Magdyno thread size

[Dean]

Hi all
Can anyone tell the thread size for the 2 screws that is part of the strap that holds the dynamo to the magneto. I have the strap and the bushes but am missing the bolts. Was thinking I could substitute for a normal bolt or if anyone has a couple of spares they can let go.
Thanks
Dean

 

[Mike40M]

Approximate measures. Taken from a Norton WD16H.
Thread is 1/4" 20 tpi. That will be 1/4BSW.
Thread length 17 mm. Length 24mm or 15/16.
Total length 28.2 mm including head.
Hex head with screwdriver slot, 7.9 mm. 1/8W, 5/16AF or 8 mm, all spanners fits.
Main difference between 1/4W and 1/4UNC is thread angle. 55 vs. 60 degrees.
A 1/4UNCx1" might work, if you don't find any Whitworth screws. Might be that an hex head is too large, an Allen screw might be better.
A washer or two might be needed. I prefer not to shorten screws used in aluminium. Not easy to grind them for a smooth entrance.

 

[oldmikew]

I am not happy about the interchangability of bsw and unf . I think the platform was lucas component not sure if they are all interchangable.. look at https://www.bt-h.biz/BT-H-Magnetos/. They have a wealth of information.. You could try
sending an Email to Les at Norvil. He used to work at Lucas and probablycould supply bolts. If you want original then try one of the specialist magneto restorers in the UK

 

[oldmikew]

Dave Lindsley - Vintage Magneto & Dynamo Repairs
www.davelindsley.co.uk

definitately would know and could probably supply

 

[Mike40M]

Update: Went out and tried various screws.
I was wrong, a 1/4 UNC screw don't fit in the magneto. In my defence, a 1/4 UNC nut fits on the dynamo strap 1/4W screw.
There must be a lot of nonworking magdynos littering your country. So a good chance to find secondhand screws. Otherwise, if you can't find a substitute in your local hardware store, a brit bike parts supplier might have it.
E.g https://www.rgmnorton.co.uk/buy/dynamo-strap_1922.htm
or https://bmhstore.bmh.com.au/index.php?main_page=product_info&cPath=38_11&products_id=30
or https://bmhstore.bmh.com.au/index.php?main_page=product_info&cPath=38_11&products_id=565
You can probably find more Googling

 

[Bernhard]

Unless someone has retapped the thread, or it has simply worn out, it will not be UNC or any metric thread, this was an English bike and all threads should either be B cycle thread, BSF,BSW ( BSP on petrol taps).

 

[Mike40M]

Bernhard, you forgot to mention the BA thread. Used in various electrical parts, and in some odd places.
Even metric threads was used (rarely). After around -68, UN threads became more common.
Norton used cycle threads (CEI, BSC) even in places where good engineering practice would be BSW. Vincent preferred BSW and BSF.

 

[Bernhard]

Of that, I plead guilty- in that I fitted one of those big horn button dip switches at the right angle to my thumb on the h/bar with a BA thread that I drilled and tapped. The BA threads elsewhere like the instruments I never stripped apart.
The mag/ dynamo and dynamo bikes are by a d large far too early for metric threads, but yes BSA was one of the first to use them.