Grease fittings and grease pins are essentially the same thing. They both allow a grease gun to be attached onto the top to inject grease inside. Both are found on virtually every type of mechanical equipment that requires grease lubrication inside of a sealed joint. Automotive suspension systems like ball-joints and u-joints are commonly fit with grease fittings or pins, but they are also found on trucks, hydraulic equipment and aircraft. They are often called "Zerk fittings" in honour of the inventor. Keep grease fittings and pins clean and clear of dirt, debris and hardened grease so they remain effective.
Spray a petroleum-based spray lube onto a grease fitting or grease fitting pin, then wipe it off with a rag.
Heat up clogged grease fittings and pins with a standard hair dryer or a heat gun on the low setting. Attach the grease gun and use it immediately; the new grease will push the now-softened older grease out of the way and unclog the fitting.
Attach a commercial grease fitting cleaner to the clogged grease fitting or grease pin. Gently tap the grease fitting cleaner with a small hammer; this will force either oil or air into the fitting, depending on the type of grease fitting cleaner, and effectively unclog the fitting.
Unscrew and remove any clogged grease fittings and pins with a wrench if they cannot be unclogged through the steps above. Clean them from the back to the front by first spraying them with lube. Allow a few seconds for the lube to loosen any hardened grease, dirt or debris and wipe them down thoroughly with a rag.
Grease fitting are inexpensive; if you have any difficulty cleaning them so they work properly, it may be more cost effective to replace the clogged fittings.