Valve guides

[cmfalco]

At the risk of going somewhat off topic, while they are called "3-point" gauges, it's not irrelevant to emphasize that their probes have finite lengths, not points. Also, this type of gauge has variations in design that can have important consequences. Both of the gauges in the photograph would be fine for an open cylinder, like that of a, um, cylinder on an engine, but only the one on the right would be able to measure the diameter of the flat-bottom pocket being machined in a head for a new valve seat insert.