilt enough transmissions to know that loud needles typically mean shaft damage - so, assuming the damage is done, I'm anticipating the need for a new mainshaft, amongst other things. Seems like Spigot repair longevity is hotly debated.

I was surprised to find that parts for these transmissions are nearly non-existent... Or, in the 10 days I've owned the car, I just haven't figured out where to look yet... I guess I'll need to have a little less fun driving it, if these parts need to last a while!

Doing a bit more digging, I've noticed some folks mention a "hybrid" gearbox using the mainshaft from a Dolomite/TR7 4 speed with an 18mm spigot, and machining out the GT6 input shaft to accept it. Then, incorporating the lay-gear cluster, 1st/2nd gear and synchro assemblies, plus the reverse gear, from the TR7, along with 3rd and 4th gear from the GT6, which results in a more robust transmission with the same or similar close-ratios to the original setup. My goal is to keep this car (fairly) original if I can, so I'm not super keen to go the 5-speed swap route, but I generally like the idea of using later parts to improve the reliability/durability a bit...

The issue is I can't find a good list or full explanation of exactly what parts I need from which transmission. More importantly, it seems like the later era parts are just as difficult to source in the US. TR7's aren't littering the scrap yards, and Dolomites practically don't exist.

- Has anyone put together a "donor parts list" to build this "improved" GT6/TR7 hybrid transmission?

- Is my best route to look for a donor TR7 4-speed and a machine shop to do the work of widening the input shaft cavity for the larger spigot bearing? Do I need anything beyond a donor TR7 4-speed and my current transmission to kluge this together?

- More importantly - is there a vendor that is a reliable source of these parts here in the US that would actually carry these components (as opposed to hunting down questionable used parts and hoping for the best)?

I'd be interested to hear from anyone who's gone this route (or has rebuilt a late GT6 gearbox in general). This has certainly been an good introduction into the Leyland "mix and match parts" approach to car building...

 

[Popeye]

Quantum Mechanics in CT might be a good place to start. They rebuild and repair; not sure if they sell parts and advice.

 

[R2daICKY]

Quantum Mechanics in CT might be a good place to start. They rebuild and repair; not sure if they sell parts and advice.

Yep, I’ve sent an email to Mr. Esposito, no response yet but I’m sure he’s a busy man. I’m not in a hurry yet.

My suspicion is that he does rebuilds, not part sales. I’d rather rebuild it myself (partially because I enjoy it, partially to save money) but from my reading so far, it seems like I’m likely going to be calling a machine shop to repair anything damaged, regardless.

I’m not surprised NOS Leyland parts have mostly dried up but I am a little surprised the aftermarket hasn’t picked them up.

 

[CJD]

Have you tried the Roadster Factory and Moss...or Revington in the UK? They should have the gears, but they are not cheap. I have never seen the bearing between the input and mainshaft go out...likewise with the main shaft itself. The exception is if you allow the wear to reach a point where the layshaft drops and you lose a tooth. Yours is not there, yet.

Noise in these boxes is almost always the needle bearings in the layshaft. They begin to grind on the shaft, and will eventually weaken it to the point it breaks, allowing the gears on the layshaft to drop and lose their mesh with the main shaft. The sound of bad layshaft bearings is a hiss (early stage) when the tranny is in neutral and engine idling with the clutch out. Once it progresses, it will be a rumble under power in 1st - 3rd gears, and a hiss or rumble in neutral.

From your description, I would say your parts required for a rebuild right now are layshaft, laygear hub, sycnro rings, all new bearings and seals, and possibly reverse gear. I would be surprised if you needed any more "heavy hitter price tag" items.

Your "hybrid" option is a huge job. It would be cheaper to find a used tranny.

 

[R2daICKY]

Have you tried the Roadster Factory and Moss...or Revington in the UK? They should have the gears, but they are not cheap. I have never seen the bearing between the input and mainshaft go out...likewise with the main shaft itself. The exception is if you allow the wear to reach a point where the layshaft drops and you lose a tooth. Yours is not there, yet.

