The problem with used lights is the reflector behind the ellipsoid eye on most 20-30 year old headlight is in sub-par dull condition which is the reason why light output ends up being less then optimal, but still acceptable. Some will have faired better then others, but none will compare to a brand new light. So far as I understand there is not a DIY solution to recoating these pieces to restore their original mirror finish. I see this as the main problem which exists independent of brand (bosch or hella). I have not seen any issues with hella units having plastic back halves.
With that said, used units are still serviceable and can look great and perform acceptable. Many of my cars run cleaned up older lights that I've pieced together from good used parts.
With that said, used units are still serviceable and can look great and perform acceptable. Many of my cars run cleaned up older lights that I've pieced together from good used parts.

Comment