A tale of two gearboxes......

[rdr952]

Hope someone can help me with this one....I recently converted my '75TR6 to OD by buying a gearbox/OD unit via eBay;the gearbox SUPPOSEDLY came out of a TR4,the OD unit is a J-type.I'd like to swap the mainshft from the OD gearbox with the mainshaft in the original TR6 gearbox and end up with a TR6 OD-compatible gearbox and a TR4(?)standard gearbox that I would then sell.The TR6'box has the serial number CC113375 stamped into it and the TR4(?)'box has CT927 as its serial number.Can this swap be done or are the two mainshafts too different to allow this to be done?

 

[TR3driver]

In a word, maybe. It depends on what magic has been done to fit a J-type to an early TR4 box. The housings are identical internally, but there were several changes to the gears along the way and the standard J-type mainshaft is not compatible with the TR4 gear set.

Most likely, the swap can be done, but you may have to move the entire gearset along with the mainshaft. Only way to know for sure, IMO, is to pull them apart and see.

There is some info at https://trf.zeni.net/TR6bluebook/index.php?page=85 on how to identify the internals.

 

[rdr952]

Thanks for the reply.I neglected to mention that the TR4(?)gearbox/OD unit mated up perfectly to TR6,same number of bolts,etc.It is marked STANPART but other than that CT927 marking,I can't be certain if it is even a Triumph gearbox!The vendor insists it's from a TR4

 

[TR3driver]

It should bolt up perfectly, they were designed to be interchangeable. It should look nearly identical to your TR6 gearbox, except the TR6 housing was reinforced in several places (most notably the mounting flange but also in the ribs along the sides and so on). And the CT927 marking would indicate that it is from an early TR4.

I'm actually running a TR6 gearbox housing in my TR3. There were some minor adaptations required since it is slightly larger in several dimensions than the original TR3 box, but effectively it bolted right up. However I had to use a TR4 gearset (early TR6 would have worked as well) to match the A-type mainshaft. And I'm using a TR3 top cover, to retain the dipstick.

Er, are you sure the TR4 OD is a J-type? TR4s only came originally with A-type; the switch to J-type didn't happen until during the TR6 run. A J-type will have a rectangular plate on the bottom, with ribs and 6 retaining screws; while the A-type has a big (like 3.5") round brass plug instead.

Here's a shot of "Frankenbox"

 

[rdr952]

Yes,I'm wondering ,too....but it is indeed a LaycockDeNormanville J-type OD....Turns out that the case of it has a crack which allows ALOT of oil loss,so I'm starting over with a Rimmer Bros.recon J-type unit...don't buy from cal-carnut,he's a slippery sort,to be kind.At any rate,I wanted to use the TR6 box with this overdrive because maybe then the odometer will read more accurately and the TR4 ratios are somewhat lower than the gear ratios of my original trans.

 

[Jerry]

This place has a kit that makes a J type fit to a Triumph.
https://www.quantumechanics.com/categories.php?op=newindex&catid=7

Jerry

 

[philstr6]

there is a jtype on ebay complete less transmission for 950.00,which will convert your trans to overdrive has everything
Philstr6
Phil

 

[TR3driver]

At any rate,I wanted to use the TR6 box with this overdrive because maybe then the odometer will read more accurately

The speedo drive couples to the output after the overdrive, so it will be the same regardless of which gearbox you use. Before 1973 (approximately), all TR 2-6 used a 5:2 ratio for the speedo drive, and different speedometer heads to account for different final drive ratios, tires, etc. But after that point, the speedos were all set to 1000 turns per mile (of the speedo cable), and the ratio was adjusted in the gearbox (or OD if present) to match.

So, once you start switching components around, about your only hope is to either figure out what speedo you have and use the gears that match; or else have the speedo head recalibrated to match the gears you used (possibly with the use of an external ratio adapter).

and the TR4 ratios are somewhat lower than the gear ratios of my original trans.

Which, BTW, was done on purpose to help the box live longer with the increased torque of the 2.5L engine. The lower ratio also puts more side force on the countershaft, which is a common failure/wear point. They also reduced the helix angle of the input gear pair, to reduce the end force on the countershaft; upgraded the bushings and so on.

The main reason I'm using a TR6 housing is because the weaker TR4 housing broke ... and that was behind an only moderately prepared 2.2L 4 banger. I'm not saying a TR4 box can't be used with a TR6 motor, only that the result will be weaker than original and the original was none too strong.

 

[HerronScott]

The speedo drive couples to the output after the overdrive, so it will be the same regardless of which gearbox you use. Before 1973 (approximately), all TR 2-6 used a 5:2 ratio for the speedo drive, and different speedometer heads to account for different final drive ratios, tires, etc. But after that point, the speedos were all set to 1000 turns per mile (of the speedo cable), and the ratio was adjusted in the gearbox (or OD if present) to match.

So, once you start switching components around, about your only hope is to either figure out what speedo you have and use the gears that match; or else have the speedo head recalibrated to match the gears you used (possibly with the use of an external ratio adapter).

Randall,

When did the TR6 change from the A-type overdrive to the J-type overdrive? Did it coincide with the change above as well?

I'm wondering if he has a Volvo J-type overdrive in the original given the mish-mash of parts which would certainly account for his speedometer issue (and moving it to the rebuilt transmission won't change that as you indicated).

Scott

 

[TR3driver]

I believe both changes happened at the same time, at the start of the 1973 model year. The pattern of the dash gauges changed at the same time as well; early small gauges have the needles pointing downward, while the later ones point upward. Tach & speedo changed as well, but I forget the cosmetic difference offhand.

Having gotten a Volvo unit is certainly a possibility; but it is also possible that whoever fitted the J-type to a TR4 changed the speedo drive gears inside the J-type to match his TR4 speedometer. That would make a 75 TR6 speedo/odo read around 11% high.

 

[MDCanaday]

I still like the Supra 5 speed swap,IMHO every one who doesnt have one now, will eventually wish they did.
I am a purist in most things Triumph, but having had more than my share of transmission failures has left me
in awe of the Japanese tranny's. I have done 2 Eaglegate installs and like them.
If you are in fact married to the J type OD, be sure you get the box completely re-built including cross shaft/bushings,
throw out fork stainless fork pin and all new bearings. I also like to add the adjustable release rod from the early slave
and brace up the bracket as was done on the tr3/4.This gives a great clutch feel that few tr6's have.....
MD