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I am trying to find the right size grease fitting for the rear spring shackle pins. They are definitely not 1/8" which is all I can find in the parts store.
I think they are 3/8-24 but am trying to confirm it. No parts stores in my area have that size.
I think you are looking for Ford Part 87918-S8 which is 1/8-27 pipe thread 90 degree grease fitting. These should be functionally equivalent: https://www.napaonline.com/en/p/BK_7151030
I have those and they are too big for the hole. I can't get the thread to grab. They just won't thread in. I will buy another set and see. I used a thread checker and it showed 3/8-24 but it probably isn't right.
There are a few things that could be going on here. First, these fittings use pipe thread which is completely different standard than fastener threats. So your 3/8-24 is utterly meaningless in this circumstance. Presuming that my Ford book is correct and I got the right fastener and you have a comparable one and you state that it doesn't fit, I would wonder if the kit you have and the pins might have been made overseas and utilized metric threads. If you're a little bit adventurous you go to your local auto parts store or your local farm supply store and get a variety pack of metric grease fittings and see if you can find one that fits.
I sent an email to Chuck where I got the parts with these same pictures. Hopefully he has the definitive answer.
I removed the old grease fitting and it threaded in but not without some effort and galling. I am definitely going to need a tap to clean that up a bit. My new fitting which still won't fit is pictured next to the old one. Maybe the threads are different. I assumed that the 1/8" pipe thread fitting are all the same? I can't visibly see a difference in the thread pitch.
The smallest things sometimes take the most time....
Just about any of your big box stores should have what is known as a "1/8 NPT Tap". These are a tapered type thread and will mesh/deform to make a seal. Don't go crazy trying to clean threads up as that will widen the taper making the fitting seat deeper (and if the taper becomes to deep, the fitting won't be able to tighten to the taper and be loose).
I think your on the right track with the 1/8-27 pipe thread. It is a tapered thread and some fittings don't taper small as much as others. Chasing the hole with a 1/8-27 taper pipe tap will enlarge the entrance just a bit. There is a straight pipe thread also, so don't get that tap. Make sure it is a taper pipe thread.
FYI
Chuck wrote me back really quickly. He is the best!
Here is his answer and it lines up with all the help you guys have offered:
This sometimes happens with pipe threads. There is a range that the threads can be cut/tapped, and sometimes the male is on the high side, and the female is on the small side. Either try a different brand of grease fitting, or get an 1/8"-27 pipe thread tap, and clean out the holes a little. You could try to run the grease fittings through a 1/8"-27 pipe die as well.
Off to the parts store as soon as I can get free to get a tap and die. Thanks!
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