Conversations about lower unit lubrication
don't typically generate much interest around the launch ramp. That is until someone
gets water in it and loses a gear case. It happens more than you may realize because
all of the power and torque of your engine turning at thousands of rpms is concentrated
on two small rotating metal surfaces. We’ve seen more than a few boaters spring
for a whole new engine because of a blown gear case. Here's what you need to know
about outboard lower units and why choosing the right lubricant is so critical.
Outboard lower units develop tremendous heat in their spiral-bevel gears and
need a high “film strength” gear oil to lubricate metal-to-metal contact and prevent
damaging wear. ASTM Timken load tests show that Royal Purple Max-Gear has a far
stronger “film strength” than both synthetic and mineral oils.
The Design Challenge
Modern outboard lower units use spiral-bevel gears designed with a curvature in
their teeth. This design distributes the load over several teeth to increase the
gear set's load capacity providing smoother, quieter operation. The downside of
this design is the increased lubrication demand due to the sliding along the curved
teeth. The sliding action increases friction and heat. These extremely demanding
conditions require specific lubrication.
The Lubrication Challenge
These spiral-bevel gears illustrate the area of contact by tremendous torque creating heat and wear.
Spiral-bevel gears require extreme pressure additives (known in the industry simply
as EP additives) to handle the strain of increased shock load and high RPMs. It's
important that a lubricant has adequate film strength to prevent the metal to metal
contact that causes heat and wear. Film strength can be measured using American
Society for Testing Materials (ASTM) D-2782 Timken Load test. The higher the film
strength, the less metal to metal contact. Performance lubricant manufacturer Royal
Purple has gained a reputation for developing lubricants that possess very high
film strength as exhibited in the chart comparing the results of the leading synthetic
gear oil and conventional (mineral oil based) gear oil with Royal Purple's 'Max-Gear'
The American Society for Testing Materials (ASTM) D-2782 Timken Load test shows
that Royal Purple Max-Gear oil has far higher “film strength” than conventional
It's important that different gear oils not be combined. If you elect to switch
gear oils you should make sure all of the old gear oil is completely drained out
before adding the new gear oil. Each gear oil brand formula is unique; mixing two
brands will result in a lubricant that is inferior to either of the two that you
Most outboard manufacturers recommend changing gear oil annually or every 100 hours
(whichever comes first). Some recommend shorter intervals when in salt water.
At least one company, Evinrude, allows up to a 300 hour/3 year drain when using
their particular HPF-XR oil. Regardless of the change interval, any accidental introduction
of water into the system demands immediate attention.
Fishing line around a prop.
Water - The Unseen Enemy
Water in an outboard's lower unit often goes undetected until it's too late and
expensive repairs are needed. One of the most common causes, particularly among
anglers, is discarded monofilament line which gets wrapped around the prop shaft
and damages seals.
When water and oil are violently mixed, the emulsified mixture dramatically reduces
the gear oil's ability to do its job. A lubricant's ability to rapidly separate
from water indicates how well it will continue to lubricate should water be introduced
to the system. (Regardless of how well and quickly a lubricant separates from water,
it should be changed immediately if water enters the system.) The chart below indicates
how well the Royal Purple gear oil separated from water when compared to the leading
synthetic and conventional gear oils.
The faster gear oil can separate from water, the faster it can start protecting
your gears. This test shows that after only 20 minutes Royal Purple Max Gear was
completely separated from water and after 60 minutes conventional oils were still
No one would ever want or plan on having water contamination in their lower unit.
Spending a few extra dollars for the extra protection from a high quality gear oil
is likely worth the peace of mind.
Royal Purple has over
18,000 dealers in the U.S. To find the closest dealer to you…
Royal Purple makes a full line of lubricating products that outperform conventional
synthetic and mineral oils.
To find out more about its full line of products go