Good question. I will do more investigating, but I have always intended to upgrade the wiring, the bulbs and fit relays.
Thanks for all the responses.
Perry
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To surmount the same problem with Sweet pea, I prised the surround of the actual old light up and outwards using multigrips and removed the original Lucas glass lens from my pitted and rusted reflectors. I then purchased a couple of 7" headlights from one of those common cheap outlets and with a stanley knife separated the glass lenses from the reflector sections. (They are glued together)
Using sikaflex it was a pretty simple job to rejoin the original Lucas lens back with the more modern light itself, fit a readily available relay that affected both beams and then at last I could both see, and kept the original glass lens look.
Whether the old lenses actually focused the newer bulbs correctly didn't really bother me as it was planned for very limited use anyway.
I am about to do a similar conversion with the new toy.
Regards
Glen
Sorry for the delay but finally had a chance to look today while changing to new crystal reflectors.
Seems to be 120mm to back of bowl and my LED bulbs with original reflector is 110mm so just make it. Can't fit the existing dust covers over the new bulbs so would have to work out a way to do it, maybe a slit on the rubber to get it over the fins and a bit of silicon to reseal. For interest sake, the crystal reflectors i've just changed too with the led's in come to 100mm so a touch more room now.
I would do as Incisor suggests. Worked marvellously on my 2a. And mounting the relay there, it is right next to the battery - just run the power supply for the headlights direct to the battery, and run an earth lead for both relay and headlights direct to the battery earth terminal. The original headlight wiring is earthed to the radiator support - which may be very poorly connected electrically to the chassis.
John