Magneto manufacturers used to discourage motorcyclists from tinkering with their products. They didn't supply any maintenance instructions other than simple points gap setting and slip-ring cleaning. And malfuctions usually invited the ominous 'return to makers' statement we all know so well.

British Thomson Houston- BTH for short - left nothing to chance when producing magnetos. Every component, not just random samples, used to undergo a functional test. Then each complete magneto underwent an exhaustive programme involving slow and high speed running for twelve hours. Every armature had to shunt a spark across a 5.5mm test gap when running at 85rpm on full advance and 200rpm when retarded. If overall performance was up to expectations the mag was tripped, cleaned and run for a futher four hours. Then, and only then, was it given the thumbs up for despatch.

I guess things were pretty much the same in other magneto manufacturers test shops. Hardly surprising therefore that they didn't want inexperienced hands working on their products at a later date.