Any manual trans rebuilders on board?

Discussion in 'U-shift em' started by 436'd Skylark, Feb 28, 2021.

  1. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    I'm working on the transmission for my Scout. It's a borg warner T90A 3 speed. The trans would grind a bit on an upshift into second but would still engage with out much drama. Third was fine and first is non-synchronized. It wouldn't pop out of any gear. I got the trans out and apart. I figured the second gear synchro would be shot and it would be an easy fix. The synchro and second gear have some wear but hardly need replacing. The only issue I found was the input shaft is scoured where it mates to the main shaft. See the pictures. Note there is no scouring where the needle bearings ride on either shaft. I had assumed the grooves were from wear and tear, but is it possible they are machined into the shafts for fluid retention? There is some evidence of scouring but i wouldn't be opposed to polishing the high spots and calling it good..

    Also, while shifting into second gear the action of the synchros would actually pull the two shafts apart, not apply pressure between the two.

    Second is the synchros. Neither showed any significant wear. The only potential issue is the surface on the gears that the synchros apply the pressure to.. they have a mirror like finish. Seems like it would hinder the synchros from doing its job.. i'm half tempted to rough that surface up with some emery cloth and throw it back together. See pics.

    I haven't found much of anything to cause the grinding issue. I did swap the fluid to the stabil GL4 specific with no change. No other trauma inside the trans. Some wear to the brass thrust washers for the cluster gear but nothing outstanding..

    Any insights?


    Attached Files:

  2. philbquick

    philbquick Founders Club Member

    All the pictures look good. That rough surface is normal, it's not a critical surface so they don't finish machine it. The surfaces the brass rings sit on are tapered so the finish is fine. The synchronization occurs when the 3 spring loaded dogs press against the brass ring which has a clutching effect due to the taper. The spring tension of the circular spring has to be enough to push the dog against the synchro sleeve so the bump on the dog creates a detent. That detent creates the pressure that pushes against the brass ring. If the bump is worn off the dog it wont press against the brass ring hard enough to
    synchronize the speeds of the shafts and grinding will occur. Things to look for: 1) worn or cracked brass rings, the teeth on the brass ring and the teeth on the gear should be 1/16 apart, if not the clutching effect will not work, also if its cracked it will bottom out and will also not work. 2) The bumps on the dogs should be pronounced and not rounded. 3) the groove in the synchro sleeve that the dog bumps rub against should not be rounded. 3) the end play on the shafts should be under .005" and the parts on the main shaft should be under .005". You mentioned that it looks like the shafts are moving in relation to each other, that should not happen. That means a snap ring is missing, broken, out of place, or a snap ring groove in the case is cracked or wallowed out. The shafts should have little lateral movement. Hope this helps.
    69GS430/TKX likes this.
  3. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    Thanks! This was the exact reassurance I was looking for. I will inspect the dogs carefully as I don't remember much of a bump on them.

    As far as the shaft movement, I will shim it as needed. I had some concern that the two scoured up surfaces between the main and input shaft would cause enough friction to somewhat jam the shafts together and not allow them to spin freely. I thought that extra friction might have been overworking the synchros and causing the grind..

    Again, thanks for the insight!
  4. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    I looked at these parts before work. The dogs have a nub of sorts still. I assembled the slider and synchros. There is a slight detent but it is easily overcome just by hand. Less than a 1/16th clearance between the synchro and gear too.

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