1/ Make sure the engine is in neutral.
2/ Undo gear selector shift linkage.
3/ Now remove the bolts holding the gearbox to the mid-section. Normally there is a hidden bolt under the zinc sacrificial anode.
4/ Gearbox should now drop down. It may need a little gentle persuasion (use a rubber mallet) and knock gently around the anti-cavitation plate to free it.
5/ The waterpump normally has a plastic cover held in place with 4 bolts. Remove the bolts and the cover.
6/ Now pull the old impeller off the drive-shaft. Note: there will be a crescent shaped metal dowel pin that locates in the impeller to stop it rotating on the driveshaft.
7/ Inspect the condition of the metal faceplate. If it's corroded or pitted - then replace. Check water-passages under the faceplate and clean if neccesary.
8/ Clean driveshaft. Lightly grease the impeller (Castrol marine grease) and slide it down the driveshaft - and engage it with the metal dowel pin.
9/ Lightly grease waterpump housing. You may require a new gasket - if it's torn. Lightly grease this too.
10/ Important: Rotate the driveshaft clockwise so that the blades of the impeller fit in the waterpump housing while pressing it into final position.
11/ Bolt waterpump down, reconnect gear shift linkage and lower gearbox to mid-section.
12/ Check the gears select properly in forward and reverse.
13/ Check thermostat housing and clean thermostat and housing (sometimes the thermostat can seize and housing gets build up of salt) - if used in salt water environment.
14/ Use strimmer wire and clean the tell-tale hole.
15/ Test engine with muffs.