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Started by rebuilding bicycles from 10 onwards, started building 'bitsa' bikes from my early teen, moved onto repairing and rebuilding motorbikes and cars
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I honestly believe that this is an early sign that shows whether a young boy is mechanically minded/interested or not. Talking of 'Bitsa' bikes, the first scrambler we built was a Villiers 197cc engine, fitted in to a Triumph Tiger Cub frame, with Renthall handlebars and Preston Petty mudguards. Can't get much more bitsa than that.
Mine was an ex-WD 350cc rigid framed
hand change Ariel, converted to foot change (about 4" travel IIRC), BSA tele front forks, AMC plunger swinging arm, and I scrambled it for 3 seasons. So about the same.
Thinking about it, it was already foot change when I got it, but the original gear box linkage was modified, probably by Ariel, from (a pre war?) hand change model, hence the exceptionally long lever travel, necessitating removing the foot from the rest, not ideal for a scrambler.