A slipping clutch had nothing to do with hitting 8000 rpm.
The stock rev limiter would stop the engine long before that (~6950rpm)
The only way you saw 8000rpm on the tach (which actually was much higher then 8000 as the stock tach is very slow!) is by mechanically over spinning the engine by downshifting at an already high engine rpm.
There is no other way to do it.
You definitely could have bent a valve or damaged a piston.
Any possible bearing damage at that time would have become fatal not long after it happened.
The real question is why did you knowingly drive it damaged for a year?
