The RX6378's round shape pairs with a 140mm metal temple arm, and matching both the curve and the length is what keeps a replacement sitting correctly against the front. A compatible arm built to a different curve can still close at the hinge while sitting noticeably off against the round front.

Round frames like the RX6378 fold to a tighter closed angle than square styles, and that repeated tight fold is what tends to loosen the hinge screw over time. Once the arm has play at the pivot, a new one restores the snug fit without needing to touch the front. It's a smaller aftermarket fix than most people expect once the actual cause of the wobble is identified. It also helps to check that the hinge barrel itself isn't stripped, since a worn screw thread can mimic the same wobble as a bad arm.

Frequently asked questions

Can I replace just one arm, or do I have to buy both?

You can order a single side if only one arm on your 6378 is broken or scratched, or buy the pair if both need replacing — either way they match the 140mm length exactly.

What material are the arms made from, and will the color match my frame?

The arms are molded in an acetate or metal finish that matches the Round style of the 6378 closely, though very slight shade variation is possible since they're independently produced as separate aftermarket parts rather than cut from the same batch as the frame.

What if the arms arrive and the hinge doesn't line up with my frame?

Because each pair is listed against a specific model, in this case the 6378, mismatches are rare, but you're covered by a 14-day return window if the 140mm arms don't align with your hinge.

My hinge feels loose — will new arms fix that, or is it the frame that's worn?

A new 140mm arm includes its own hinge barrel, so if the wobble is coming from worn threading on the arm side of the 6378, replacing it usually restores a snug, tight fold.