My pinion clutch stays in but the pinion gear unlatches, despite a new spring and the end-bolt correctly set up. I'm wondering if the latch sleeve and/or gear are worn, and as the gear and ring gear rotate, the components are "wiggling" and causing the latches to work their way out. Pinion is going to come out again so I can study it in depth. I have a 3J engine sitting in my shed and the pony is good so I will test it on there where I can get a good look at it to see where it's going wrong

Nathan are you talking about the four bolts that hold the pinion sleeve on to the pinion gear? I thought they were more like 5/16 or 3/8 UNF. In any case, if you could establish that you could cut down four new bolts the next size up, and you can drill and tap the gear accurately, then I don't see why that wouldn't work

Which o-ring did you go with? If it's not too late, you could get them to add the 8H o-ring in, then, if the default one is just too darned hard to install, then you could try the thinner one