I am in the process of converting a Stag non-OD into a Stag J type OD and have researched this so wanted to get it down for myself and in case it helps others. Corrections gladly received! Maybe a moderator would consider this for the Tech section? its easier to edit it to look good here!
To convert a Box to Overdrive a few things are needed.
An Overdrive, an OD mainshaft, the Adaptor plate, Top cover with switches and the longer Release bearing carrier. You may want to change the speedo gearing as the TR6 is different.If the OD was not from a Triumph, e.g. Volvo you may need an Output flange. You will need to work out the rear gearbox mounting as well. Finally the exhaust downpipes are different for the J type. I think thats all!
There are a couple of ways to do this.
1. Use the TR6 OD box to rebuild a 4 speed Stag box. Definitely the best option, especially the post late 73 TR6 Box as they have at least some common parts. As long as its a good Stag box, the parts you add from the TR6 to make it Overdrive are the same.
2. If you have a Stag gearbox, buy the parts.. Both types of Mainshafts and adaptor plates are available, you just need to get the Overdrive and top cover (or convert yours - see the next post..)
3. Use a TR6 OD box, you only need to swap out the input shaft and possibly the Constant gear (they are a matched pair) but neither of the Stag input shafts are available new. But remember you are using weaker components from the TR6, it may not be strong enough.(see comments below)
Getting the right TR6 Donor OD gearbox.
The first thing to look for in a donor TR6 OD Box is the "imperial" or "metric" mainshaft. This refers to the bearing between the input shaft and mainshaft the imperial mainshaft/input shaft has a pressed in bearing, metric has an open cage bearing..
ALL Stags are Metric so you must get a TR6 metric box to provide the Overdrive mainshaft as the stag input shafts you must use are Metric. No problem for J type they are all metric but for A type you need to get TR6 gearbox serial# CD20282-CD21768 which is where the metric mainshaft came in. Keep in mind that the gearsets in these boxes are different to the Stags.
The next consideration is the bushings.
Before late 73, TR6 boxes had Bronze bushings After that the gears and steel bushings are the same as the Late Stag boxes (after LD7889). I have not pinned down a TR6 Gearbox number for that change except Commission# CF12500 and CR5000 or "late 73". The earlier Stag boxes (before LD7889) have a Stag only 2nd gear on the mainshaft with non-tophat steel bush and thinner spacer. After that The Stag box has the same second gear as the post 73 TR6 box with a steel Top hat type bushing and thicker spacer with a grove for a ballbearing that sits in the mainshaft. This prevents the spacer spinning. The 2 types of steel 2nd gear Bushings are not interchangeable, i.e they need the correct 2nd gear, but with the correct bushing/gear either can be used, as both use the same mainshaft. The only problem may be when you try to use a tophat type bushing on an older mainshaft without a ball bearing in the mainshaft.
Drew confirmed that there is indeed a steel tophat and ball bearing in the mainshaft of a late Stag box.
The ball bearing is not shown in the Rimmer parts listing or the factory part listing. However the factory part listing DOES show the part number for the thicker spacer with groove for the ball bearing and the later top hat type bushing with a groove, whilst Rimmer does not.
On the left is the earlier type stag "non-tophat steel bearing" on the right the later Stag and post late73 TR steel tophat bearing. The gears are different in that the recess at the synchroniser end is deeper in the tophat type to account for the thicker spacer.
On the left the shaft at the top has the ballbearing for the tophat type spacer, on the right is the tophat type spacer showing the grove.
The final thing is the layshaft. The Stag has a stronger Layshaft that is thicker at the 1st gear end and uses double cage bearings there as well. It also has roller thrust bearings which are said to be stronger. This also means you cannot put Stag internals into a TR6 casing or visa versa, unless you want to get into messing with the rear layshaft hole. Even with the different layshafts TR6/Stag leygears will fit either.
Doing the conversion
The big difference between a Stag and TR6 box is the input shaft. With the correct input shaft any metric TR6 box can be pressed into service in a Stag. Having said that the Stag box from the start was built stronger, especially the Leyshaft and thrusts. All Stags had steel gear bushings (but there were 2 types for the second gear) whereas the TR6 boxes used bronze bushings until late 73. (The TR6 Bronze top hat bushing on second gear is famous for breaking) There are 2 types of Stag Input shaft, the earliest type (Below LD5818) has a different Helix angle on the gear, so it needs to be matched to the constant gear on the leyshaft. Either pair will work but keep in mind they should really be a matched pair as different wear patterns can cause noise.
Note:- TR6 input shafts changed the helix as well. Gearbox serial# CD20282-CD21768 (A type?) should have the earlier helix angle. CD21769 and up should be the later helix angle.
In the pictures above the left shaft is an early type stag input shaft, the right a late TR6. note the extra length of the Stag shaft and the slightly different gear Helix used in both boxes.
If you are just converting a TR6 OD box and are using it whole (option 3) then all that is needed is to drop the leyshaft, remove the input shaft and install a helix matching stag input shaft and use the longer Release bearing carrier.. And hope its strong enough
For the other options You need to dis-assemble the Stag gearbox, and rebuild it on the shorter OD Mainshaft. (Note:- Aside from the input shaft the post late 73 TR6 gearbox seems to have all the same gears as the later Stags)
Please correct any glaring errors/omissions
To convert a Box to Overdrive a few things are needed.
