Ray-Ban built the RB3603 around a 140mm metal temple arm matched to its square metal front. A compatible replacement needs to hold that same length and finish, since the RB3603's fit depends on both arms closing evenly against the front rather than one running long or short. Even a small mismatch in either measurement tends to show up as a visible tilt once the sunglasses are worn.

Metal-on-metal contact at a squared hinge tends to show wear sooner than on curved styles, with the finish thinning right where the arm meets the barrel. That's usually the first visible sign an RB3603 needs new arms, well before the rest of the frame looks tired. Replacing the pair of temple arms at that point is a straightforward aftermarket fix, and it leaves the metal front and lenses untouched.