I think the toothed pulleys in the timing case are all sintered (produced by a technique known as Powder Metalurgy) and therefore have a lower mechanical strength than if the pulleys were cut from steel and then suitably hardened
If they are not a really good fit on the shafts and their associated keys, any rotational clearance will allow the pulleys to rock backwards and forwards and the keyways in the pulleys will widen due to the on and off effect of the load. This widening of the keyways will allow angular movement which will alter the timing of the valves.
Only a theory but maybe a possibility?
Keith
