KC8QVO

If you can see the wall thickness you can measure it. As to how precisely - if you were trying to make an emergency repair I don't think a precise measurement is required. If you can see the wall thickness you can see that it is pretty thin. If it is pretty thin that will be a good guide to whoever is doing the repair as to how much heat the parts can take.

One technique for "measuring wall thickness" through a vent hole of the like is to use the head of a safety pin - the kind with a mushroom top like a carriage bolt. You can lock the lip of the head on to the inside wall of the part at the edge of the hole. Then mark the position on the pin where it is level with the outside of the part. If you want to get even closer precision - factor in the paint thickness (not much on modern bikes). Then measure the distance between the lip of the pin head and the mark on the pin shaft.

Machinists rules have marks on them down to about the .010 mark. When you get past that you really need something like dial or digital calipers or a micrometer to resolve (but if you are measuring a rather crude mark on a pin shaft you're not going to be even within .010 probably - maybe .025 if you're good - .0625 is 1/16",.03125" = 1/32 - thats a pretty fine increment).

Unless a fabricator has identical material to practice with to dial in their heat settings and techniques there isn't any good way to ensure great results. It is kind of a "try it and see what happens" approach. If there is too much heat then, yes, the welder will blow through some of the metal. A good welder will be able to minimize that risk - and knowing an idea of how thin "thin" is will give them a better idea of what they're up against, but still no absolute.

If they do blow through something - a good welder can also fix a blow out. It might not look pretty, but that is a pretty important skill in fabricating.

When you are talking TIG welding, torch welding, and brazing (joints other than slip joints/sleeve joints) - you have to be an expert on part fitment. That is because these processes can't handle any slop in the fitment. You don't build up metal to close a gap with these processes. Slip/sleeve joints only pertain to brazing - and you do want a tight slip joint and if you're talking a slip joint you're not talking "filling a gap". In some respects, the depth of the part inside the slip joint doesn't really matter - you could cut it at an angle and still get a good brazed joint - just you minimize the surface area of the brazing. With any other joint - like a butt joint (think stay welded to bottom bracket) - part fitment is imperative for TIG welding, torch welding, and brazing - because these processes aren't to add much metal, only fuse the joint. If there were the requirement to build up metal (say you needed to cut out a cracked section of a stay) then you would be best off to replace the base metal/add to the base metal (with sheet metal, another tube, etc). If that isn't available - you can "build up" metal with MIG, Flux Core, and Stick welding - if done carefully. It won't be a pretty repair, but it might do the trick to get you back riding.