Noise in these boxes is almost always the needle bearings in the layshaft. They begin to grind on the shaft, and will eventually weaken it to the point it breaks, allowing the gears on the layshaft to drop and lose their mesh with the main shaft. The sound of bad layshaft bearings is a hiss (early stage) when the tranny is in neutral and engine idling with the clutch out. Once it progresses, it will be a rumble under power in 1st - 3rd gears, and a hiss or rumble in neutral.

From your description, I would say your parts required for a rebuild right now are layshaft, laygear hub, sycnro rings, all new bearings and seals, and possibly reverse gear. I would be surprised if you needed any more "heavy hitter price tag" items.

Your "hybrid" option is a huge job. It would be cheaper to find a used tranny.

That’s sort of what I’m discovering too… though I like the idea of building a more robust box if I can figure it out…

I wouldn’t be surprised if it was the layshaft needles. Seems to fit your description pretty well, thought it’s definitely more a rumble, and less a hiss. I’ll have to pay attention if any noises disappear when I shift to 4th gear…

1. Should synchronisers be replaced as a matter of preventative maintenance, always? I’m not really having an issue with mine. Ironically, in the FJ40 transmissions I’m used to, synchros are hard to come by, so you reuse them if they’re serviceable.

2. If the needles fail, and eat the layshaft, shouldn’t I expect similar damage on the inside of the laygear cluster? Or is that what you’re calling the laygear hub. Seems like that’s a fairly difficult part to find.

Honestly, I don’t expect to drive this car hard enough to really necessitate strong GEARS, but if there’s a way to improve the robustness or longevity of the layshaft/bearings and spigot that would probably be enough.

Edit to add: I did speak to a gentleman at Quantum Mechanics (not Mr. Esposito himself) - they could build the TR7 hybrid I seek, but it would certainly cost.

 

[CJD]

These boxes are not overly strong, but they will last if you drive them like a sports car instead of an American hotrod. They used the same gearbox in everything from Triumph, to MG, to Jags...so it is not horrible, just not one to drag race with like a Muncie rock crusher!?! If you want strength, there is a conversion to fit a toyota box. It, to, is pretty expensive.

Yes, when I said hub, I meant cluster gear. The needle bearings need to be replaced in every build. If they are bad enough to make noise, then you will also need the cluster gear and layshaft. Synchro rings can be reused. They fall into the category of "while I'm there", since they are available and not expensive compared to the labor to get the box out and apart when they go bad. Reverse is required if your PO was a ham fisted shifter...and can take out the 3-4 synchronizer hub if he/she was really bad at shifting.

The input shaft is usually alright, but look very closely at the splines. If they are not perfect, they can prevent the clutch hub from sliding, and you will get a slight drag, even with the clutch fully disengaged.

 

[R2daICKY]

These boxes are not overly strong, but they will last if you drive them like a sports car instead of an American hotrod. They used the same gearbox in everything from Triumph, to MG, to Jags...so it is not horrible, just not one to drag race with like a Muncie rock crusher!?! If you want strength, there is a conversion to fit a toyota box. It, to, is pretty expensive.

Yes, when I said hub, I meant cluster gear. The needle bearings need to be replaced in every build. If they are bad enough to make noise, then you will also need the cluster gear and layshaft. Synchro rings can be reused. They fall into the category of "while I'm there", since they are available and not expensive compared to the labor to get the box out and apart when they go bad. Reverse is required if your PO was a ham fisted shifter...and can take out the 3-4 synchronizer hub if he/she was really bad at shifting.

The input shaft is usually alright, but look very closely at the splines. If they are not perfect, they can prevent the clutch hub from sliding, and you will get a slight drag, even with the clutch fully disengaged.

I had it out for a quick spin this weekend to show my dad (who hates all project cars, but has a soft spot for British sports cars). After the drive, he told me if he ever drives it again, he's driving it home. So the GT6 has claimed it's first convert.

