I bought 3/4" black pipe from a hardware store. I measured the diameter as best I could in-store with a measuring tape, and thought it was 25.4mm. I got it home, and with my micrometer, I see that it’s more like 26.5mm. Adding insult to injury, I can’t find the receipt (home depot was supposed to email it to me) and it was pricy.
I will try to return it and find EMT conduit. But if I can’t, am I out of luck, or is there a way to make 26.5mm work?
Welcome!
HomeDepot are usually pretty good about giving you store credit if you don’t have a receipt. If you bought on credit card recently they can look that up.
Personally I’d go back and get smaller pipe like 3/4" EMT. However, if you’re looking for a stiffer 1" DOM pipe then consider Metal Supermarket if you have one in your area, contact them directly for price, their website prices can end up being higher than if you work with staff at the store 
Good luck with your build!
EDIT: looks like you’re building MPCNC Primo and not LowRider 3. So… Maybe ignore me, someone that knows things will chime in shortly…
Sorry. There is no way to make the 26.5mm work. As a point of reference, Ryan created separate modeled parts for 25mm and 25.4 (1") tubing, so even 0.4mm is too much of a difference. Your black pipe is 1.1mm larger than the 1" parts were designed for.
No.
Long answer: Consider that Ryan re-engineered the machine for 23.4mm, 25mm and 25.4mm. If that 0.4mm needed comlletely different pieces, 1mm is more than enough to require re-doing the whole thing again. As I understand, people have assembled machines with other sized tubes, but I’ve never seen results from those machines.
In the end, you will be way better off with 3/4" conduit or 1" steel tube. (The conduit is cheaper.)