Tyres. Do old tires with plenty of thread left increase risks?

I recall a friend bought a Sunbeam once. He became a regular visitor to Pollards used spares at Speed on Garage, Dinnington to buy distributors as his kept snapping the shaft. Must have been something fundamental wrong but he never founder out iwhat.

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Mine, and a mate's S7, used to shear the rivet holding the drive dog to the distributor's shaft. The trick was to fit a new rivet without hitting the shaft. He had a broken cam chain almost a factory job to replace it. I managed to devise a way to do it using some 1.5mm welding rod and a split link - then came the fun trying to time it on a sloping garden, as there was only a slight angle between OK and 360 deg. out. The engine was very 'soft' - it would fire briefly at 360 deg. out!

When Lucas do this, they use a roll pin. Much stronger than a rivet which by nature is made from soft steel. And never known one fail.

I had a Bedford van with a Delco distributor driven by an offset tang on the shaft. That broke. Twice.