You absolutely have to have a radius on the probe for accurate results-I just profiled mine on a belt sander until it fit semi-snugly on a 4"-ish bore. I haven't seen any mention of calibrating off of a known thickness of the block you are working on. You do this with the velocity adjustment. Example: You measure something directly with calipers or a micrometer like from a waterpump bolt hole to the deck--make a mark with a Sharpie exactly where you measured and then you place the probe there WITH ultrasound gel (which I didn't see mentioned) and you dial the velocity up/down until it reads the same as the caliper/mic measurement. This is to account for different material densities. You can measure just about anything as long as you can qualify it first with a direct measurement. There are probably hundreds if not thousands of different cast irons. BTW grey iron is very different than the usual cast iron used in engine blocks.
Also of great importance is the shielding gel--You MUST use a generous amount to get accurate and repeatable measurements. Also I take the intial few measurements 3 times till they all measure the same--too little gel will skew the results--too much costs extra money in waste. FWIW All G3 Hemi 5.7's will easily go to 4" with tons of thickness to spare--Oddly enough the BGE and Hellcat blocks aren't quite as beefy in cylinder wall thickness. Oh and I also taped my probe to a 5" long piece of TIG welding rod to make kind of a wand to reach into cylinders more easily. J.Rob
