It is important to keep the entire system in mind.
Shearing a properly supported 5/8" pin of decent quality in a receiver hitch is a very uncommon situation. No matter what strength material your pin is made from, the outside supporting structure is still made of mild steel. This will typically be something like A500/A513 hot roll tube. The hitch receiver generally goes through one more drawing process, similar to A513 type 5 DOM tubing, to remove the internal seam. This will give us a strength of about 65-58kpsi tensile with a 3/16-1/4" wall thickness. What I typically see is that if the pin is VERY strong material like a 150kpsi+, you will start to see tearing in the hitch tube before the pin fails.
This would be on test rigs where you can isolate things to get results in one specific area. The forces this requires are much higher than anyone would typically be willing to put on their vehicle through and would feel frankly frightening. Forces of 40-60k+ to break a decent quality 5/8" hitch pin would likely result in a very bent hitch cross tube. That bent cross tube is going to be a VERY good indicator that you are doing something you shouldn't.
Note: Beware of hitches with a very short distance from the pin hole to the face of the hitch. 2.5" from the hole to the end of the hitch would be what I would call 'normal' for a 2.0 hitch. There are definitely hitches out there with concerningly little material between the hole and end.
What would I recommend for a hitch pin.....
A Curt 21507 is rated and tested to operate it's entire life at 21,000lbs by the manufacturer. It has a lifetime warranty against breakage. That 21k number may seem 'low', but that is including what I would say is a HEALTHY factor of safety to account for the service life and fatigue cycles. These are typically $10 or less....commonly $5 shipped on amazon.
3" x 3" receiver hitches are built for towing heaviest of trailers, and a hitch pin with the proper size and rating is essential for safe towing. This heavy-duty hitch pin has been stringently tested at our Detroit engineering facility to safely tow 21,000 lbs. It easily secures heavy-duty ball...
www.curtmfg.com
MasterLock 1465DAT. These do not have an official rating, but they still do still come with a lifetime warranty against breakage. I like these units because of the integrated retainer that you can't lose, but they still retain a solid pin. These are listed as stainless steel. They are $20 at your local tractor supply, Amazon has them for $11 commonly.
5/8in (16mm) Class III/IV Stainless Steel Barrel Head Hitch Pin. Model No. 1465DAT, product features, specifications, service and support from Master Lock. Shop now.
www.masterlock.com
Not many manufacturers I have found offer a 'rating' or a warranty. I don't think the hitch pin is honestly THAT much of a point of concern as long as it is properly supported. If you want to use something like a grade 8 bolt, make sure that the unthreaded shank extends completely across the hitch. You do not want a threaded section loaded in shear. The reduction in minor diameter makes it much weaker along with the sharp corners of the treads will lead to fatigue cracking over time. ( not to mention the threads chewing up the host tube ).
I hope that helps.
