Paint Marker which is really useful as it writes on most components. The nib is quite wide but great on disassembly of your project so you can mark each large part. Even better if you've got a few running and need to know which flywheel fits which (can you tell we've been there?!)
Also use to mark bolts and fastener positions and for making timing marks on flywheels and crank pulleys.
It works like a glue stick, simply twist the bottom for more solid 'paint'.