Changing Outboard Motor Gearbox Oil
An outboard motor gearbox is located behind the propeller. It is a ‘right angle drive’ type gearbox that changes the drive from the motor from vertical to horizontal. Some gearboxes have a clutch mechanism behind them, often with forward/neutral or forward/neutral/reverse gears.
If the oil leak is persistent, it may be from a damaged ‘O’ ring behind the plate behind the propeller. This is a rare occurrence but sometimes O rings are damaged or not seated properly when the unit is assembled.
The gearbox is full of oil – usually a 90-grade gear oil commonly available in one-litre plastic bottles from automotive supply stores such as Repco, Super Cheap Auto or your local garage. A multi-grade gear oil such as 75-90 will do the job also.
The seals around the vertical and horizontal shafts are usually very effective. There are usually two seals per shaft – one to keep the oil in and one to keep the water out. If one of these ever gets damaged water may get into the gearbox so check the oil and if it looks ‘milky’ you have a problem that needs attention before the outboard is used again. Oil and water mixed together usually produce a thick milky looking fluid and the cause of water ingress must be found and fixed before the oil is replaced.
This Tohatsu website has a good article on changing the gearbox oil on bigger outboards. [Click Here]
Changing The Oil On A Small Outboard Motor
For most outboard motor gearboxes, you can remove the lower plug and let the old oil drain out. Note: Also take the top plug out so a potential vacuum does not form inside the gearbox that prevents the oil from draining out. There is not much oil in a small gearbox – from about 43 ml upwards depending on the model but larger gearboxes will hold considerably more oil. Have a drain tray under the gearbox to catch the oil.
Oil can be squirted up into the gearbox from the bottom hole with a plastic bottle that has a spout. Push the spout into the hole so the spout makes a seal around the hole and squeeze the oil in. Once the oil starts to flow out the top hole replace the top plug. (You may be able to hold the spout in place while you finger tighten the top plug.)
Remove the spout after the top plug is in place – this will create a slight vacuum inside the gearbox that will hold the oil in while you rapidly put the bottom plug back in. If you use this method you are less likely to get a bubble in the oil and thus get a false oil level reading.
For small outboard motors that only have one oil drain plug…. or if you can’t manage to squirt the oil up into the gearbox from the bottom drain hole …………
Replace the lower plug when the old oil has all drained out. To refill the gearbox from the top hole slowly squirt new oil in the top filler hole with an oil can or a large veterinary syringe obtainable from vets. To ensure the oil goes in and the air goes out, push a very small diameter plastic tube onto the syringe. Cut the plastic tube to length so it is long enough to push past the gears to the very bottom of the gearbox.
Make sure there is room around the tube in the filling hole for air to escape around the tube, then push the plastic tube right to the bottom of the gearbox so the oil displaces the air from the bottom of the gearbox up as it fills the gearbox. If you are forcing thick oil through a small diameter tube just exert a slow steady pressure – the oil will flow slowly.
If you get an airlock and oil does not seem to be going into the gearbox but, instead, just flows down the outside of gearbox, use a clean thin wire to burst the bubble in the oil. Check the oil level and give it a few more squirts until the oil is full to the underside of the fill hole.
If you put too much oil in let the surplus flow out. Check there is not another bubble in the oil and then reinstall the plug. Tighten firmly but not so hard as to damage the alloy washer or strip the thread in the plug hole. A stripped thread will mean an expensive repair job.
Make sure there is some oil on the plug thread to make it easier to get out in the future.
It’s worth having a small dedicated oil can/ syringe for the job. The cost is minimal compared to other costs of owning a boat.