Mercury Mariner 8/9.9HP 4-Stroke Factory Service Manual, Page 122Get this manual
Timing Belt Installation Timing Belt Installation Notes:
3Install timing belt as follows aAlign both gears with their alignment marks (Steps and 2)Make sure that they don't turnbPlace the timing belt around the drive gear and around the driven gearIMPORTANT: Make sure the Part Number on belt is not inverted4Reconnect hose to fittingb
Protect the timing belt from water and oilUse care not to scratch the beltDo not use any metal device to help stretch the belt onto the driven gear