741 & Sport Scout with British 4-Speed Gearbox - some basic thoughts.

The photos below show a 741 chassis with a Triumph 4-speed. Note that the Triumph gearbox is from a pre-unit swingarm bike (approximately 1954-62). The rigid frame Triumphs used different upper mounting points for the gearbox and are a little harder (though by no means impossible) to adapt for Scout use.

Many other British bikes of the 1950s used very similar gearboxes which could equally easily be adapted to Indian use. Some of them were: BSA swingarm pre-unit singles and twins from, roughly, 1955-62. Norton singles and twins from the 1950s up to and including the Commando models of the 1960s and 70s. Various AJS and Matchless singles and twins (I think both rigid and swingarm frame models have the same Burman gearbox). Ariel singles, twins and fours have slightly different Burmans with the upper mounting hole elongated for adjustment [see photo 6 below]. Someone mentioned Royal Enfield, but they don't really match up to the Scout as well as any of the above. For more info, you can buy a booklet on the subject from Victory Library in New York.

Most of these gearboxes are probably easiest mounted rigidly in the Scout rear engine plates (like the Scout trans) and the chain/belt tensioned by a slipper/roller (like Indian used) or similar, but you could rig up something like the Brit system where the box pivots around one of the mounting holes and has a tensioning device at the other [see photo 7 below].

Using the clutch that goes with the gearbox is probably much easier than trying to adapt the Indian clutch, and you might be able to use the primary cover that goes with the gearbox too (probably needing some sort of adapter at the front end where it meets the L/H crankcase half).

The Indian and Brit chains don't match, so unless you want to make up a belt drive, you'll need to grind the teeth off the Indian engine sprocket and weld on a Brit size sprocket blank (or the other way around with the Brit clutch drum, but a #35 three row blank in that size is more expensive and maybe not as easy to find). A single row chain should be enough for even a healthy Scout engine (but go with what the clutch drum has). Lots of 500/650 Brit OHV bikes ran single row primary chains, and the major difference on the Scout is the lack of engine shock absorber (compensating sprocket) which will probably shorten chain life - but it's not hard to change a chain once in a while so I don't think this is a real problem. Besides, if you end up with a Triumph trans/clutch, the clutch centres on the alternator models (from 1950 or so) had a rubber-based shock absorber built in.

If you end up running a semi-open chainguard instead of a closed primary cover, you may want to consider adding a ScottOiler for chain lubrication. The clutch bearing rollers also need lubrication, but will survive ok with regular greasing if you rig up a simple sealing system (bits of felt and scraps of metal goes a long way if you don't have the tools to make something more elaborate). If you go with a belt drive - or actually no matter what - you will need a real oil seal where the drive shaft comes out of the engine. Both Rick Abbott and Indian Parts Europe have bearing nuts with oil seals.

If you can get a rear wheel from the same bike that donated the gearbox, clutch and maybe primary cover, it may be a good idea to get that as well. It will save you turning over the Indian rear wheel and possibly messing around with chain-to-centerline distance for the rear sprocket. On the other hand you will have to make up spacers and axles so there doesn't appear to be any clear advantage with either solution. It shouldn't be too hard to adapt the Indian wheel if you choose that; mainly a question of flipping it over and designing a new linkage and anchor for the brake. In most cases, no matter which wheel you use, you will need to come up with a new brake linkage.

Click to enlarge photos.

1. As can be seen in this shot, the mounting points and general outline of the stock Scout transmission and the Triumph gearbox are much the same. Red lines indicate the mounting points.

NOTE: The Triumph gearbox is in backwards (the visible side should be on the other side of the frame) - this photo is just to show the mounting points. 

2. Here it is the right way around. Note that mounting points almost line up - you could probably weld and drill the stock engine plates to make them fit. I didn't want to take out the lower frame stud (which would have the frame collapsing) but the blue lines indicate what goes where.

The clutch fits on the mainshaft coming out through the sprocket for the rear chain. Just like on an Indian, but on the other end of the mainshaft. Clutch is operated by a pushrod through the centre of the mainshaft. The pushrod is being pushed by a cable operated mechanism on the R/H side of the gearbox.

3. R/H side of the bike with the gearbox loosely fitted (lower mounting point still not lined up, see caption above). Kickstarter, clutch cable and shifter pedal mounts indicated in photo. Depending on the rear wheel used you might find the stock brake crossover tube (just visible under the gearbox case towards the rear) useful; the brake pedal would probably be easiest to fit on the L/H side - unless you want to make some sort of crossover for the shifter - or convert to handshift? In that case the stock clutch pedal could easily be adapted to cable operation.
4. I don't think the Triumph gearbox looks out of place? This Scout has a BSA Gold Star front wheel and a Triumph rear wheel too, so maybe that makes the gearbox feel more at home! It is ironic that the Triumph rear wheel would have to be turned around to its original position if a Brit trans were to be used on this bike...
5. The other side. I need to sell this bike when it's finished (this summer?). I have a freshly built 45" stroker engine for it and everything else is new or rebuilt. The first person giving me a deposit on the total price of US$16000 (at the current exchange rates) gets to decide if the bike is fitted with a regular Scout trans with foot clutch and hand shift, or a Triumph 4-speed with the opposite setup! 
6. Ariel Burman gearbox showing elongated upper mounting hole. For tensioning the primary chain, the gearbox pivots around its lower mounting hole, drawn rearwards by a tensioning screw. The upper mounting bolt fits in round holes in the engine plates and the oval hole in the gearbox case, and is tightened after adjusting the chain. This setup was only used by Ariel. Other bikes using Burman gearboxes (AJS, Matchless, Panther and lots of lesser known ones) used variations of the system in the photo below.
7. The most common Brit primary chain tensioning setup, where the gearbox pivots around the lower mounting hole and the upper mounting bolt slides in elongated holes in the engine plates. This is an approximately 1955 pre-unit swingarm BSA single (but with an Ariel engine in it, though that has nothing to do with the transmission).

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April 18 2004