[embee]

Could be OK, depends what it was designed for.

The thing with valve train is that it will work up to the design speed before loss of contact occurs, the limit is clearly defined. Valves break usually when it's been over-speed and there's been piston contact causing a slight bend in the underhead stem. This then flexes when it seats and eventually the repeated flexing causes fatigue and the head pops off.

The problem with bottom end is that the limiting factor is usually fatigue limit of rod-caps, bolts, or sometimes the rod itself. The critical areas are usually in the thinned section near the bolts. It can reach a point where clamping is lost at exhaust TDC and shells can rotate in the rod (many modern rods don't have bearing tangs). If the stresses are within the design fatigue limit the life is more or less infinite (except with light alloys, not common these days for rods). If run beyond the fatigue limit the life becomes finite, the greater the stress cycling the shorter the life. You can run beyond the design stress limit, but plan on replacing the rods fairly regularly. Even crack testing doesn't help much, once a crack starts the life is very short so unless you're rebuilding after every few races you probably won't catch it. If you know the material and stress you can predict the fatigue life, otherwise you're guessing.

Modern pistons rarely suffer fatigue fractures, being light alloy and running at high temps they have to be designed with a high safety factor anyway.