From the J&P Cycles Blog Archive

One of the things many do in preparation for hot weather is 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 the correct unit for your application isn't selected, or 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).

Buying A Motorcycle Oil Cooling System

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 its 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 are 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 system 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 that 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!

Mounting An Oil Cooler

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.

Keep It Moving, Keep It Fresh

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!