For 1973 & later Mercury/Mariner 2-cyl, 20-45 Hp | 3-cyl, 50-90 Hp | 4-cyl, 75-125 Hp | 6-cyl, 90-115 Hp | V6, 135-300 Hp outboards.
ID: 1.375″ | OD: 1.5625″ | W: 0.0937″

also crosses with:
Sierra: 18-7164-9
GLM: 81260