A selespeed I had a while back, had a new head under warranty at 50k. It was regularly serviced with all the stamps up to date (mostly main dealer).
I picked it up (with a dead selespeed system), with 80K on the clock.
Fixed the selespeed, new cambelt kit, a full service and ready for sale.... No sooner had I advertised it, taking it for a drive one afternoon, the almighty clatter appeared and lost all power. Big end failure!!
I then had to plough in whatever profit I would have made (and more) into the engine rebuild
Whether this is down to the belt failure at 50k or not stays a mystery, (would the damaged shells last 30k??), or down to poor maintenance (all the services upto date)??
The last few twin sparks I've worked on, I replaced the shells as a matter of coarse. I can say they were worn down to the copper coating and would not have lasted long.
What I conclude about this is when the belts go and the pistons smack the valves, the shells distort, or get a flat spot smacked onto them. This in time will wear the coating, as they are not round anymore, and in time cause debris pick up and bearing failure.
You can get a set of shells for under £50, so for me it's a no brainer