5/16-18 to 5/16-24 X 1-11/16. Note: Exhaust studs as used in the M8 engine (OEM 10900015, 10900015A) are of a slightly different design, with a small protruding hex-shaped outer-end to aid initial stud-installation, but these early Evo/Twin Cam style studs will still fit. OEM replacement reference 16715-83
