View Full Version : Grease Zerk Threads

07-13-2006, 02:42 AM
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.

07-13-2006, 03:31 AM
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.

07-13-2006, 03:46 AM
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...

07-13-2006, 04:27 AM
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 (http://www.newmantools.com/taps/bsp.htm)


07-13-2006, 08:52 AM
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.

07-13-2006, 09:18 AM
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.

07-13-2006, 01:39 PM
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.


07-13-2006, 01:44 PM
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.