gear shimming and gear backlash

[wcsparky05]

i've had my boat now for almost a year (yes, my first) and i just replaced the water pump impeller on my lower unit. its a '89 merc. inline 4 cylinder 3 litre 140 hp i/o engine. i am confident i replaced everything correctly. i shifted into full forward first before removing and followed the instructions and everything turns correctly in neutral. however, i've been reading about shimming and gear backlash but its a bit confusing. can anyone shed some light on the subject in lamens terms? thx.

 

[WillyBWright]

Re: gear shimming and gear backlash

In layman's terms, if you're not inside the gearcase, you don't need to know.

For maximum lifespan, the gears need to mesh appropriately. The pinion is the key. It has to be set at the proper height. That's the first step. Then you set the forward gear so that the teeth mesh with the pinion gear, leaving enough room for expansion from heat. This is done by measuring the lash (the amount of freeplay between teeth). Changing the pinion height and forward lash is done with shims between the case and bearings. Reverse is checked, but not as critical as Forward because it isn't engaged long or under nearly as much stress. 98% of the time, it's just along for the ride.