hey I have a set of 31.7 clamp bars. do I NEED to get a 31.7 stem or could I fudge with a 31.8? I'm not great with measurements but that seems like a nominal size variation. thanks for the help.

I'm gonna go out on a limb here and say that there is no such thing as a 31.7mm handlebar. You have a measuring error. That bar is a 31.8.

Yep. In any case, such teensy differences really don't matter. A tenth of a millimeter is only 4 thousandths of an inch, about the thickness of a human hair.

Sheldon "Splitting Hairs?" Brown

I wouldn't be surprised if the bar or stem actually says 31.7mm on it. In the early days of the oversize standard, a couple of manufacturers were calling their bars and stems 31.7mm. I think Deda was one of them. They might still be doing it. It thought that it had something to do with 1-1/4" coming out to 31.75mm. Some manufacturers rounded up, others rounded down.

At any rate, this is really just a nominal designation, and 1/10mm difference isn't going to matter at all. In fact, it's pretty normal to measure a 31.8mm bar and find that it's over or under by at least that much.

Deda oversize bars and stems are listed as 31.7, while most of the rest of the industry lists theirs at 31.8. It's purely due to rounding. Deda chose to round down, everyone else rounded up. The oversize standard is based around the magic size of 1.25 inches, or 31.75 millimeters.

While I haven't done it (I can tomorrow at work), I bet if you took a caliper to a bar marked 31.7 and one marked 31.8 they'd measure the same.

It seems a lot of bar/stem manufacturers try to make their sizes sound proprietary hoping customers will feel compelled to buy both from them. The 25.8 and 25.9 mm advertised bars/stems sizes in places of the "standard" 26.0 mm was a similar situation.

Actually no. 25.8 and 26.0 is different.

You can force these different parts to work together since the difference is small but if you compare the fit carefully, there is a definite difference - hold a open faced 25.8mm stem up against some 26.0mm bars and the bars will not seat down into the stem evenly.

31.7 and 31.8 ARE the same since the true size is 1-1/4" (31.75mm)

Pretty sure it would void any warranty, though.