Hi have a late 70's early 80's Peugeot U08 and I want to give it more modern parts, I have already started on the bottom bracket and crank set, but what I want to do now is install a threadless stem with out replacing the forks.
My question is, the adapter I plan on buying is a 22.2mm, but I dont know the dimensions of the bike. If you know the dimensions that would be great. I also would like your opinions on what sort of thread less stem size I should use? I plan on keeping the same head set.

https://i.imgur.com/ZRIm2LG.jpg

this is her here.

I would use a digital caliper and measure what you have but I believe that Peugeot used an unusual sized 22mm stem as did many French bikes. If it is from the 1980s that might be different. I would not order anything especially for a French bike before measuring at least twice. Everything I have taken apart on Peugeot Px10 has not been a standard part. You can buy a good enough digital caliper from Harbor Frieght for under $20 and way under if it is on sale which happens often. You are going to need it for your bike. Roger

Why do you want to install a threadless stem? The Peugeot is not a bad bike but certainly not high-end, but given that the bike was designed for a quill stem, why not just stick with a quill stem? If removable faceplate is your desire, you can get quill stems with removable faceplates. The only reason I can see to convert is if you really want to use a bar with 31.8mm clamp diameter.

Peugeots of the late 70s and early 80s typically had handlebars with a 25.4mm clamp diameter, and given that the quill may be weird size, I would just stick with the quill stem that's original to the bike unless you want to get the bars further away or closer to you (horizontal adjustment). I have the basic Nitto B115 25.4mm clamp diameter road bars on two of my bikes (both rebuilds of late-70s or early-80s frames), and I'm a big guy.

I ended up going in a totally different direction. The quill was strange, but I found an adapter for threaded to thread less, but I came across this Peugeot, ( which is the same model that started my obsession with them) under the impression it was my size. I showed up only to be disappointed, too small, but the wheel set was worth more than the asking price of the whole bike so i picked it up anyway.
this is what I got.
https://i.imgur.com/0VHgrty.jpg

if any of you are from the central CA area and have a 61cm frame like this one, or if you have the decals, please let me know, I would love to buy it from you.

but as for the confersion went I decided to go this way with it.

https://i.imgur.com/t8KskJw.jpg

I plan on taking the fork off the black one and putting on the red.