Easy Oil Question

beaug244

Cadet
Joined
Jul 6, 2009
Messages
7
On an 1970's 20hp merc, there is a single half inch screw on the side of the lower unit that looks like an oil drain. Next to it says "flush". I unscrewed it and no oil came out but there was a little residue on the end of the screw.

What type of oil do I replace this with and where do I pour it in? Thanks in advance
 

dajohnson53

Lieutenant Commander
Joined
Apr 28, 2004
Messages
1,627
Re: Easy Oil Question

On an 1970's 20hp merc, there is a single half inch screw on the side of the lower unit that looks like an oil drain. Next to it says "flush". I unscrewed it and no oil came out but there was a little residue on the end of the screw.

What type of oil do I replace this with and where do I pour it in? Thanks in advance

Usually "lower unit" refers just to the part at the very bottom that contains the gear case and where the prop is. It also contains the water pump.

The entire vertical part below the engine powerhead is usually called the "leg" and contains the long drive shaft, the gear shift shaft and the main water tube going from the water pump to the engine.

I believe what you're looking at is towards the top of the leg - not at the bottom where the gear box and prop are, correct?

That is a flush fitting to flush fresh water through your engine. They make fittings for garden hoses that fit in there. You can flush your engine's innards to get salt water or silty water out of there.

It is not intended to use to actually run the motor on the hose. Water going into the flush fitting circulates around in the engine, but doesn't properly wet the water pump mechanism (which is located down in the lower unit itself). Therefore, if you actually run the engine using only water into the flush fitting, it will indeed cool the engine, but you can destroy the water pump.

The oil change holes are on the lower unit itself. One is way down at the very bottom, the other is higher up, around the "anti ventilation" (sometimes called anti-cavitation) plate. You drain the oil by removing both screw/plugs. You replace the oil by pumping oil into the lower hole until it comes out the upper hole - at which time you quickly replace the screws before much oil leak out. Replace the washers (gaskets) on those screws every time you change the oil as bad gaskets are the primary reason for water getting into the gearcase.

You use outboard motor gear oil available at any boat shop and most hardware stores.
 

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Easy Oil Question

The early 70's Mercs were equipped with flush ports on the LU in addition to the VENT and DRAIN ports.

Not entirely sure how to use the FLUSH port, but you DO NOT add oil there. Later models did away with the flush port on the LU.

On the "bullet" shaped gear housing you will find two ports marked VENT and DRAIN. The VENT port is located just below the antiventilation plate on the side of the LU. The DRAIN port is located underneath and towards the front of the gear housing. Can't really see it unless you tilt the motor up or get down on your hands and knees.

Contrary to what you may think, you fill through the DRAIN port while the VENT screw is removed. Do a search in this forum on how to fill the LU.

Use marine Extreme Pressure gear oil SAE 90.
 
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