One of the things many will do in preparation for hot weather is to purchase an
oil cooler. It make sense. The oil in an air cooled engine performs a major role
in cooling the engine, so an oil cooler will enable the oil to do a better job,
right? Well, yeah, kinda sorta... to a point. Although oil coolers do a
wonderful job of cooling the oil, if you don't select the correct unit for your
application, or it's not installed properly, it can do more harm than good.
Oil has a tough row to hoe in Harley® engines. It has to be able to flow
through very small holes, into very small passages at very low temperatures and
must be able to resist thermal breakdown at very high temperatures. It must be
slippery enough to lubricate high stress points, such as piston rings and
skirts, valve stems and tappet bodies, but not so slippery as to promote
hydroplaning or roller sliding. When the engine gets hot, the oil begins to
deteriorate. As thermal breakdown occurs, the additive package (the chemicals in
the oil that give it particular properties or characteristics) begins to break
down. A good, high quality oil cooling system can do much to prevent this
petrochemical China Syndrome from occurring too soon (at least for awhile... in
the long run, it's inevitable).
The first thing
you need to consider when purchasing an oil cooling system (notice I said
system), is the design, along with mounting hardware. A good oil cooler should
be well constructed, with heavy fins in a solid frame and a heavy mounting
bracket. The unit should be small enough to mount unobtrusively (Harleys® look
funny with radiators) and still contain adequate surface area so as to promote
efficient thermal transfer. Surface area is the key factor here. The more
surface area you have, the greater it's heat transfer properties. In the past,
some oil coolers have been designed with cooling fins on the inside of the
passageways. Sounds good on paper, but there are some inherent problems with
this feature, as pertaining to Harley-Davidson® engines specifically. Since the
oil cooler must be installed on the return side of the oil pump, the scavenge
system often is overtaxed trying to overcome the additional resistance to flow
created by the oil cooler. When that resistance is compounded by internal fins
in the oil cooler, the result is usually oil carryover from the crankcase vent.
Sometimes an oil cooler may not even be necessary. There a few devices
available that simply slip over the screw on oil filter, thereby adding surface
area to the canister. Since more surface area transfers more heat, VOILA!, the
oil filter becomes an oil cooler as well. There are additional units that
install between the filter and its mount providing the same effect.
Whatever type of oil cooler you choose, make sure it's installed in
conjunction with a thermostat, designed to open at no less than 180°. This is
where the ÒsystemÓ in oil cooling system comes in. Many folks have actually
damaged their engine by installing an oil cooler straight up without a
thermostat. The result is oil never reaches operating temperature and thus fails
to scavenge contaminants from the engine. If the engine is using a heavy grade
straight weight oil, there may be additional damage from oil starvation at
critical lubrication points. If you can't for some reason install a thermostat,
at least put a cover over the cooler in temperatures below 80°. It's not as
effective (or easy) as a thermostat, but it's better than nothing!
When choosing a place to mount the oil cooler, look for a spot that will be
in the path of unrestricted air flow. Most systems install on the down tubes of
the frame or just below the neck. Areas of concern are clearance to the front
fender when the forks compress during hard stops or bumps. If you mount it low,
make sure you have adequate ground clearance. Make sure the oil lines are secure
and routed away from any sharp edges or exhaust pipes. While routing the hoses,
try to avoid sharp bends that could possibly cause the line to collapse or be
pinched.
And finally, don't assume that because you've got an oil cooler, your engine
can now withstand idling for extended periods of time without any negative
effects. In order for an oil cooler to function, the bike must be moving,
keeping air flowing through the cooler. Also, don't try to get more miles out of
your oil. The oil will be more resistant to thermal breakdown with an oil
cooler, but it should still be changed on a regular basis. After all, fresh oil
is the cheapest insurance you can buy for your engine!
Ride safe and stay cool!