Why would a brand new dogbox pop out of gear?

Built a brand new dogbox. Everything setup fine, tested fine, works fine.

10 minutes into its first race, it starts popping out of 3rd gear. Issue happened when in gear, and getting on and off the throttle very abruptly, it would pop out.

All other gears are fine, 1st, 2nd, 4th, 5th, they all held firm, no problem with any of them.

Post-race, tear it open. 3rd gear dog teeth are rounded off, not all the way. They looked like they still had usable life. But there was very obvious visual wear on those teeth.

4th gear had some slight wear too, but not nearly as bad. All other gears still had sharp edge on the dogs.

Rebuilt the box with new 3rd and 4th gears.

It tested fine, shifted great, felt great, no obvious issues of any kind.

First minute of the first race on the rebuilt box, same problem starts happening again. This time it's happening even when hard on the throttle, when the gear should be holding the strongest, it still pops out. Again only 3rd gear, no other gears.

Trying to hold the shifter in gear doesn't work, it forcefully pushes itself out of gear. Even

I don't think it's driver error because it's only happening on one gear, rest of gears still look new. And it's happening so quickly, I don't think bad shifting habits would ruin a gear THAT fast.

I'm thinking it's a build/assembly problem, but don't know what could be causing that.

Before spending time and money rebuilding the box again and risking another wasted race, I'd like to figure out the root cause on the bench.

I was going to say torque reversal (like getting on/off the throttle). Since that is exactly what you said was happening. You might want to look at the fork detents preload) if it's an H pattern, or perhaps the forks aren't positioned / aligned correctly. A normal rebuild may not examine/adjust the fork positions.

As David said, look at the forks and where they are holding the gear, it may be that the forks are holding the dog ring slightly proud of the gear so it isn't getting full engagement. If it is a Sequential box, it may be that the selector barrel hasn't been machined correctly and isn't allowing for full travel for the forks in that gear.

Which gearbox is it, there may beknown issues with it.

On top of the issues mentioned by the others, is it possible it has a bad bearing, or the bearing isn't being held correctly in it's housing, allowing the shaft to float axially?

Yes, it is an H-pattern. I was thinking about the shift forks, they're the only thing that makes any logical sense. So that helps, I'll take a closer look at those.

Bearings are new and good, I measured the preload myself, probably a dozen times to be sure.

Box is a Ford MTX75.

Yeah, I managed to partially assemble the box and take a look with a mirror. There's the problem, that is "fully" engaged in 3rd gear. But the ring is around 3mm short of where it needs to be. And you can see the chip out of the dog tooth, that just happens to exactly align with the engagement point of the ring. So it's clear that's exactly where it's making contact under operating conditions.

Next question is what to do about it. Nothing about the shifter is adjustable. The only solution I can think of is to get some new custom parts made.

Perhaps the gear stack was assembled with an incorrect spacer (thickness and/or location). If the fork position can't change, then it must be the gear location that is wrong.

Here's a pic of the kit. 1 & 2 are machined into the input shaft, they don't move. #3 is the problem gear, it just butts up against #2. There are no spacers or anything that go there.

To solve the issue, #3 would have to move to the left. But, Everything else lines up on both shafts though and moving #3 would throw everything else off.

I'm starting to conclude that it was designed or machined wrong, and I need to go back to the manufacturer.

It's more likely the fork is bent, but what has the gearbos/chainset supplier said?

That drawing looks like the spacing between 1&2 and 5&Reverse are slightly narrower than the spacing between 3&4. The spacer for the driven side of 3&4 looks like it has two smaller spacers to make up the total gear spacing. If the dog-driven hub also has a spacer, then I wonder if one of those spacers is supposed to go between 2 & 3 on each shaft.