More importantly, it gave me some time in the passenger seat to confirm some of the sounds I'm hearing. There is some whine in first through third gears, and it does get much quieter in 4th. I think this goes further to point the finger at the layshaft bearings, and probably means I should stop driving it until I can afford a transmission...

I don't necessarily plan to drag race the GT6, but "driving it like a sports car" to me still suggests some high rpm downshifting and aggressive up-shifting. I have plenty of experience driving stick, in general, but if there's any guidance onto how to drive a British transmission with success, I'm open to suggestions! I don't think I want to go the full 5-speed conversion route - I've already built a frankentruck with my FJ40, and this car is too original to molest that badly... My thinking is, if there is a way to make these gearboxes stronger/fix some of the weak points, and it will prevent me from requiring a rebuild (ideally, ever again) then now is the time to do it.

Some arbitrary math: I really have no idea what transmission is in the car currently and if any similar work was done - but based on the paperwork that came with the car, the transmission was either rebuilt (or replaced) in 1987. Not sure if that was with a used unit, a rebuilt unit, or how the math works out in terms of mileage, but the first transmission made it 16 years. The second transmission made it 25 or so under power (plus a decade in a barn). The car has 138,000 miles on it so if you apply it evenly, that's just shy of 70,000 miles per transmission. I'm 40 years old, and hopefully my knees will continue to work well enough to press a clutch and get in and out of this car until I'm in my 70's. (Conservatively, let's say 30 more years). It won't be driven daily, but it will hopefully see 2-3,000 miles per year.

All of this to say, theoretically, even a standard, rebuilt transmission could get me there... but if TR7 gears or otherwise sourcing stronger components will be more likely to survive 90,000 miles, it's safe to assume that the next transmission rebuild will be someone else's problem

 

[CJD]

Sounds like a plan!

To better describe, with the old Muncie tranny we would start by putting it in first and just drop our foot off the pedal so the clutch hammered the tranny at full throttle and 5000rpm. The shift to second would be with a quick slap of the clutch and jam the shifter to 2nd, while your right foot remained on the floor...and same for 2-3 shift. We rarely made 4th, as we were already well over 100mph. The muncie did not like this treatment, but it would take it (until you missed your clutch slap). The Triumph box will not take abuse like that. But, so long as you use your clutch as it is supposed to be, and do not drop the clutch at full throttle for starting, it will last the life of the car.

What seems to kill the British boxes are many missed shifts. Every time you grind gears, metal is left in the box. It floats around until it settles into the sludge in the bottom of the case and becomes locked there. If anything churns up that sludge, though, all the missed shift metal makes it's way into the weakest bearing... the layshaft needles, and starts to break them down. I know this sounds crazy...but on my last TR2, I changed the tranny fluid at every engine oil change. When I sold the car at 8 years, the tranny felt like new.

It is important to either change your tranny fluid regularly, or not at all. The absolute worst thing you can do is go for 50k miles without a fluid change, and then change it. That releases all the filings that were being held in the sludge in the bottom of the case...and will destroy the layshaft needles in short order. Most owners never change standard fluid, and that's fine. Most buyers want to change ALL the fluids when they get their new baby home. Unfortunately, with a tranny, that is the worst thing you can do. If the PO does not tell you he regularly changed the tranny oil... then do not change it.

 

[TR Tom]

Interesting,
So how would a magnetic drain plug fit into the equation? Do the brass bits do any damage, or just the steel ones?

FWIW I have an OD trans, so am religious about oil changes.

 

[R2daICKY]

Sounds like a plan!

To better describe, with the old Muncie tranny we would start by putting it in first and just drop our foot off the pedal so the clutch hammered the tranny at full throttle and 5000rpm. The shift to second would be with a quick slap of the clutch and jam the shifter to 2nd, while your right foot remained on the floor...and same for 2-3 shift. We rarely made 4th, as we were already well over 100mph. The muncie did not like this treatment, but it would take it (until you missed your clutch slap). The Triumph box will not take abuse like that. But, so long as you use your clutch as it is supposed to be, and do not drop the clutch at full throttle for starting, it will last the life of the car.

