Author Topic: Thomson and Bennett Magneto Drive gear wanted  (Read 673 times)

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Offline Scomoto

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Thomson and Bennett Magneto Drive gear wanted
« on: 06 Nov 2022 at 09:19 »
HI,

Has anyone got a suitable magneto drive gear to fit the Thomson & Bennett (looks very similar to the EIC) magneto

it appears the gear that sits in my timing chest (1920 W) is incorrect

Please can anyone help to get another machine on the road ?

Thanks

Offline cardan

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Re: Thomson and Bennett Magneto Drive gear wanted
« Reply #1 on: 06 Nov 2022 at 21:24 »
More or less, magnetos were built to a standard. In principle different magnetos of the same type should used the same gear.

The Thomson-Bennett flat twin magneto came in either "M" or "K" sizes - usually the smaller M would be 35mm armature height with the small M taper, with the drive gear held in place by a BA screw that threads into an internal thread inside the armature shaft. I think Thomson-Bennett called this the DDMD. The larger size was 45mm armature height with the large K taper, with the gear held on by a nut that screws onto the threaded end of the armature shaft. Called the DDD?

I'd guess the W20 used the smaller magneto with the smaller taper. The usual magneto would be an EIC, but Thomson-Bennett could have ended up on some bikes. Have you got the wrong magneto and the correct gear, or vice-versa?

Another possibility is that your "magneto gear" is just an extra idler gear, in which case the hub will be parallel rather than tapered.

Leon

Offline Bob M

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Re: Thomson and Bennett Magneto Drive gear wanted
« Reply #2 on: 07 Nov 2022 at 04:23 »
Just to clarify Cardans words a little further I am attaching a scan of the British Standard of the day pertaining to magneto sizes.
You can look at all the various measurements to ascertain where your Thompson-Bennett magneto fits and what alternatives you have.
It even tells you the size of the BA screw Cardan mentions.
Be warned about Woodruff key size numbers. Woodruff key dimensions remain the same as always but the system of numbering them has changed over the years so the Number 3 Woodruff key mentioned in the text will have a different number today even though the physical dimensions have not altered. This is part of the joys of setting proper Standards for people to work to. Over a hundred year period the technical reqiuirements of the explanatory paperwork involved has evolved dramatically but what is actually done by the tradesman concerned is just the same. God Bless Bureaucracy.
This scan was made of the relevant book pages as they are printed so a little twiddling, twirling and rotating is required to read them but the information contained is solid gold.
Best of luck'

 

Offline Scomoto

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Re: Thomson and Bennett Magneto Drive gear wanted
« Reply #3 on: 07 Nov 2022 at 08:12 »
Thanks to you both for your prompt replies.

feeling a little better about it as the mag does indeed have the gear retained by a BA screw!  So we are looking correct in that area. the gear definitely has a taper but also a key way, (no key way apart from a small circular recess on the armature). The taper is the correct angle but would not tighten. however I took the mag off last night to investigate further and note that the bearings seem pretty worn and that there is a good 2mm of inline movement, I'm wondering if the armature is just moving away too far to get the taper to bite. more investigation later today.

it is also possible that the taper is a little worn.

Mike

Offline cardan

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Re: Thomson and Bennett Magneto Drive gear wanted
« Reply #4 on: 07 Nov 2022 at 08:28 »
Sounds exactly right Mike. If you have a look at Bob's diagrams you'll see that the M taper has a round peg in the armature, rather than a Woodruff key. Of course the gear still needs a slot in its taper to engage over the pin. Not that the K taper at 13mm is 2mm bigger than the M taper, so you'll know if the gear has the larger taper. There is no "official" taper in between.

Leon