Author Topic: thread size on dynamo  (Read 1648 times)

Offline mike667

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thread size on dynamo
« on: 17.05. 2010 23:27 »
Hey all
 so in my wisdom i thought i'd replace the dynamo chain setup w/ srm kit as long i am in there doing the magneto thing - it appears that my dynamo thread for the pulley bolt is pretty mucked up as the srm kit will not bolt on it - joy -

i am hoping if i run a thread chaser over it it will remedy the situation - anyone know the correct thread size it will be -  since it is some British  size i will need to purchase it  via ebay or some specialty shop and don't want to buy the entire range..

 agggrhh i wish i had left good enough alone



Online RichardL

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Re: thread size on dynamo
« Reply #1 on: 18.05. 2010 16:59 »


BSA, Triumph & Norton Parts Price List http://www.burtonbikebits.netFile Format: PDF/Adobe Acrobat
19-1473. NUT - 3/8" X 20 BSW - DYNAMO

I think most folks here would recommend a BSW thread file but, if you're like me, you want it done now (maybe yesterday). As an alternative, if you have an extra one of these nuts lying about, doing you no good, you could try sawing through one flat to the hole. Then, see if it can be started straight on the thread and turned onto the male thread while squeezing it together. In this manner, if your thread is not too bad, the leading edge of the saw cut may act like a die. The tricky part will, no doubt, be holding the armature steady while doing all this. Another approach like this one would be to cut the nut all the way in half, but then you must fuss with the halves.

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Offline beezalex

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Re: thread size on dynamo
« Reply #2 on: 18.05. 2010 17:00 »
Yea...that armature shaft is soft as putty, innit?  Even a brass hammer will kill it.  I'm gonna have to go look to make sure, but I'm fairly certain I've cleaned them up with a UNC 3/8-16 die.  The true thread is probably Whitworth, so it'll be a bit of a "bodge", but the threadforms are close enough and it'll be better than having it dinged up.

Edit:  I see now that I may be wrong....pretty sure I didn't buy a special die....I will check this evening.  Sorry if I've confused you so far.

Too many BSA's

Offline mike667

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Re: thread size on dynamo
« Reply #3 on: 18.05. 2010 17:25 »
thx for thoughts/input guys -  i have been told its a 3/8 BSF by magneto rebuilder here in states- of course size unobtainable on US EBAy - (UK bay has it though....) -


Offline MG

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Re: thread size on dynamo
« Reply #4 on: 18.05. 2010 18:43 »
yep, 3/8 BSF (20TPI) is the correct size
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Offline A10Boy

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Re: thread size on dynamo
« Reply #5 on: 11.06. 2010 13:44 »
For clarity; 3/8 Whit is 16 tpi, 3/8 BSF is 20tpi


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Offline muskrat

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Re: thread size on dynamo
« Reply #6 on: 11.06. 2010 15:05 »
Just did mine 2 hours ago, 3/8 BSF for sure.
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Offline trevinoz

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Re: thread size on dynamo
« Reply #7 on: 13.06. 2010 00:54 »
Most of these threads are butchered by owners trying to bash the sprocket off.
The easy way to remove the sprocket is to loosen the nut by a few threads and insert two large screwdrivers between the sprocket and the bearing retainer.
Lever upwards and the sprocket will release. The nut being in position prevents the sprocket from disappearing into the dark recesses of your shed.
When I get a thread beyond repair, I machine it flush with the taper, drill and tap the end and loctite a stepped stud in place then machine the stud to 3/8" and cut a BSF thread.
Some armatures have a keyway in them and the drill will break through but this is not a problem as no key is used.
I fill the slot with epoxy "metal" and clean up.
Works for me.

Offline wilko

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Re: thread size on dynamo
« Reply #8 on: 13.06. 2010 01:03 »
Good fix Trev! Handy having a lathe isn't it!

Online Brian

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Re: thread size on dynamo
« Reply #9 on: 14.06. 2010 07:52 »
I've done two now by cutting off the damaged thread, machine flat and simply drill and tap a 1/4" BSF thread into the end, the same as the magdyno ones are done. A decent washer under the bolt head and a touch of loctite. Both of these have been in the bikes for many thousand of miles and work fine. The sprocket doesnt seem to need much to hold onto the taper, anyone that has had to remove one will know what I mean, they can be right little buggers to get off sometimes.