How to Check the Lower Unit Oil

Explore America's Campgrounds

Lower units house the gears, drive shaft, propeller shaft, water pump and raw water inlet for both outboard motors and inboard/outboard motors, where the lower unit is known as a sterndrive, gear case, outdrive or simply a "drive." Lower units use an oil bath for lubrication; that is, the entire unit is filled with oil that requires regular replacement, in most cases, every 100 hours. The gear oil change gives you the chance to inspect the oil for evidence of internal damage.

Items you will need

  • Garden hose

  • Flushing attachment

  • Standard slotted screwdriver

  • Clean, lint-free rag

Lower Unit on an Outboard Motor

Ensure that the motor is vertical and is level from side to side, whether on your boat or a motor stand. If the motor has not been run within 30 minutes, connect a garden hose to the motor's flushing connection or to a motor flushing attachment.

Place the flushing attachment's discharge ends over the raw water inlets of the motor. Turn on the water full force. Start the engine and allow the motor to warm to its normal operating temperature.

Shut the motor down. Remove the lower unit's vent plug, a large bore, countersunk screw which you may remove with a standard slotted screwdriver. If oil does not leak from the vent plug when you remove the screw, the lower unit oil level is low.

Lower Units Used as Inboard/Outboard Drives

Ensure that the lower unit/out drive/stern drive is fully lowered. As with the outboard lower unit, the oil is best checked when the motor has been running recently, or is allowed to warm to its normal operating temperature.

Locate the dipstick, usually screwed into the gear case oil filler on top of the gear case, marked "Oil." Unscrew the dipstick and remove it from the drive. Wipe the oil from the dipstick with a clean, lint-free rag and screw it back into the oil filler.

Unscrew the dipstick from the oil filler and note the gear oil level. The oil level should be at or near the center of the dipstick, and above the lower mark on the dipstick.


  • When you're working on your motor or lower unit, use a 5/16-inch open-end wrench to remove the nut from the negative post of a marine battery. The positive post uses a 3/8-inch nut. If the lettering or cable color is obscured by dirt or oil, you can identify the terminals by the fact that the 5/16-inch wrench won't fit on the positive post.


  • If a bit of oil drains from the vent hole of the outboard lower unit, rub it between your thumb and forefinger. There will be a few tiny metal shavings, smaller than a grain of sand, in the oil; that's normal gear wear. If it feels like more than a hint of sand is present, there may be gear damage.
  • If the oil coming from the vent is milky, that's the result of water in the oil. You need to change your propeller seals, or damage will result.
Gone Outdoors