Author Topic: Dynamo driving sprocket nut  (Read 599 times)

0 Members and 1 Guest are viewing this topic.


  • Star
  • *
  • Posts: 2
    • View Profile
Dynamo driving sprocket nut
« on: 11 May, 2017, 11:06:13 »

I'm doing maintenance on my BSA Golden Flash Plunger from 1954,
When removing the dynamo sprocket (the large one) I found that the nut was in quite bad condition.

I bought a new one and asked for spare part number 2-449 then I got a 3/8 26 TPI nut.
But it was wrong, when checking the cutting I found out that I need a nut 3/8 28 TPI.

That was a little odd and my supplier in Sweden have never heard of that size.

Now I wounder if anyone know where I can get a 3/8 28 TPI nut.
Second question is if anyone of you Ladies and Gentlemen knows anything about this odd size nut on a BSA?

Best regards


Mike Farmer

  • Silver Star
  • ****
  • Posts: 486
    • View Profile
Re: Dynamo driving sprocket nut
« Reply #1 on: 11 May, 2017, 11:47:38 »

Hi I cant find the nut size that you need but if you contact   they sell the the taps and dies to the size that you require. so you may be able to make one. I will continue looking



  • Empire Star
  • *****
  • Posts: 1155
  • A10
    • View Profile
Re: Dynamo driving sprocket nut
« Reply #2 on: 11 May, 2017, 11:49:48 »
Sounds like someone has cut a different thread on the shaft. They can get damaged due to efforts to remove the sprocket. It is definately not the original thread.

The original was certainly 3/8 x 26 tpi cycle thread.

As for a 3/8 x 28 tpi thread then someone could have recut it usig a 1/8 x 28 tpi BSP pipe thread which has an outside diamter of 0.383 inch according to my Zeus charts.

Tracey tools list a 3/8 x 28 tpi UNS tap. I do not think that is a very common thread.

I dont think you will find a nut with that thread, just pipe fittings.

BSA used mainly Cycle threads in 20 or 26 tpi, BSW and some BSF. Oil pipe and petrol pipe threads were usually BSP.

The only slightly odd ones are the pressure relief valve to crankcase thread which is 7/8 inch x 16tpi UNS (Unified National Special) and the speedo head to cable which is 12mm x 1 mm pitch.


  • Star
  • *
  • Posts: 2
    • View Profile
Re: Dynamo driving sprocket nut
« Reply #3 on: 12 May, 2017, 22:11:34 »
Thanks for your answers.
When looking at the shaft I can't see that it has been modified.
Maybe someone at the factory was in bad mood and made it just for fun.

But I have been lucky a mechanic workshop had that tap so they made me a nut.