Hello,
Wise to be wary of alloy ET37 valve spring cap, had one fail 40 years ago. All that saved a disaster was the rocker pad being tight on the valve stem.
Had the head off before the engine was cold, no damage other than the ET37.
This was in the era of poor quality spares, it was perhaps made of the wrong material.
The Vincent valve gear is amazingly robust, have seen 9000 revs twice, not by intension but by accident.
Regards John.