Shop: Lubricating O-rings

When searching for a “will fit” O-ring for a non-pressured, noncritical assembly such as a hand-pump sprayer, the best choice is to look through the green and purple parts that are made of HNBR synthetic rubber and lubricated with pure silicone oil. There’s no way to know what materials were used for a collection of O-rings left over from previous jobs. Photo by Scott R. Nesbitt

Having wrongly thought that O-rings were simple things, I’ve added a small container of 100 percent pure silicone oil to the toolbox. The liquid lubricant is making life in the shop easier and increasing confidence in the longevity of many repairs. No longer will I squirt just any old spray oil onto any rubber-like gaskets, seals, O-rings, or the adjoining metal or plastic parts. What follows are some basic terms and concepts worthy of inclusion in the technician’s knowledge base.

The term “rubber” was first used in the mid-1700s, when Europeans observed that dried sap from a South American tree could be rubbed across pencil marks to erase them. Working with petroleum, chemists developed synthetic versions of the sap. Today, “natural rubber” comes from trees or oil wells and is called latex or isoprene. It is used for pencil erasers, car tires and many other things. More oil-based synthetic rubbers were developed, and others were created from rock-like minerals. These are called fluorocarbons (Teflon) and silicone.

From string trimmers to tractors, a few dozen synthetics are currently used, but there’s no color code or other system to tell them apart. You’re gambling when replacing an old black O-ring with a new black one. When working with a critical pressurized system, such as hydraulics and fuel and coolant systems, it’s safest to use parts from the OEM (original equipment manufacturer) or a name-brand gasket kit. When I need a “will fit” part, I check my supply of HNBR (hydrogenated nitrile butadiene rubber) O-rings designed for auto air conditioning systems. These resist most automotive fluids over a wide temperature range. They work, for example, to seal fuel injectors mounted in intake manifolds.

Pure silicone oil helps resolve many of the unknowns in the rubbery domains. It helps prevent mishandling of O-rings or other gaskets, and helps avoid their failure. Rolling an O-ring down a shaft or across screw threads can cause damage or distortion. Ditto for putting a dry O-ring into the groove cut in a machine part. (That groove is called a “gland” in many service manuals.) The lube lets rings slide into place. Because it doesn’t dry over the long haul, it helps gaskets flex when heat changes and when vibration causes movement in places where a rubber gasket acts as a seal between metal or plastic parts. The pure silicone oil reduces galling and improves accuracy when fasteners are tightened to a specific torque into plastic or metal components.

Chemically, pure silicone oil is almost completely inert and neutral — it won’t cause deterioration, swelling or shrinkage of virtually any synthetic or natural materials. It retains lubricity over a wide range of temperatures and stays put under pressure. There is a small chance silicone oil might partially dissolve a gasket made of pure silicone, but such gaskets are used mostly in food processing equipment and rarely in systems found on engine-powered machines.

It’s the chemical issue that should limit use of spray or liquid lubricants on anything other than purely metallic components, such as chains. Petroleum-based chemicals pose a challenge because they can deteriorate oil-based synthetics. Many lubricants labeled as silicone contain 5 percent or less of the polydimethylsiloxane or polymerized siloxane chemical. The rest of the product is a liquid carrier. If the package says the stuff leaves a dry lubricant film, it’s not the pure oil.

You likely have some nearly pure silicone oil on hand as a component of the dielectric tuneup grease used on spark plug boots. However, the heavy body of the grease could interfere with some assemblies, and it should be a second choice. Be guided by experience and common sense.

You probably won’t find pure silicone oil in a hardware store. Look for it online. It may be carried by gun shops, as it’s the required lubricant in certain firearms and air-powered pistols.

My half-ounce bottle cost $8, including shipping. It takes just a tiny bit and is worth the investment.

Scott R. Nesbitt is a freelance writer and former GCSAA staff member. He lives in Cleveland, Ga.