There were fixes that can be done to stop it jumping out of gear if it does, and some of them can be done pre-emptively if you want to be safe. There were a few factors that contributed to gear popping on Allis's:
- The shift forks didn't have much meat on them, and were prone to both bend slightly and wear down over time. Especially if it saw a lot of hard forcing/grinding into gear. Not helped by the fact that on the earlier 190 (non XT) and smaller units they had such long sticks that you could get a lot of leverage on. Allis came out with beefier 3rd-4th shift forks later on that had a little more meat in the right spots to reduce wear and be harder to bend. Any shift fork you buy these days for one will be the newer style. They're readily available - both from Agco and aftermarket sources (DJS Allis is a good one, this site also offers them). And pretty easy to install. It's the same fork for the 180 through 200.
- The stock detent locations barely allowed full tooth face contact when new. And as soon as they started to wear, when linkages started to get a little sloppy, when shift forks and collars started to bend/wear, and when the gear stack got a little loose, they wouldn't engage for full face contact. This caused the gears to want to pop out when being pushed by a load/running downhill. Allis issued a service bulletin and offered 'deep shift' rails to get a little more stroke and tooth engagement. The Allis service bulletin and 'deep shift' rail they offered only had something like 60-thou additional stroke. Most folks fixing them these days go further than that: 0.100" or so is more common, to get nice deep engagement. The deep shift rails aren't available from Allis anymore, so folks these days just turn the original shift rail to get a new face and die-grind a new detent location. You want to make sure you have enough stroke before you assume you can go 0.100" or so. Usually you do, but it's worth checking.
- Allis issued a service bulletin for a stronger detent spring to better hold the gears new detent location. The specific spring Allis offered isn't available anymore, but it's pretty easy to find & source a slightly stiffer spring. Just make sure you get a good quality one that will hold its stiffness over time. "Dr. Allis" is an Allis expert who floats around here sometimes and more on the 'Unofficial Allis Chalmers Forum' website, and one of his fixes is to put a second smaller spring inside the original.
Other fixes were/are available (new gears, new shims on the shafts & countershafts for better engagement, etc.), but the above fixes will solve 90% of Allis gear jumping issues. As long as the gears get full face contact, they should last forever. The check Fritz mentions above is good advice. If the gear stack is loose, it really should be shimmed accordingly and the thrust washers checked. You'll also note in the service bulletin below that Allis offered a thicker snap ring to tighten up the gear stack. But those are a bigger, tractor-splitting and tranny tear-down jobs. If your 200 isn't jumping yet, you're probably fine. Just don't force/grind gears. If you want to play it safe, it's pretty easy to change the shift fork for a newer/beefier one. And while you're in there, you might as well grind a deeper detent location and put a stronger spring in. Just be careful when removing the shift rail, as the spring-loaded detent ball will go flying across the shop if you're not careful and ready for it. All fairly cheap & easy to do without splitting the tractor. Only the top cover has to come off. Just make sure you have the shifter in the correct gear when taking the cover off (I can't remember which gear it has to be in, but it has to be in a specific gear to come off & on). You always hear that its a problem with 3rd/4th gear, and thus the 3rd/4th shift fork & rail are the ones that are typically changed & modified. This is because those are the most common gears to use for almost all field work. It's not unheard of for other gears to have this issue, but very uncommon.
Below are some pics from one of the Allis service bulletins for reference. I wouldn't get too fussed about reading them and following them precisely, however: The fixes mentioned above are often all that's required.
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