One of the most common errors when sonic testing is the use of incorrect probes.Since we do roll cages as well as blocks we have various size probes.For cast iron we use .500-1MHz,for steel .250-7.5MHz amd for aluminium .250-2.25 MHz.The small diameter probes should be used when testing concaved or convexed surfaces as to not have any chance of gaps between the probe and surface.
We have seen some really thick blocks that have exceeded .500 and .400 in areas but they are certainly rare.Uncle Barry bought a hemi block from us that exceeded these numbers.
As for using hardblock on a short fill,it has more advantages than disavantages.As we mentioned we find casting irregularities at the bottom of the cylinders more than anywhere else.This is why you see many cracks at non thrust sides and below the ring travel area.It certanily doesn't hurt and is a bit of insurance.