"Mark, Mercury started produci
"Mark, Mercury started producing the 9.9's for the 1986 model year.
The line continued with one major change coming in the 95 models up until the they were discontinued in 2005 (when Merc stopped producing "conventional" 2 strokes, save the 50, 90 and EFI 150's).
Earlier Mariner 9.9's (78 thru 86) were Japanese built, 100% Yamaha engines.
The 86-94 9.9's (whether Merc or Mariner) were built using a 12.8 cu in Yamaha powerhead and Yamaha 2.08:1 gears. That combined with Walbro carbs and Merc's Thunderbolt 4 ignition made them the most reliable and durable outboards ever built. You have to really neglect and mistreat them before they will break. The 6, 8 and 9.9 motors of that year range were identical save the carb and exhaust.
In late 94 they switched powerheads on the 9.9 and replaced the 12.8 with the 16.0 cubic inch power head that was being used on the 15 horse (since it's rebirth in 1988). The rest remained the same.
99% of all parts for any 6/8/9.9/15 produced from 1986 on is interchangeable (save the carb, exhaust and powerhead itself) and there was thousands of the motors produced.
I personally own 3 motors from that series. An 86 9.9, 87 9.9 and 91 15 horse. Both of the 9.9's have well over 1500 hours each and the 15 is just approaching the 1000 hour mark. Aside from spark plugs and impellers, not a single component has been changed or needed repair (of course with proper winterization and lower unit oil changes thrown in as required).
Additionally, I have never worked on one that required more than regular maintenance. So either all those owners have been lucky or maybe it's just an exceptionally built motor (I would lean towards the latter). It is difficult to find a 20 year old OMC that hasn't replaced a powerpack or other ignition component. I have heard of 1 or 2 Mercs that required a switchbox, but never seen it myself..."