One thing many of us do in preparation for hot weather is buy an oil cooler. It makes perfect sense, because the oil in an air-cooled engine performs a major role in cooling the engine. Does this mean an oil cooler will enable the oil to do a better job? Well, yeah, sort of — to a point. Fact is, oil coolers do a great job of cooling the oil, unless you pick the wrong unit for your application, or the unit’s not properly installed. In either of those cases, an oil cooler can do more harm than good.
Oil has a tough role in Harley engines. It has to be able to flow through very small holes, into very small passageways at very low temperatures. And it must be able to resist thermal breakdown at high temperatures. Plus, it’s got to remain slippery enough to lubricate high stress points, such as piston rings and skirts, valve stems and tappet bodies, but not so slippery that it promotes hydroplaning or roller sliding.
When the engine gets hot, 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 go a long way in preventing this process from happening.
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) yet still contain adequate surface area to promote efficient thermal transfer. Surface area is the key factor here. The more surface area you have, the greater its heat transfers properties. In the past, some oil coolers were designed with cooling fins on the inside of the passageways. Sounds good on paper, but there are some inherent problems with this feature, especially when it comes to Harley-Davidson engines. Since the oil cooler must be installed on the return side of the oil pump, the scavenge system is often overtaxed in its effort 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.
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 degrees. This is where the “system” in oil cooling system comes in to play. By not having a thermostat, oil is prevented from reaching normal operating temperatures and thus fails to scavenge contaminants from the engine. If the engine is using heavy-grade straight-weight oil, there may be additional damage from oil starvation at critical lubrication points. And if it looks like you can’t install a thermostat, there are optional bypass valves available.