Ok well I brought an ultrasonic thickness gauge from ebay, model UM6500. It comes with an 8mm diameter flat probe. I used a file to shape it to the curve of the cylinder wall. Doesnt take much, probably removed 0.2mm at the extreme edges, tapering to nothing near the centre of the probe.
None of the preset calibration adjustments for different materials matched up with measured thicknesses to well. It has a 5mm thick piece of metal on the unit that is used for calibration. I found that the side of the block down near the sump is approx 5mm thick so calibrated it against that. I then worked my way through the preset calibration adjustments till I got a very close match between measured thickness. Best place I found to confirm accuracy was measuring at the very top of the cylinder bores where they are closest together. At that location its solid metal between the bores and easy to check against a measurement with vernier calipers.
Using this approach, I was able to match the results from the machine shop pretty well. I also had a good look around for any more thin spots. I mapped out the areas with a thickness less than 110 thou and marked a white outline around them.
It found pretty consistent results showing the thinnest areas where 1 and 2, along with 3 and 4, get closest together. On a b230 cylinder spacing isn't equal, the gap between 2 and 3 is slightly larger and correspondingly doesn't appear to suffer the same issue of thin spots.
For reference of depth, I used a vernier caliper to reflect approx where the top of the piston would be when at the bottom of an 86mm stroke.
Cylinder 1
Cylinder 2
Cylinder 3
Cylinder 4
The thin areas reflect that at the top of the bores the cylinders are 'siamesed', followed by thin spots directly below that. I didnt find any particular thin spots outside of these locations. Given the depth of these thin spots and as they arent on the thrust side of the bore, perhaps its not an issue?
I also have a AQ171 and another B230FX which I measured up for comparison. As per below, there is strong trends between the different blocks. Did some colour coding to highlight the trends. Red is 109 or less, orange is 110-120, yellow is 121 to 130.
The spare B230FX was actually particularly thin in and not a viable alternative. The AQ171 is pretty good, however its already bored out to 96.5mm and has a bit of corrosion in the bores. Not sure how well it will clean up with a hone and if in the end it would still be in spec. Also doesn't have oil squirters. Given these findings I'm less concerned about using the freshly machined block but may see how a hone of the AQ171 goes.