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The maintenance of Lower Unit

Views:105     Author:Site Editor     Publish Time: 2019-09-25      Origin:Site


The lower unit (or “foot”) of your outboard motor is the bottom part that houses the prop and gear case. Taking care of your lower unit is vital to make sure your outboard runs smoothly and has a long life. The main keys to lower unit maintenance are keeping fishing line and other debris from getting wound in the prop and changing your lower unit oil regularly.


lower unit


The gears inside your lower unit are constantly turning, and the only protection they have is the lower unit lubricant you use. Its job is to form a micro-thin film between the metal component parts to keep them from actually touching. Under the large loads these gears exert, proper lubrication requires extreme resistance to pressure, foaming and molecular shear. And, since the lower unit operates underwater, its lubricant also has to be capable of maintaining necessary lubricity even when a significant amount of water is present, should some leak inside. Ordinary gear lubricant can’t do all of this, so always avoid automotive or tractor gear oils. Use a quality, marine-based formula. It can perform all of these required protection and lubrication duties, even with a full 10% of water present. 


Visually inspect the exterior of your lower unit for any signs of damage every time before you go out. If you fish, it’s a good idea, every fifty hours or so (or when necessary whichever occurs first), to remove the propeller and check for any fishing line or other debris that may be wrapped around the propeller shaft, which can cut into prop shaft seals and potentially allow water to enter the gear case. If found, visually inspect the prop shaft seals for damage or leakage. Then check the color of your lower unit lubricant by cracking the “drain screw” on the bottom of the bullet and the “vent screw” on the side of the lower unit. Allow a small amount to drain into a clean container. If the lubricant is milky in color, you’ve got water in the lubricant. Have the lower unit seals inspected through a pressure test, and if necessary, have the seals replaced. 


boat lower unit


Replace your lower unit lubricant after the first 20 hours for a new outboard, then every 100 hours after that. Also, if it’s milky in color or smells or looks burnt when you’re checking it, it’s time for a change. To do it yourself, place a pan under the bottom drain screw, remove the screw, and then remove the upper vent screw on the side of the lower unit. Allow all of the lubricant to drain into the pan. While you’re waiting, check the drain screw. It’s magnetic, so any metal particles or shavings present in the lubricant will be attracted to it. Fine metallic dust is normal, but if you find larger chips of metal on the screw or in the pan as the lubricant drains, check possible internal damage. Also at this time, replace the small crush washer present on each screw (commonly known as “drain plug gaskets”). If you don’t see one, it may still be adhering to the outboard. Use a pick to pull it free, as these must be located, discarded, and replaced each time these screws are removed.


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