Every time it went up & down the bore it would have been expanding and contracting as it went in & out of the worn area of the bore.
The rebore & new pistons will fix it & it should be good for another 100k.
There's multiple reasons it could have been, reading the various posts by yourself and Darren, the most likely cause was just wear and tear, I suspect the ring groove would have been excessively worn & the ring was chattering in the piston.
It's great to see youngsters taking an interest in classic cars. Hopefully you'll find a suitable project car.
The clubs are here to help, we all share an interest in keeping the cars running & there's a wealth of knowledge you can tap into.
It's good to find the fault & know the crank is fine. A rebore and matching pistons will get the bottom end sorted.
You'll be back behind the wheel in no time. 😊
I personally wouldn't bother with them, as DVD3500 says, they can be a **** to remove & 4 wheel nuts give much better clamping force than a couple of little screws.
The wiring colours were done to BS-AU7, there was a later revision BS-AU7a which was introduced just after Triumphs stopped being produced. As for wire gauges, you'll have to calculate current draw/voltage drops for your circuits.
BritishWiringColours-BS-AU7.pdf