There is no real "zerk" in the CV joint,at least on the older trucks I'm used too--most have a inverted cone shaped fitting and a needle point tip on a grease gun goes onto it,you have to hold pressure against the tip to get any grease to go in,and its a bear to get at..the inverted cone fitting might be threaded,if it is it might me 1/4x28 thread like a typical zerk fitting,as is used on the sliding slip yoke end,and most front end components like ball joints ,tie rod ends,etc..