How do zerk fittings work?

Question

I recently purchased an ATV with a blown engine that I'm converting to electric. The frame has zerk fittings which were encrusted with old grease. They cleaned up fairly well but now I'm conflicted. Should I add a few pumps of fresh grease with a grease gun or pull apart the frame and clean all the joints before adding fresh grease? How do these zerk fittings work?

Answer 1

A Zerk fitting is a hollow piece of metal with a one way valve in it. This one way valve (as long as it is working correctly) allows grease to flow in through the center and into where it's needed. The one way valve is a spring with a small ball on the end of it, which is captured in the end of the fitting. When the grease pressure from the gun stops pressing against ball, the spring presses it back up into place and seals the end, so grease cannot escape and dirt/grime cannot enter. Here is a diagram of what it looks like:

In most cases, you should be able to pump new grease into through the Zerk and displace the old grease without issue.