An Overdrive, an OD mainshaft, the Adaptor plate, Top cover with switches and the longer Release bearing carrier. You may want to change the speedo gearing as the TR6 is different.If the OD was not from a Triumph, e.g. Volvo you may need an Output flange. You will need to work out the rear gearbox mounting as well. Finally the exhaust downpipes are different for the J type. I think thats all!
There are a couple of ways to do this.
1. Use the TR6 OD box to rebuild a 4 speed Stag box. Definitely the best option, especially the post late 73 TR6 Box as they have at least some common parts. As long as its a good Stag box, the parts you add from the TR6 to make it Overdrive are the same.
2. If you have a Stag gearbox, buy the parts.. Both types of Mainshafts and adaptor plates are available, you just need to get the Overdrive and top cover (or convert yours - see the next post..)
3. Use a TR6 OD box, you only need to swap out the input shaft and possibly the Constant gear (they are a matched pair) but neither of the Stag input shafts are available new. But remember you are using weaker components from the TR6, it may not be strong enough.(see comments below)
Getting the right TR6 Donor OD gearbox.
The first thing to look for in a donor TR6 OD Box is the "imperial" or "metric" mainshaft. This refers to the bearing between the input shaft and mainshaft the imperial mainshaft/input shaft has a pressed in bearing, metric has an open cage bearing..
ALL Stags are Metric so you must get a TR6 metric box to provide the Overdrive mainshaft as the stag input shafts you must use are Metric. No problem for J type they are all metric but for A type you need to get TR6 gearbox serial# CD20282-CD21768 which is where the metric mainshaft came in. Keep in mind that the gearsets in these boxes are different to the Stags.
The next consideration is the bushings.
Before late 73, TR6 boxes had Bronze bushings After that the gears and steel bushings are the same as the Late Stag boxes (after LD7889). I have not pinned down a TR6 Gearbox number for that change except Commission# CF12500 and CR5000 or "late 73". The earlier Stag boxes (before LD7889) have a Stag only 2nd gear on the mainshaft with non-tophat steel bush and thinner spacer. After that The Stag box has the same second gear as the post 73 TR6 box with a steel Top hat type bushing and thicker spacer with a grove for a ballbearing that sits in the mainshaft. This prevents the spacer spinning. The 2 types of steel 2nd gear Bushings are not interchangeable, i.e they need the correct 2nd gear, but with the correct bushing/gear either can be used, as both use the same mainshaft. The only problem may be when you try to use a tophat type bushing on an older mainshaft without a ball bearing in the mainshaft.
Drew confirmed that there is indeed a steel tophat and ball bearing in the mainshaft of a late Stag box.
The ball bearing is not shown in the Rimmer parts listing or the factory part listing. However the factory part listing DOES show the part number for the thicker spacer with groove for the ball bearing and the later top hat type bushing with a groove, whilst Rimmer does not.
On the left is the earlier type stag "non-tophat steel bearing" on the right the later Stag and post late73 TR steel tophat bearing. The gears are different in that the recess at the synchroniser end is deeper in the tophat type to account for the thicker spacer.
On the left the shaft at the top has the ballbearing for the tophat type spacer, on the right is the tophat type spacer showing the grove.
The final thing is the layshaft. The Stag has a stronger Layshaft that is thicker at the 1st gear end and uses double cage bearings there as well. It also has roller thrust bearings which are said to be stronger. This also means you cannot put Stag internals into a TR6 casing or visa versa, unless you want to get into messing with the rear layshaft hole. Even with the different layshafts TR6/Stag leygears will fit either.
Doing the conversion
The big difference between a Stag and TR6 box is the input shaft. With the correct input shaft any metric TR6 box can be pressed into service in a Stag. Having said that the Stag box from the start was built stronger, especially the Leyshaft and thrusts. All Stags had steel gear bushings (but there were 2 types for the second gear) whereas the TR6 boxes used bronze bushings until late 73. (The TR6 Bronze top hat bushing on second gear is famous for breaking) There are 2 types of Stag Input shaft, the earliest type (Below LD5818) has a different Helix angle on the gear, so it needs to be matched to the constant gear on the leyshaft. Either pair will work but keep in mind they should really be a matched pair as different wear patterns can cause noise.
Note:- TR6 input shafts changed the helix as well. Gearbox serial# CD20282-CD21768 (A type?) should have the earlier helix angle. CD21769 and up should be the later helix angle.
In the pictures above the left shaft is an early type stag input shaft, the right a late TR6. note the extra length of the Stag shaft and the slightly different gear Helix used in both boxes.
If you are just converting a TR6 OD box and are using it whole (option 3) then all that is needed is to drop the leyshaft, remove the input shaft and install a helix matching stag input shaft and use the longer Release bearing carrier.. And hope its strong enough
For the other options You need to dis-assemble the Stag gearbox, and rebuild it on the shorter OD Mainshaft. (Note:- Aside from the input shaft the post late 73 TR6 gearbox seems to have all the same gears as the later Stags)
Please correct any glaring errors/omissions
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