[blykins]

The hand-held sonic testers will work fine, but if you're going to try that, I would suggest trying it on some known thicknesses. Another thing that tends to throw results off is that cylinder walls are curved and most of the sonic tester transducers are flat. They really need to be shaped so that they sit flush up against the cylinder wall.

When you test, take a reading all around the wall, but concentrate on the thrust side of the cylinder where most of the load is under the power stroke.