Ax kicker, Your are right that they are two different planes. The outside flatness needs to be within 1/16" and the inside an 1/8". That's a different measurement technique than measuring total thickness. I'd lay a straight edge across each face, put a light behind it and see it it's writhing spec. If you're not positive stick a feeler gage in the gap to make sure. That's .0625 on the outside and .125 on the inside - that's how flat it needs to be. As far as thickness (21/8-21/4?) you could use a a caliper, plastic or steel; you can find them a lot cheaper than micrometers. I'd use one that does inches, hundredths of inches and thousandths of inches but for this, one that does 1/32" is good enough. If you get one and can't read it I'll help you; the common ones use a vernier scale, takes a little getting used to, the dial calipers are a little more intuitive but more expensive. A good machinist could measure tolerances like this with a six inch scale (a ruler). I found the air hammer really speeded up transom removal but if the hull was really thin it could be dangerous. Like you it worked for me. All of your pictures are showing up as x's, at least on my ipad. Don't know what's up with that. I have plenty of dry glass - in the transom and the tabbing for the stringers. Based on the pics I've seen it's not normal but it certainly does happen. I think it just means a little more grinding for those of us so blessed.
