Speedo drive threads ?

.....to be British Standard Brass, I believe:

18mm = ~45/64 or .703"

19mm= .748"  

11/16" = .6875"

12/16" =3/4"  or .750"

I would doubt that there was anything made in metric dimensions on a 16H

I am going to guess that it is 3/4"x 26

Can anyone confirm this? 

Mike

I believe that the thread is 0.718"  x 26tpi - (23/32" )

There are lots of odd sizes on these instruments and drives, probably due to their French (pre-ISO metric) origins.

Cant measure it accurately without dismantling. I think Richard is probably right. Highly unlikely to be M18x1 because it's a bit more than 18mm.

A bit off topic, found that a 650SS/Atlas tacho cable is maybe a tad longer but can replace a WD16H speedo cable.

I agree with Richard, 23/32"; 0.718" x 26tpi Whitworth form.

Does anyone have any idea as to why Norton would have chosen such an oddball thread?

Was it common to something else at the time?

Mike

I've always thought the speedo gearbox was a proprietary item bought from Smiths. As the chronometric was initially a Jaeger design and metric, it's possible that Smiths made a small thread change to the nearest imperial measures they could figure out. Does not matter if you make 100,000 or more parts if a thread is a bit odd. Nowadays car manufacturers seems to purposely fit odd screws just to force owners to buy genuine spare parts.

Just guessing.

" I would doubt that there was anything made in metric dimensions on a 16H "

Hi Mike and all,

   Norton were always trying to save a few bob, especially in the Depression, and from 1931 all Norton singles and most of the twins were fitted with metric wheel bearings. The SOHC racing engines used a metric timing side main bearing and imperial drive side main bearings. And also all the bore and stroke measurements are calculated in metric.

   The thread pitch of metric 1mm is very close to 26 cei with just 0.016" per inch difference in length, which might be easily confused. Then there are a variety of different imperial threads on the rest of the bike, so check thoroughly, then check again !