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Grease Zerk Threads

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  • Grease Zerk Threads

    I've searched the forum and didn't find anything about this but i'm sure i've seen it come up before...

    I have a set of assorted grease zerks from HF. The ones marked "1/8 - 27" are easy to figure out: 1/8 npt and they work fine. The other ones are marked "1/4 - 28" which is, of course, NF mechanical thread, except that the actuall diameter of the fitting is way over 1/4 inch. There are no pipe taps that were 28 tpi from what i could find. Just by chance i happened to find that it fits perfectly in a "3/16 female tube" threaded end. That is to say i tried it in a fitting that adapts from a "3/16 female tube to M11 1.5 brake line" that i had sitting on the shelf. Fit perfect and tightens up great to the 3/16 female side. What size tap should i be looking for here?? I looked at McMaster for something like this but the only 3/16 listings i could find were mechanical threads and these are more like pipe thread that seals as you tighten it.

  • #2
    For 1/4"x28 grease fittings I use a regular tap as I would if it was a bolt going in the tapped hole, it always seems to work fine. Now I'm wondering if I've been doing something wrong???
    Hopefully someone who knows for sure will chime in.


    • #3
      I know mechanical threads don't seal as well but if i had a mechanical thread tap that would work even semi ok with the zerk, i'd use it. There won't be any appreciable pressure behind the fitting. I just need the fitting to shoot some grease in a hard to reach area. Thing is, the 1/4 -28 zerk is nowhere close to a 1/4-28 hole. Its tapered so the outside diameter changes but at its smallest point its about 1/2 inch! Maybe they were mislabeled...


      • #4
        sounds likt it could be a BSP thread - 1/4 BSP is however 19 TPI. 1/8 BSP is 28, so probably no help... Have a look here on info on BSP threads



        • #5
          Sounds like a mis-label to me. Probably no big surprise, as you say the assortment came from HF. I use 1/4-28 zerks frequently, and they are just that...1/4-28.
          Arbo & Thor (The Junkyard Dog)


          • #6
            Grease fittings are usually available in 1/8-24 NPT pipe thread and 1/16"-27 NPT pipe thread. They are probably 1/16" pipe, which is almost 5/16" OD.

            Zerk fittings are available in straight or tapered pipe thread and also in fine thread machine screw sizes such as 10-32 and 1/4-28. There are also press in fittings that are close in size to the screwed fittings. The straight thread fittings bottom out on the shoulder for a seal, some may have a thin gasket.

            They were probably out of 1/4-28 when they filled the box, and used what ever was handy.
            Jim H.


            • #7
              I bought some packaged (also made in china it turns out) 1/4-28 grease zerks that carried the Lincoln brand recently and used one on the back gear housing on my bridgeport mill, so I had to drill and tap the hole. They were, indeed, standard 1/4-28 machine thread. I think you have something in the wrong slot in the package....or else they doubled up on some pipe-threaded zerks and forgot the 1/4-28 zerks.

              Paul Carpenter
              Mapleton, IL


              • #8
                Thanks guys - PCarpenter you hit it square on the head ... "I think you have something in the wrong slot in the package"

                I'm a moron! After i found that the zerks labeled "1/4-28" fit the 3/16" tube adapter i decided to find out what size thread that would be. Hey, what do you know, 3/16 tube fittings have 3/16 hose barbs and 1/8-27 pipe thread. Thats when it dawned on me ... they mixed the two zerks up. I assume who ever filled the package didn't realize one was a pipe thread and one was mechanical so they filled the package based on appearence. The zerk that has the smaller thread must be 1/8 while the larger must be 1/4. Of course this is not the case with 1/8 pipe thread compared to 1/4 mechanical thread. I can't believe i didn't see that last night; i must have been tired i guess.