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By Adrian
#88304
Really pushing the boundaries of irrelevance with this one, but hey... Some equally deranged soul might find this of interest. The usual apologies to those who have seen this posted elsewhere.

Long story short, I wanted to fit a magneto with a manual advance and retard to a set of Indian Bullet crankcases for a project bike's engine. The Lucas SR1 on late 50’s Bullets can simply bolt straight onto the later Indian pre-EFI Bullet crankcases, but unless you want to run fixed advance they need the fairly hard to come by auto advance unit as well, not what I’m after.

Pre-56 Redditch Bullets were either fitted with magndynos for road bikes or Lucas N1 magnetos for the competition versions, both platform mounted, ie fitted on a specially machined raised section of the crankcase casting over the oil tank, AFAIK secured in place by pegs screwed into base of the magneto and a steel strap holding it down, the last of these being the Fury models, 1959-61. Early Indian Bullets still had magdynos but soon followed the post ‘59 Redditch models, for which the SR1 had been replaced by the so-called distributor allowing for cheaper coil ignition. Fast forward to my set of 2001 Electric start Bullet crankcases, and with any need for one long gone, the magneto platform is no longer part of the casting. It is possible to sort one out with a milling machine and an alloy spacer plate, the castings still have provision for the magneto strap, but I don’t have the kit or the skills. Shame, because a Lucas N1C or BTH KD1-TT competition mag would look rather nice in there.

So is a Lucas K1F or BTH KC1 flange-mounted magneto an option? Well, they’re both available with anti-clockwise rotation, and while many of them were fitted to Velocette singles with auto-advance units, some manual advance versions were also made. OK, this sounds promising, so I bought a worn-out BTH at autojumble (didn't see any K1Fs) for overhaul and modification. Now why aren’t more Bullet owners using them, or at least the more experimentally-minded among them? Because they don’t fit, that’s why. Here’s the problem.

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Both have fixing holes at the 10, 2 and 6 o’clock positions, but the three holes tapped ¼”W on the SR1 flange on the left are a lot closer together than the three 5/16” holes in the flange of the mag on the right, in this case a BTH KC1, but the Lucas K1 (and K2) mounting flanges have the same dimensions. Both magnetos have a locating spigot, the SR1 41mm/1⅝” diameter by about ⅜” deep (the ‘56-on Bullet crankcase machined to suit), the KC1 50mm diameter by only 3mm deep.

The are different ways around this, but with careful minor mods to both the crankcase and the KC1, you end up with a match, but you still have a KC1 that could just go back on whatever model of Velocette it came off, and a Bullet crankcase that could still be refitted with a “distributor” or SR1 if needed.

Hang on, didn’t RE make an adaptor plate for fitting Lucas K2Fs to the later twins? Yes they did, they’re still listed in Mr. H’s stash too, but they move the mag too far away from the crankcase for the Bullet’s drive pinion. So part 1 involves a shallow 50mm counterbore at the back of the crankcase by whatever machining options are available, a 50mm holesaw guided to preserve concentricity was what I used with a die grinder for a bit of trimming. If it only goes 3-4mm deep you still have plenty of metal left for refitting the SR1.

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You can now slot the mag into position, well ALMOST… The bottom of the KC1 flange just fouls top of the oil tank if you want the top of the mag level, a quick few strokes of a file to trim 2mm off the very bottom tip still leaves enough for the original mounting stud. Now you can position the magneto and use the original three mounting holes in the crankcase used by the SR1’s ¼” W screws to mark the back of the magneto for drilling and tapping (not too deep) for three ¼” threads, I went for BSF. What you end up with is this.

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Grab three ¼” screws and you can have this.

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Now if you have got everything properly centred you can fit a drive pinion and it will mesh happily with an idler pinion.

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Meanwhile, as an added bonus, from the drive side it seems to be held on with fairy dust, which will be good for messing with folks’ heads…

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The advance and retard cable is a bit close to the cylinder fins, though if you fit an end housing with the cable exit on the right (swapping from slack wire to taut wire advance in the process) it keeps it well out of the way.

This works for fitting the flange-mounted versions of the newer BTH electronic magnetos (FM1R) too.

A.

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