Drill and Tap Size for Grease Zerks

[Rayster]

There are many different types and with that are different thread pitches. 1/4-28 is most common (that I'm aware of).
Drill for 1/4-28; I would use a 7/32

 

[Pooh_Bear]

It will probably be 1/4-28. I'll have to pick some up tomorrow.

Pooh Bear