Round metal frames such as the RB3546 leave less margin for error than boxier shapes: 140mm is the length that keeps the arm sitting flush against the curve of the front. A compatible arm has to match that measurement and the metal build already on the frame, since a mismatched finish stands out more on a round design than on a squarer one.

Thin round-wire temples flex more than heavier square ones, and the most common failure point is a gentle bend that develops right behind the ear over years of folding. The front rarely takes any damage from this, so a new aftermarket arm is usually the only fix needed. Checking for that kind of bend before assuming the whole frame is worn out can save the cost of replacing more than necessary.