The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Lucas, Ignition, Charging, Electrical => Topic started by: Roger (Doomtrainbarx) on 29.08. 2024 19:14
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Anybody here familiar with the 5 brass roller auto advance mechanism ?
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G'day Roger.
I can't picture a "5 brass roller auto advance mechanism". A google search gave nothing.
What is it fitted to? Can you post a pic?
Cheers
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Will try and post a pic tomorrow - thanks for your reply.
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That would be a BTH unit I guess.
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https://www.ebay.fr/itm/186451671093
Scroll down on the link and there should be a tired BTH one, now sold, with pix from both sides.
Clock-type spring on 'inside' face, rollers in the sandwich between driving and driven members.
Same principles of basic operation as the Lucas, and interchangeable. I've mostly seen them on 5T and 6T Triumphs. New fibre gears are available (for Triumphs anyway.)
Wouldn't like to argue the merits of these ones versus Lucas, but the BTH has a very solid feel. The springs are not easy to find though, I seem to recall when needing one a while back.
See also page 7 of the BTH booklet here for a fuzzy drawing: https://library.brightsparkmagnetos.com/library/BTH/BTH%20KC%20BKH%20KD%20KH%20Instruction%20Book%201377.pdf
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Trev and Bill are quite right - now I've cleaned the rust off a bit I can see it's stamped BTH.
The problem is that when it's in my hands the the spring returns the weights to retard as normal ,but when I install it and tighten the centre bolt it causes the unit to go tight and bind up.
It looks like it's rivetted together so maybe a throw away unit ?
Could I replace it with the later more common AAU ?
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No way throwaway Roger! Riveted, yup - so are Lucas - but correct rivets can be had.
If you don't want the BTH item, I'm sure you could get some useful £$€ for it, even if it's tired, as they don't grow on trees. Triumph owners particularly look for them, as a lot of the '50s pre-units had BTH KC2 magnetos. I've seen some wear on them, sure - flats on the brass rollers and so on, springs with a busted hooky end as I mentioned (and we know how trying to form new hooky ends on clock springs goes - badly, almost always!) but there's a demand for them and there's a steady market for new fibre pinions for them.
On the binding front, depending on exactly 'how tight' etc, could be it's not actually abnormal - CheeserBeezer's Priory Mags website has the best summary of some of the issues, under'A Bit of ATD Theory'. (Of course, if it jams solid, it's definitely not normal though!)
You could replace it with the Lucas version, yes, as the tapers and threads are the same. I'm pretty sure pinion alignment would remain good but I don't know what the degrees of movement are on BTHs, or the choices that were available. But I do know that the Triumphs had 15-18° on their Lucas-equipped versions, which is more than Beesas' 11-13°. That's at the mag, so double it for the crank.
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Thanks for your help Bill - I'll have a look on Andrew's page.