ALTERNATIVE ELECTRICS

I have acquired a dismantled 1955 Triumph Tiger 100 engine but it is obvious that it is definitely not standard. It has apparently been fitted with Thunderbird primary drive casings including an alternator. The hole in the timing side of the crankcase that would have accommodated the dynamo has a blanking plate and a magneto has been fitted in the normal place.

This is an arrangement I have not come across before, yet I was assured the engine was functioning fully prior to being dismantled. I can find no reference to this set-up and wonder how unusual it may be. Obviously the wiring diagrams I have access to show either dynamo or alternator systems, but not alternator with magneto!

So my question is, how did this work? How would it have been wired and what is missing?

I hope to revive the engine which seems basically sound, but currently have three choices. The first to acquire a dynamo and wire it as original. Second; to utilise the alternator and incorporate coil ignition. Thirdly; to work out how this unusual combination of magneto and alternator worked together.

The latter would presumably be least costly as the bits are already there, however originality would be my preference – but at what cost? Something of a dilemma. The benefit of your combined expertise would be really appreciated. Malcolm Miller, member 3363

It’s not as uncommon as you might think, and I’ve owned several machines down the years modified in similar ways. Worry not about running the bike as it is – Norton, AMC and indeed Triumph built lots of bikes using a magneto for the sparks and an alternator to drive the lights. The advantage of converting a dynamo engine back then when bikes were cheap and we all had no money was that if the charging side failed the magneto would still keep the engine going; a flat battery was not a push’n’hope job. Another approach – most famously by BSA on their police bikes – was to fit both a dynamo and an alternator to power extra kit, as indeed fitted to police machines. Fitting a dynamo would be perfectly simple, but reverting to the original primary drive would probably involve replacing the chaincases and the crank, as the dynamo engines used a crankshaft-mounted shock absorber, whereas the alternator units built that into the clutch – so the crankshaft is different. Check which crank you have! Frank W

Another nail struck firmly on the head. I suspect we are all better at buying bikes than selling: I certainly am.

I have both bought and sold bikes I really shouldn’t have but I am happy with my current motley collection (1972 Tiger 650, 1922 AJS Model B, 1961 G80CS and 1975 Mk3 Commando).

The Triumph is probably the only one I’d have trouble giving up because it was my introduction to the wonderful world of old motorcycles. Besides, my daughter would not forgive me for parting with all those shared memories. The Norton has some claim because of the cost, time and trouble rebuilding and the potential for memorable rides to come (going forward as they

say) and the flat-tanker is in a class of its own, a reminder of where we came from.

The bike I should have kept? The 1948 AJS 16M that helped fund the much shinier and sportier G80CS, no doubt a better bike and great deal of fun to ride but it lacks the charm and calm of the old chuffer. Bet I’ll not find another for ten Franks!