From Big Al's Carponents
Our round mirrors are 3" in diameter.
Mirror stem options are a 5" Long Flat Stem, 4" Standard Flat Stem and a 4" Drop Arm Stem, which include mounting hardware of two 10-24 x 3/8" set screws. Round Stem with 1/4-20 threaded screw included or a Round Stem with a 5/16-18 threaded hole and no hardware.
Please note the round stems are designed to be mounted to the windshield frame, wind wing bracket or window post.They cannot be mounted to the car door.
Our mirrors are attached to the stems with a mirror ball that has 10-24 threads.