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Figuring Out The Grease Fitting Size You Require

A Zerk fitting ready to receive grease.

Once upon a time, cars required lubrication every time the oil was changed. Garages routinely advertised “oil changes and lube” services jointly, with both the engine and the chassis receiving attention. Modern vehicles do not require regular lube service, as most of their parts are self-sealed or come “lubed for life.” Even so, your owner’s manual will likely mention a few areas you’ll need to keep lubricated, and knowing the correct grease fitting size can help you do so.

The Zerk Fitting

A grease gun for a Zerk fitting.If you’ve ever done any work on an old car, the term “Zerk fitting” might be familiar to you. Named for Oscar Zerk, an Austrian inventor and entrepreneur, a Zerk fitting is another term for a grease fitting, which is used to lubricate points on bearing housings or other mechanical equipment.

Zerk didn’t invent the process that would incorporate his fittings, but he did refine it, and it’s still used today. His design modified the then-common Alemite pin type method by adding a seal between the hose coupler and fitting. That seal is maintained by the pressure that results from the user applying the coupler to the fitting.

Later, an engineer modified Zerk’s invention by adding a small bulb at the end which enabled the grease gun to seal to the fitting without requiring pressure. Nearly a century later, this design is still widely used, with grease bearings appearing on driveshaft u-joints, tie-rod ends and steering joints.

Grease Fitting Size

There is no one-size-fits-all grease fitting. They come in various types, sizes and thread identifications. Your owner’s manual may mention the correct size for your situation, and the Haynes Repair and Maintenance manual can also point you in the right direction.

You’ll need to identify the correct connection fitting before you apply the grease. You can pinpoint this by examining the threads that wrap around the cylinder or cone to form a coil. There are six types of threads: British Standard Pipe Parallel (BSPP); British Standard Pipe Taper (BSPT); metric parallel, metric tapered, National Pipe Thread/Fuel (NPT/NPTF) and United/United Fine (UN/UNF).

You can identify the grease fitting thread type by sight, but this won’t guarantee you’ll get the right size. This is where a grease fitting gauge comes in handy — it’s the easiest and fastest way to figure out what thread type you’re dealing with. Why is this important? Because fittings don’t last forever, and when a nipple gets damaged, you’ll need a new grease fitting.

Although grease fittings aren’t expensive, ordering the wrong size would be a waste of time. You can order a grease fitting assortment, which routinely includes eight sizes, but if you only need one, then why spend the extra money? You can always contact a NAPA associate to help you find the right fittings as well.

Other Applications

Personal vehicles aren’t the only machines that have grease fittings. Notably, riding lawn mowers also have them. Check the front spindles of your mower for these, as the fittings are located on the insides of the front tires. Each of the front wheels might also have fittings, and the steering sector may include one that’s reachable by lowering the mower deck.

For owners of farm equipment, it’s worth noting that tractors, combines, harvesters, lawn and grounds equipment, and construction equipment will likely include Zerk fittings as well. Reference the maintenance manuals for more information on their types and locations. You may discover that the greasing intervals for these are as short as 25 hours of use. Don’t neglect lubricating your equipment at these intervals, or you may risk damaging your equipment and facing expensive repairs.

Check out all the grease fitting products available on NAPA Online, or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on the different types of trucks, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photos courtesy of Flickr.


Matthew C. Keegan View All

Matt Keegan has maintained his love for cars ever since his father taught him kicking tires can be one way to uncover a problem with a vehicle’s suspension system. He since moved on to learn a few things about coefficient of drag, G-forces, toe-heel shifting, and how to work the crazy infotainment system in some random weekly driver. Matt is a member of the Washington Automotive Press Association and is a contributor to various print and online media sources.

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