The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Lucas, Ignition, Charging, Electrical => Topic started by: Billybream on 27.07. 2022 19:24
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My bike packed up on a recent run, had to get recovery to get me home.
Starting checking through fuel, ignition and discovered the timing had slipped.
First thought was the pinion taper had slipped, so invested in a buzzer, and already had the head off, so timing was via dial indicator.
Set timing several times, but everytime I tightened the pinion nut up, then rechecked the timing it had moved by quire a bit. Really stumped now, could the points plate carrier plate bolt be stripped?
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Timing moving as you tighten the nut is par for course. You have to estimate (from the previous attempt) how much it moves and then apply that as a correction factor before tightening. Lapping the taper and tapping it on before tightening helps.
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Billy, the points carrier is also on a taper, an internal one into the armature shaft. There is a keyway to locate the two which is relatively shallow. Some need dressing for a positive fit. A stripped retainer for the central bolt is a simple fix, but the armature has to be split for access.
Brass points plate needs a brass screw. For a steel carrier it's a steel one. Don't mix them, the screws are different lengths.
Usual trick, endorsed by previous posts to locate the timing gear or ATD is to GENTLT tap it onto the dry taper, then tighten while still applying hand pressure to the gear. Hitting magnetos with big hammers is a rich man's pastime. Drive end steel taper shaft is a crude fitting into the brass armature, very fine splines. Wear starts with a mysterious wobble on this shaft. A prelude to what happened to Julian's armature pictured some time ago......taper shaft and armature came apart.
Swarfy.
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Thanks Gents for your advice, after resetting timing a dozen times I got abit ***** off, so will try again tomorrow after a night's sleep
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I have experienced the armature drive shaft shearing; forward of the taper.
Richard
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Yep mine did exactly the same thing. Seemed it found a sweet spot on the taper that was considerably too advanced. I managed to lock it into place by tapping the taper on firmly before tightening and making sure the nut and washer were a spin on fit and well lubed. It has moved since so a success.
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There's an old trick from back in the day when these bikes took people to work every day, and that is to put a tiny smidge of grinding paste on the taper and push the ATD home. It provides enough bite for the ATD to stay put put while you tighten the nut.
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Tried again today to get the armature pinion to grip on the taper, again no joy, decided to remove the Magneto.
Discovered the points plate tapered bolt appeared to have stripped, and was unable to remove the bolt or points plate.
Quick phone call to Andrew @ Prory Magneto,s for advice, he,s suspects the armature to have snapped, so Magneto sent to him for operation.
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Quick phone call to Andrew @ Prory Magneto,s for advice, he,s suspects the armature to have snapped, so Magneto sent to him for operation.
Dunno if that's wise - been trying to order from him for two weeks. Three emails and two PM' s but no reply.
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It's been said before, but a good way to hold the armature still Slymo is to remove the earth brush from its holder, insert a hard wooden or soft metallic bung, even a too-long carbon brush (snug, so it won't drop into the mag, but not so snug you can't get it out again) and do the thing up till it nips the armature. Works a treat, for me anyway, when dealing with one of those 'won't sit still' ones.
However, that's easy compared to BillyB's possible ailment . . . they do break sometimes sadly as Orabanda said. At least you've put yourself in the right hands!
I have a theory that the vulnerable ones are the early 15mm drive end diameter ones, when carrying an ATD. Only observational, but the broken ones I've seen - there have been a few - have all been the small shaft diameter ones, with fixed camring and ATD. I consequently am reluctant to use an early one with an ATD myself.
The trouble first arose, or so legend has it, at the TT, on a Porcupine, which had shaft failure when in a leading or podium position . . . and Lucas reacted by upping the diameter at the drive end on all versions thereafter. That might be apocryphal, and there are no doubt other legends, because they already had Racing mags with one piece stainless steel drive ends in the K series, also in NR1s I think . . . but the point remains . . . early ones are more fragile.
Some repairers will machine out the earlier bare housings to take the later armature, bigger bearing & oil seal etc - but it's a bit of a labour of love (DAMHIK) unless you set yerself up to do it on a regular basis.
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I use this earth brush holder/wooden peg to hold the armature.
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Again thanks guys for the earth brush peg idea to act as a lock, got a spare so will adapt something similar
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Get some Brasso and lap the taper to pinion with it.
Then clean it off
pop the pinion in the corect position and give ot a TAP and I mean Taap not whack with a dead blow hammer
And by dead blow we are talking either leather faced or solid lead . not a wooden or rubber mallet
This will lock the tapers together
This is one place where an electric nut driver running full speed works really well
A bit of lube under the nut also helps.
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Shows how fragile the armtures can be.
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The stainless one piece drive end of a K2FR.
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So why did they make them that way? Purely to cut costs or was there another reason? Certainly looks like a failure waiting to happen. Not even silver soldered.
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Shows how fragile the armtures can be.
Exactly! I've got a box of them too Julian.
Why did they make them like that RD . . .?? Well, I dunno! Everyone did similar, although I've only seen broken ones from Lucas, and K2Fs mostly.
Hard to compare the exact construction between makes without deliberately pulling good ones to bits, which isn't a good plan! The stainless R versions bucked the trend - very tasty. They made those in 15mm too, early on, and they are very solid. Last one of those I saw was on an early KVFR, late '40s iirc..
That said, BTH never upped their shaft diameter, 15mm on all their bike mags with rotating coils, and I've never seen a snapped one. Nor a Bosch, also at 15, with steel-in-brass. If BTH flange-mounts were an easier fit on a wider range of machines, then I think we'd see more of them out there. The KC2 as fitted to many 5 and 6T Trihards is a darn good thing, as is the KC1.
A while ago I was hoping to have a batch of stainless armatures CNC machined up, but only got as far as 2 prototype MO1 magdynamo ones. Designed them so that the through screws (capheads, counterbored) had their heads at the accessible end, not under the slipring, so in theory dead easy to get to bits. But like many things, a project that fell by the wayside. Design needed some tweaking to get a bit more thread depth at the 'wrong' end (which required some small tweaks to the coil design in its turn), and the usual things, life etc, got in the way. Here'sa pic of what could have been!
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In my case it wasn't that it wouldn't stay still while I was setting it up but that it would shift to a sweet spot on the final tighten. I definitely think that the lapping in with brasso would be the answer. Although it has been perfect since 2018 and as the Glaswegian engineer next door used to say "If it works leave it alone!." I did clean the points for the first time since then the other day though and blimey what a difference. Much smoother at speed, better idle and one kick starting again.
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Well it was a snapped armature shaft, quickly sorted by Andrew at Priory Magnetos, now on its way back to me in in less than a week, he,s hoping to get back to enjoying some happy runs out again. Some good advice on maintaining backlash on pinion.