Journal Bearing Clearances, Con Rod ID/Out of round, Piston to wall clearance, cylinder out of round/concentricity, wrist pin clearances etc. Snap gauges can have some repeatability and accuracy issues based on technique, your micrometers you're measuring with, etc. Even then they're still slightly less accurate than your micrometers that you're using to measure the gauge with. Typically the best practice is to measure the actual part with the actual tool. I'm not sure what level of accuracy/precision you're trying to obtain, but my experience at work is all down to the 0.001mm. FWIW, if you can pick up a nice set of ID mics, they are much easier to use and produce a more repeatable measurement.