Q: I have two valves out of a classic motorcycle (a Triumph) one and the other are the same size in all respects. The parts book gives a different parts numbers for each valve (inlet and exhaust). The problem is knowing which is which. I have looked in all my service books but cannot find any reference to this matter, Which is the inlet and which is the exhaust? As I have said they are in all respects identical! In normal circumstances, even allowing for an exhaust valve being more ‘burnt’ etc than the inlet, the inlet is normally on engines (generally) larger. The only thing that may help is that one is ‘ferrous’ i.e., a magnet sticks to one valve but not the other, the valve head.
A: As a general rule, exhaust valves suitable for modern fuels are non-magnetic (i.e. austenitic) whereas inlet valves are magnetic (i.e. martensitic). However, your valves are probably not designed for modern fuels, so that is probably of no help. I was reminded by this letter that some older Triumphs did have inlet and exhaust valves of the same size, so that would make life more difficult as far as identifying which is which is concerned. You could try scratching the valve faces with a scriber or similar to test the room temperature hardness — which might give you an idea, but otherwise it could be time for new valves and sets anyway as this will prevent any worries about unleaded fuel. I shall be looking at this topic in the future.