type of tacho drive

The rev-counter you require was as supplied by Smiths which was BG1508/05. This was a 1.1 drive with a 2.1 head. This rev-counter was also used on the all the later 750 Norton twins. Try to get a genuine Smith one, (this may be a tough ask) as the pattern ones can leak from the timing cover joint and the part where the cable screws into. I had one of these pattern ones, and I found a Smiths one, and all the oil leaks went away.

Hi Tomas, Look at my post on '650SS Clocks' from 1 week and 6 days ago - some good information generally on tacho/speedo. It seems that your Magnetic RSM 3003 00 takes 1:1 drive number BG 1508/05 (same number as one of the Mercury owner's posts) but I'm not completely certain. I suggest you talk to Stuart Walker at Classic-Speedometers, who is renovating some clocks for me (1967 650SS) - he may be able to help you ( info@classic-speedometers.com ). Possibly some of the guys who responded to my post will answer your slightly different question. I  wish you luck and hope you get sorted out. Cheers,

Just to be clear - my '67 650SS also has 1:1 drive number BG 1508/05, same as the Mercury which was mentioned.

The rev-counter BG1508/05 will fit all the post 1963 bike with the Smiths magnetic rev-counters. I also have a BG1508/05 for the Smiths Cronometric rev-counter on my 1963 Norton Atlas scrambler. If you cannot get the correct rev-counter drive you can always get an electric digital one for your bike.  That rev-counter drive was used on the Royal Enfield Crusader's that used rev-counters.

If the clock is 2.1 then the rev-counter needs to be a 1.1 drive.

I had to fit a  1to 1 REVERSING  Tacho Drive to my 650SS. The drive is taken from the camshaft nut,  which is 1/2 engine speed (2-1 reduction) and is anti-clock wise.