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Grease fitting sizes and threads
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JeepSaffer View Drop Down
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    Posted: 19 Sep. 2016 at 10:09am
Just about all the grease (zerk) fittings on my project are missing or otherwise need renewal. I am planning on replacing most of them.

I found a thread on the topic 

www.thecj2apage.com/forums/zerk-grease-fittings_topic26930_post237948.html?KW=Zerk+thread#237948

and in it Lee (MN) says the following:

Originally posted by Lee (MN) Lee (MN) wrote:

1/8 pipe threads, smaller ones are 1/4-28 machine thread, same as a 1/4 fine thread bolt.

                      Lee

I am a little confused because he suggests that the smaller fittings are the 1/4, not the 1/8??? Maybe this was just a typo. I also found the following table on the web:







This suggests that 1/8" NPT is 27 tpi, and 1/4" are 18 tpi?

Now i'm even more confused Confused

Can anyone clarify this for me?
1. What are the correct zerk sizes?
2. What tpi is correct for each size?
3. Are they indeed cut to National Pipe thread standards?

I have to get this right because we are not in a standard Imperial system here in South Africa. I may well have to order these and so I need to know exactly what they are so that I can specify them in the order.

Thanks,

Mike



Edited by JeepSaffer - 19 Sep. 2016 at 10:12am
1948 CJ2A #204853 in South Africa
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PaulEdm View Drop Down
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Points: 130 		Post Options Post Options   Thanks (1) Thanks(1)   Quote PaulEdm Quote  Post ReplyReply Direct Link To This Post Posted: 19 Sep. 2016 at 3:21pm
The trick is that 1/8 NPT really is *bigger* than the 1/4" machine thread (which is actually 1/4").  The 1/8 NPT is a tapered thread, while the 1/4" is a straight thread.  I think.

-Paul
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JeepSaffer View Drop Down
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Points: 1181 		Post Options Post Options   Thanks (0) Thanks(0)   Quote JeepSaffer Quote  Post ReplyReply Direct Link To This Post Posted: 19 Sep. 2016 at 4:06pm
LOL

Thanks Paul. So If I've got this straight, this is what I currently understand:

There are two zerk sizes on the CJ2A:
1. "Larger" - This size has the technical name of 1/8" NPT and is a tapered thread with 27 TPI.
2. "Smaller" - This size has the technical name of 1/4" UNF (unified fine) and is a straight thread with 28 TPI.

Rather confusingly the 1/8" NPT is not actually 1/8" but more like 13/32", and is therefore larger than the 1/4".

Let me know if I've got the right end of the deal here. I've already crossed one set of threads by guessing incorrectly Ouch

Thanks,

Mike
1948 CJ2A #204853 in South Africa
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Points: 130 		Post Options Post Options   Thanks (0) Thanks(0)   Quote PaulEdm Quote  Post ReplyReply Direct Link To This Post Posted: 19 Sep. 2016 at 5:17pm
I think that is correct.  At least, I bought a pack of zerks that contained those two sizes, and so far I've been able to replace every zerk I've encountered on my Jeep during my rebuild.

The NPT designation come from pipe sizes (often labelled NPS), which are equally confusing.  a 1" NPS pipe has an OD of 1.315", for example.  The 1/8" NPS OD is 0.405".  The actual size matches the nominal size only at 14" and above.  Each size is available in different weights, with different wall thicknesses, and therefore different inside diameters.  I think that the nominal size generally is close to (but not equal to) one of the available inside diameters.  But that's not true in the sizes under 1/2".  Since a 1/8 NPT thread is cut on a 1/8 NPS pipe, it has a diameter substantially larger than 1/8".

-Paul
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SE Kansas 46 CJ-2A View Drop Down
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Points: 3190 		Post Options Post Options   Thanks (0) Thanks(0)   Quote SE Kansas 46 CJ-2A Quote  Post ReplyReply Direct Link To This Post Posted: 19 Sep. 2016 at 5:30pm
Some history. Waaaaaaaay back when the pipe threads were being established the measurement was based on the inside diameter of the pipe. The pipe at that time was mostly made of cast iron and the walls tended to be thicker than modern pipe steel because cast iron did not hold the pressures that steel pipe does so it needed to be thicker. Now you know.

Of course even now there are different thicknesses of pipe walls depending on what it is used for. Standard Schedule 40 1/8" pipe has a wall thickness of .068" while Schedule 80 1/8" pipe has a wall thickness of .095"
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Points: 106 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Stray Bullet Quote  Post ReplyReply Direct Link To This Post Posted: 19 Sep. 2016 at 6:39pm
I purchased a few 50 piece grease fitting assortments from Harbor Freight #67570. I've replaced the fittings on a dozen or more vehicles with these and they work just fine. They come with 30 1/4" TPI and 20 1/8 NPT.
Rog
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Points: 551 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Jeff_Davis Quote  Post ReplyReply Direct Link To This Post Posted: 19 Sep. 2016 at 7:40pm
Related to this replacement topic, a bit of a warning on some of the reproduction "stuff" coming out of China and India... I have noted in several instances that the zerks are metric threaded, and fitting a proper SAE replacement zerk was impossible. This was noted on some of the suspension threaded leaf spring shackle bushings and pivot eye bolts supplied from either Crown or Omix-ADA.
So, if you find a unit that a SAE threaded zerk may not go, but the threads appear fine, it maybe one of these crappy reproductions.

I have also noted this same problem on imported brake wheel cylinders - the bleeding screws can't be replaced with SAE threaded bleeders, and a metric wrench is required to turn the bleeder! Why would a responsible producer of reproduction JEEP parts allow the mix and match SAE and Metric standards??!!

OK... I feel better now...
Jeff
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Points: 1181 		Post Options Post Options   Thanks (0) Thanks(0)   Quote JeepSaffer Quote  Post ReplyReply Direct Link To This Post Posted: 20 Sep. 2016 at 9:04am
Thank you all for your responses!
1948 CJ2A #204853 in South Africa
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