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So I have a 1991 U1250. I know that it has the 8 speed gearbox with the splitter engaging between fourth and fifth. The issue I have is that I'm missing first and second gear. I know this since I can only engage two gears before the splitter kicks in and then I go through the final four gears. The other issue is that it doesn't like to stay in fifth and sixth gear which is essentially first and second. It never really seems to slide in; I have to hold it there. I want to know if this is a problem with the linkage or if it could be a lot worse? How would I diagnose this?
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I've lost my file on the Synchro pieces. I might have to wade through the search function and try and find some pieces of it. Ben has been trough his trans and is probably one of the best sources for you.
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OK, I didn't actually lose my file with the synchros, I was just looking in the wrong place. Sorta the same thing.

The files I have with the falling out bits, refer to the 404-406 transmissions, but the square cabs have something very similar. I just don't have photos of them.
The same deal, however, they hide under the gears, and the forks slide the said gears over them as you shift. They do fall out on the 404-406 gearboxes, but I don't know if it is a problem with the SBU (UG3 / 40 and UG3 / 65) transmissions. Things are more robust in these transmissions, but that does not guarantee that the little buggers can't get loose.

Hillbilly logic : If there are pieces of them in the bottom of the case, they got loose. If not, they didn't get loose. Then, they could still be in place but worn or otherwise screwed up so they are hindering or preventing gear engagement.

An entry form 10 years ago, from tmodler .
Also, I thing I have found the correct term for the little do-hickeys.

Schaltmuffe = Shift Sleeve
Gleitsteine und Kugeln = Sliding Stones and Balls

So, if there is stuff at the bottom of the case, it'll be Sliding Stones and Balls for you.
Somehow, I doubt those terms will launch into common usage.



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OK, photos of the Sliding Stones and Balls from a 404 transmission, in this case. Note that the OP on the thread in question found the pieces at the bottom of his case, but still had shifting and synchro function - although that is with a 404.
Other wear points, as mentioned by Ben, are the guide shoes at the tips of the shift forks. Ben's transmission had some fairly slaughtered examples of them.
There are two forks that shift the gears, and another one above for the Forward / Reverse function. The two lower forks are the issue here, I'd think.

So, If the Sliding Stones and the Balls escaped, then the possible reasons ( I'm Guessing) would be :

-Bent Forks that move the gears 'too far', exposing the SS&B units so they can escape from their pockets. Pretty stout looking forks, however.
-Worn Shaft or Forks (interior of bore) that allows for slop that moves the gears 'too far', etc.
-Worn or out of place detent balls on the Fork / Shaft assemblies, allowing the forks to lose registration, and slide around out of place, again, going 'too far'.
-Worn or missing guide shoes on the tips of the forks, which allow the Forks (and hence the sliding collar) to move 'too far.'
-Worn or broken SS&B assemblies that allowed them to escape in pieces. I think this is less likely, but that is another guess.




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Also, from this photo, your truck is not equipped with the Fording system. If your truck had the conventional Fording system, this fitting (Red Box) on top of your case would have an air supply hose attached to it. As it is, this is a vent. While you are at it down there, I'd pull it off and soak it, brush it, blow-it-out, etc. to get rid of any crud, making sure it is clear and open.

In the Blue box, I'd lube up both ends of the gear selector arm, as well as the shaft that enters the box.

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OK, I was looking for some more information and I came across this chart.
I noticed item No. 6 in the legend - the ‘synchronizing body’.

Hmmm. Could it be that the square cab transmissions use a different system in order to synchronize the gear shifts ?

It would explain why I have not yet come across people finding
Sliding Shoes and Balls in the bottom of their SBU transmission
Cases.
I don’t know.


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One thing that you might be able to check is the condition of the guide shoes on the tips of the forks. They might be hard to see through the window that opened up when you took the side plate off, and you can probably only see one per fork, but it could give you an idea of their condition. You might be able to move the forks and gears around a little by manipulating the lugs on the forks.

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A look at some of the parts, focusing on the synchros and the forks. The design of the 'Sliding Stones and Balls' has changed from the 411-404-406 style shown earlier. The UG3 / 40 and UG3 / 65 use a spring in the assembly.

I'm guessing here, but I don't think these pieces tend to fall out as was seen in the earlier transmissions. Purely a guess, based on the sketchy theory that they don't seem to be an online issue in the photographic record.

The terminology is English is 'Pins and followers'. The drive shaft first. No.s 26, 29, and 32 are the spring / pin / follower assembly . The screen grabs have some overlap in the parts listings, so there is duplication at the seams.



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I notice that the Forward / Reverse Fork has 'Slides' on the tips, but no parts are shown there for the gear shifting forks.
The photos I posted earlier, however, do show a separate piece in that position. It might be a difference between the UG3 / 40 and the UG / 65. Or ?

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