I dunno where your parts originated, but over the years, I've found a lot of
mating threaded parts from China or India to be completely unsatisfactory.

You were probably fortunate that you had the threads let go at low torque,
as I suspect that failure was a predetermined fate. Roadway use of trailers
can produce some harsh shock loads, so it's better to have the parts fail
before use IMO.

It seems retailers will stock anything that has a decent profit margin, now
a'days.

When I examine new parts, I find threads which exhibit so much free movement
that the parts aren't suitable for any purpose I can think of.

When machine parts have oversize female threads, I'll see if I can retap to
a slightly larger fastener, even if the fastener may need to jump from
metric to inch or verse visa.
Loctite and other "fixes" are often a poor choice where there isn't enough
engagement of the threads.

I don't know what the popular overseas methods are for making threads in
holes, but I do know that taps don't grow as they wear.

Poor axial alignment is another issue that puzzles me. I could only tap a
hole that far off if I was attempting to run a tap by hand, behind my back.
Since these overseas manufacturers don't reject parts due to poor quality,
it's up to the end user to make the right decision wrt proper/safe use of
those parts.

--
WB
..........


"Winston" wrote in message
...

I looked it over and decided that I could slice through
the middle of the nut flats with some Dremel discs.
(It took 5 discs, one shattered).

After slicing through two opposing flats and attacking the
gaps with a chisel, I managed to split the nut into two
semicircles and free the hitch ball without damaging the
bumper.

The autopsy shows the nut thread and the stud thread
look surprisingly intact. The plating on the stud
is down into the copper for the length of the nut
on both walls of the thread and the crown of the thread
is much shinier and flattened in relation to the crown
of the non-stressed part of the stud.

The minor diameter of the nut is flattened for about
100 degrees and much sharper for ~260 degrees

This is supposed to be a 3/4-16 thread.

So the major thread of the stud should be no less
than 0.75" dia. It measures 0.744". The nut is no
longer with us as such but it appears that the minor
diameter of the nut widened and began slipping over
successive crowns of the major diameter of the stud.

I measured the thread on the replacement ball and
found the major diameter of the stud to be 0.743"
instead of 0.75". The minor diameter of the nut
should measure no more than 0.6823". The new nut
measures 0.689" I.D.! So our fasteners appear to
be sloppy to the tune of about 0.007" per side!

The tightening specification is 160 ft. lbs.
I don't know how much force I was applying to the nut
but I would be very much surprised to learn it was
much over ~40 ft. lbs when the fasteners failed,
given the short lever arm of the ratchet and the
remaining muscle tone of a weak old man.

Given that the thread on the replacement ball appears
to be even sloppier than the thread of the ball from
O'Reilly Auto, I will assume that it will fail at
somewhat lower torque than did the O'Reilly part.

I conclude that Ed is right.

These parts are junk.

Thanks for your patience and advice.



--Winston