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0B120974 is an 1986 MERCURY 8 HP (M) Model Number 1008206....OEM Parts Manual Part Number 90-811286 covers the 6/8/9.9/15 HP engines. http://www.marineengine.com/parts/mercury_marine_parts/sn/0B120974
You get your choice of service manual part numbers you may want to buy. I searched the first one out for you.
OEM Service Manual Part Number 90-13449--1 / 1187
6/8/8 Sailmate/9.9/9.9 Sailpower/ 210 cc Sailpower/XR10/Magnum 10/10 Marathon/ 10 Sea Pro/15
Serial Numbers (U) 0A197112 thru 0D280999 (B) 09503295 thru 09792199 (C) 0A708341 and Above..., https://www.ebay.ca/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=90-13449--1+&_sacat=0
Manual Part # 90-827242R02 / February 2003
6/8/8 Sailmate/9.9/9.9 Sailpower/XR10/ Magnum 10/10 Marathon/ 10 Sea Pro/15
Serial Numbers (U) 0A197112 and Above (B) 09503295 and Above
Manual Part #90-891600R02 / February 2003
6/8/8 Sailmate/9.9/9.9 Sailpower/XR10/ Magnum 10/10 Marathon/ 10 Sea Pro/15
(I) All Serial Numbers.**