As title states, I was wondering if the GI plastic canteens can take being filled with boiling water without losing shape?

I figured someone here probably knows the answer!

Mark

I wouldn't do it.

If you did, and then screwed on the top, any air inside would expand and create a lot of pressure.

What type of plastic is used ? If it's in the polyethylene family that's on the borderline .
Me ? I'm a metallurgist so it's hard to admit plastics exist !

In my use of the USGI plastic canteens on active
duty c. 1977-84, we were instructed not to heat
them to anything more than "somewhat warm" -- no boiling water. In the field, at times there were chow setups that had big (clean) galvanized 40 or 50 gallon trashcans with heaters on them that had boiling water available to clean the old WW2 style messkits. IIRC, they had a process of 3 successive cans -- pre-wash, detergent scrub and rinse. No canteens ever came close to them, that I knew of.

I do know that I never wanted to take a chance on hot (HOT) water in one of them.
Hope this helps,
John

Are you trying to sterilize it? If so just use a cold solution of Clorox and water. Rinse three times with clean water and you are done.