Marine Lower Unit Needs Regular Service

Normally, your marine motor gets all the attention, your boat isn’t going anywhere without the lower unit. Its combination of gears, bearings and other components turn horsepower into movement. Lower units are resilient and can last for years – provided you service them annually. We reveal the inner workings of a marine lower unit and show the common suffering points. Marine Gear Lube is recommended for this service.

The Shift Shaft allows the operator to select forward or reverse.

Like an automotive differential, a marine lower unit contains a Pinion Gear that drives a ring gear (either a Forward or Reverse Gear). The pinion gear concentrates extreme pressure on the ring gear teeth, especially during throttle bursts. If the lubricating film weakens (due to water intrusion, most likely) and fails to keep the gear teeth separated, they can chip and lead to failure. This can also occur if the boat owner fails to service the lower unit annually and allows the lubricant to deplete to a dangerously low level. The lubricant’s extreme-pressure properties are key to providing superior gear protection.

The bearings, housed in the Bearing Carriers, require consistent lubrication to prevent failure. The number one cause of bearing failure is degraded gear lube or lubricant starvation due to improper maintenance.

The Shift Dog contains large teeth that mate with similar teeth on the forward or reverse gears, connecting the drive shaft to the prop. If your outboard frequently pops out of gear, it’s likely due to rounded-over shift-dog teeth as a result of wear. The shift dog is typically made of softer metal than the gears so it wears out instead of the gears, which are more expensive to replace. Strong anti-wear properties are vital to ensuring the lower unit shifts properly and stays in gear.

The Drive Shaft transmits engine power to the prop, propelling the watercraft.

The Shift Shaft, Drive Shaft & Prop Shaft Seals prevent water from entering the lower unit and contaminating the oil. Water weakens the lubricant film and can lead to gear and bearing wear.

That’s Why:

  • The number one thing you can do to protect your lower unit, is change the gear lube every autumn prior to storage.

If the lubricant appears cream colored, water has breached the seals and entered the lower unit. For an extra measure of protection, Amsoil designed the Synthetic Marine Gear lube to provide excellent wear protection even when contaminated by up to 10% water


Change your marine gear lube every fall prior to boat storage. It’s the single best way to keep the lower unit running strong for year. The marine gear lube pump eases installation and reduces mess when changing gear lube.