Ava alloy stem and bars diameter
 

I have a 1971 px10 in the mail. Id like to replace the Ava stem. It should be a 22.0 diameter stem but I'm wondering what the clamp size would be if I want to reuse the original Ava bars. 25.0? 25.4?

Also, some of the stems I am looking at are open at the back (you can see the quill bolt). Is this normal? Seems like a great place for water to get into the steerer.

Yes, that was normal, and, yes, please get yourself a forged, not cast (like the AVA) stem. They are especially vulnerable to breaking when raised up. The consequence of a broken stem is not pretty...

Gotcha. Open back is ok.

What about the clamp diameter?