Old Stewart Warner Shop Mechanical Tach ?

Commonly used for small engines, but can be used on anything that spins. The tip is held against the face (the taper centers in the centerbore of the shaft) of whatever shaft you want to measure, and RPM reads directly on the gauge. If the end of the shaft is not accessible, you can hold it against the outside surface of the shaft (or flywheel, balancer, pulley...) and do some math- diameter of shaft vs. diameter of the rubber gauge wheel- to figure out your RPMs.