Mercury Mariner 8/9.9HP 4-Stroke Factory Service Manual, Page 120Get this manual
Timing Belt inspection Replace timing belt if any of the following conditions are found Cracks in the back of the belt or in the base of the belt teethExcessive wear at the roots of the cogsRubber portion swollen by oilBelt surfaces roughenedSigns of wear on edges or outer surfaces of beltStretching by .40 in(10mm) or more when belt is pushed in with your finger
2Remove flywheel cover by loosening thumb screw (a) and pull out lock pin (b)a
aThumb ScrewLock Pin
3Hold flywheel from turning using flywheel holder (91-83163M)Remove nut and washera