What seems to kill the British boxes are many missed shifts. Every time you grind gears, metal is left in the box. It floats around until it settles into the sludge in the bottom of the case and becomes locked there. If anything churns up that sludge, though, all the missed shift metal makes it's way into the weakest bearing... the layshaft needles, and starts to break them down. I know this sounds crazy...but on my last TR2, I changed the tranny fluid at every engine oil change. When I sold the car at 8 years, the tranny felt like new.

It is important to either change your tranny fluid regularly, or not at all. The absolute worst thing you can do is go for 50k miles without a fluid change, and then change it. That releases all the filings that were being held in the sludge in the bottom of the case...and will destroy the layshaft needles in short order. Most owners never change standard fluid, and that's fine. Most buyers want to change ALL the fluids when they get their new baby home. Unfortunately, with a tranny, that is the worst thing you can do. If the PO does not tell you he regularly changed the tranny oil... then do not change it.

Got it... so I'll avoid smokey burnouts but otherwise, I don't think I'm particularly hard on the transmission. I'm usually off the gas and on the clutch for a gear change, then back on the gas. Just wasn't sure if I had to be carefully rev matching/double-clutching all the time!

I have no idea when (or if) the gear oil has been changed in this transmission, or even if it's full. Something I should definitely check. I know the previous owner mentioned a bad leak at the tailshaft oil seal that he had fixed, so I suspect the oil was changed sometime between 2023 and now (if not simply topped up).

Interesting,
So how would a magnetic drain plug fit into the equation? Do the brass bits do any damage, or just the steel ones?

FWIW I have an OD trans, so am religious about oil changes.

Do magnetic drain plugs exist for these cars? (Yes, I know I will google it). This seems like a common addition to my Toyota gearboxes but I have not seen them universally applied or recommended on the British cars. This would certainly help somewhat with the shrapnel.

 

[TR Tom]

Drain plugs for all of the Triumph running gear use common pipe threads…so in addition to being able to easily change to magnetic versions, there are other much more user friendly head configurations than those annoying agricultural square headed plugs.

In my opinion, a magnetic drain plug belongs in ANY car’s engine, transmission or differential. I use them in everything I have ever owned and can’t think of a reason not to. My question to CJD is how their use fits in to his theory of changing oil often, or not at all, which I have never heard before.

 

[R2daICKY]

Drain plugs for all of the Triumph running gear use common pipe threads…so in addition to being able to easily change to magnetic versions, there are other much more user friendly head configurations than those annoying agricultural square headed plugs.

In my opinion, a magnetic drain plug belongs in ANY car’s engine, transmission or differential. I use them in everything I have ever owned and can’t think of a reason not to. My question to CJD is how their use fits in to his theory of changing oil often, or not at all, which I have never heard before.

Got it, I ask because I have encountered at least one instance where the slight protrusion of the magnet would interfere with the gears inside the case, so they can’t be run. Specifically, putting larger, lower gears (3:1 or 4:1) in a Land Cruiser split transfer case, where the box itself has to be machined to about the structural limits to fit the gears (and aligns with the plug position).

Wasn’t sure if this was the case here as well. They are very small boxes!

 

[CJD]

A magnetic plug couldn't hurt. I don't think it is a cure, though. My first car was an ex-US Border Patrol 1970 IH Scout, with a 3 speed manual, dual range 4 wd and 130k miles. As a super-excited 16 year old, I changed all the fluids right off. 1 week later I was driving to school and the counter-shaft broke. That's when the old mechanic who helped me fix it told me to NEVER change the oil in an old gearbox. It did have a magnetic drain plug...but if you do not change the oil regularly, the plug loads up and gets overwhelmed. It happened to me again with my first TR3A. I changed the oil in the tranny, after being told many times (even by some on this forum) that it was "poppycock", and that changing the fluid wouldn't hurt anything. 1 week later, the box started making the terminal "hiss". When I rebuilt it, the layshaft needles were wearing through the lay shaft.

Note: I still have the stub of the broken Scout countershaft in my toolbox. It comes in handy as a punch!