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I am in the middle of rebuilding the differential on my 33 p/u. What grade of ring gear bolts should I use ?

I would use at least grade 8 bolts though I doubt the originals were that good but it would be embarassing to have them sheer off in the middle of the intersection.
Tony

Originally rivets I think.

I hesitated to respond to this because my axle (1936, very similar design but not the same), is not on the road and proven yet. You would want at least grade 8. No matter what you do it is going to be a bit shaky.

The trouble is that the holes are probably swelled a little oversize in the case as it is softer than the ring gear. Also, the hole size is not super-consistent, and it is likely that a few bolts will take the brunt of the abuse. It was designed to be riveted and should be riveted, but in my experience, no one will do it.

I used an old trick that was often used on threaded ring gears that would loosen up. I put loctite on the ring gear flange. I started with some grade 8 bolts that had a long unthreaded area so it would align the holes as good as possible. I stacked washers to get good pull, and assembled and torqued it QUICKLY so the loctite would not have a chance to start setting up. It happens faster than you think. I torqued them to all the grade 8 bolts could stand, and let it set a few days to cure up.

Then, I took those bolts out one at a time, and replaced them with aircraft close-tolerance shear bolts and more loctite. The shear bolts wont take as much torque as grade 8 (though they are about grade 8 tensile strength), but they are slightly larger in diameter and fit the holes a little better. They are also available in many more lengths than grade 8. I used shear bolts, washers at both ends, and jet nuts. For best strength you don't want any threads up inside the hole, and you want at least 2 threads sticking out the end of the nut. There are a bunch of available lengths of the unthreaded area on the bolts, and there are 2 thicknesses of AN washer available. You need a washer on the nut side, I also have one under the head because there is not as much contact area under the head as I would like.

The nuts have to be on the ring gear side to fit on the 36. They look like they would fit the other way, but they don't (probably on your 33 too).

On the 36, I used NAS6605-14 bolts, MS21042-5 nuts, AN960-516 (thicker), and AN960-516L (thinner) washers. This is 5/16-24 hardware. The -14 on the bolts indicates the unthreaded shank length, 0.875 inch. A -13 would have less unthreaded length, a -15 would have more. You can adjust things a bit with washer thickness.

There may well be better choices from the aircraft world, but I don't know how to figure it out.

Or, you could get non-aircraft close-tolerance shoulder bolts. Most of these take a nut one size smaller than the shank. I'm not crazy about it. McMaster-Carr has some that take a normal sized nut, but you still don't have nearly the choices in unthreaded length that you do in close-tolerance aircraft shear bolts.

I don't have a lot of confidence in this. Maybe someone else has better ideas.

Thanks for the the info bloo. I will use your method . this will be a new ring gear against a used differential case. I found the bolts online (shipping costs more than the bolts)

Measure carefully. the parts may not be the same thickness as mine.

Bloo's approach is a good way to go. The key is that the clamp force developed by the bolts has to create enough friction force so that the ring gear will not move on the carrier.

That is the way most bolted joints work. The bolt is not loaded in shear, only tension. The friction force is what prevents movement.