Leaf Spring Center Bolt / Pin - what diameter?
 

Napa should have them, either 5/16 or 3/8.

I've found those sizes, but which one is the correct one? If I go with the 5/16" then I have a 50/50 shot, but they could be too thin and with the 3/8" they could be too large. I could buy both sizes, but I need two sets and dont want to get stuck with special ordered parts I cant return.

just measure them.

My local Napa keeps them in stock, no special order needed. They should be 3/8 if it is a later model. You should be able to see the bottom of the bolt through the leaf spring to verify what you have, same front or rear.

Am I some.kind of maroon to you Tony? If I could get a tape on them I wouldn't be here posting up.

Napa here doesnt have them in store. They offer Rockford brand center bolts; never heard of them before. I clicked on each individual item they show and they dont give the bolt size for some weird reason. And like everybody else it says they have to be ordered for in-store pick up. I guess floor/shelf space has become more and more of a premium. At least around here.

If your local NAPA causes that much trouble, I suggest you get them online. I've only needed these bolts for my CJ-6, and they were 5/16" size. The CJ springs are really narrow (1.75") and I would guess that the FSJ springs would use 3/8" bolts.

I bought these - https://www.amazon.com/Rancho-RS8127...80M/ref=sr_1_6
and these https://www.amazon.com/Rancho-RS8128...3A/ref=sr_1_11
and threw the 3/8" bolts in the junk bin, IIRC.

eTrailer has them cheap, but maybe not much cheaper than the Rancho ones when you add shipping.
https://www.etrailer.com/Trailer-Lea.../516-25tb.html
https://www.etrailer.com/Trailer-Lea...ne/38-4TB.html

Then stick a 5/16 or 3/8 bolt up there and compare it if you cannot tell the difference.

Jeep used both, depending on year/spring rating/etc.
Who knows if your springs are still stock?
I have seen both sizes used on SJs

Yeah, the head goes into the seat on the axle, no? Meaning the end of the bolt should be visible in the hole in the clamping plate???

Nut could even be removed?

Yes, on the rear. would need to clean out the hole to see in in there, easier if you take off the wheel also.

3/8
fine thread.