How many teeth?

[Black Sheep]

I suspect my 1951 A7 Star Twin could do with a new gearbox sprocket, the last one having clocked up 30,000 miles. Should it be 19T or 18T? Various books have differing opinions.

[Swarfcut]

 Black Sheep.. My parts book, published in 1953, covers model years '49/53. Lists A7 and Star Twin as 18 teeth standard. 19 Teeth optional for A7 & ST, 19 Teeth standard for A10.
 
  Early boxes have a metal type of cylindrical seal and a scroll on the back of the sprocket. If your box has this, then you will need to change it for the later conventional oilseal as new sprockets are  smooth to run on the later seal. My '53 Plunger had the early metal  seal and scroll type sprocket when I checked it way back in 1971.  The box appeared never to have been off the motor, so I assume it was as originally fitted. Installing the later oilseal  is a straight swap.
 
 Swarfy.

[Greybeard]

I put a 20 on my bike, (A10) Cruising at 60mph is comfortable.

[muskrat]

G'day BS.
I got 4 and 1/2 left on the bottom.
OH, you mean sprocket! 19 on my 51 A7. Just right.
Cheers

[a10 gf]

For the A10 pl, did try 19, 20, 21. Can say 21 was not kept for long, was a clutchdestructor to get going. Maybe if there was 6 gears and\or 25 more hp's it could have worked. 20 was an ok compromise.

[Black Sheep]

Thanks for the info Swarfcut. I had to add the conventional oil seal the last time I changed the gearbox sprocket so all set up for that. So the choice is mine, 18 or 19 teeth. Swither, swither.

[a10 gf]

As an anecdote from when I had the Triumph 900, did some experiments, ended up with approx 60 mph in 1 st gear :O) That was nice (but the clutch had a hard life of course).