Sm420 Synchro Damage Photos

My CJ6 has an SM420 transmission that has been shifting poorly. I pulled the transmission and found a bit of wear on one of the synchros. I'm pretty sure it's not supposed to look like that.

I bought this transmission used (a long time ago) and it appears to have multiple wear issues that make me this it was just a well used transmission. Fortunately I have a replacement transmission that looks to be in much better condition, so I'm just installing that one instead of trying to fix this one.

I don't think I've seen a photo of a worn synchro before so I thought I'd post this one to let others know how it looks.

 

Looks like your dog got loose.
My t-90 did that, the spring broke and the dogs got loose.

 

The only good thing is you can definitely see the problem. It's tough when you tear one down with shifting problems and everything looks good.
Thanks for posting the pictures.

 

I see multiple possible issues here.

First, the syncro is worn. It is riding up almost against the gear. This will cause poor synchronization and possibly the damage you see to the front of it.

Assuming you haven't removed the front bearing retainer ring, there is too much of a gap between third gear and the 3rd/4th clutch sleeve. This will also cause the damage you see. Possible causes are input/mainshaft bearing failure or a snap ring failure.

One or a combination of these issues will allow too much clearance between the shift plates and the syncro. This will allow the shift plates to ride above and/or on the outside edge of the syncro instead of inside the slot which will eventually allow the syncro to spin and the plate to eat it up.

 

jeepdaddy2000, I did remove the front bearing retainer, and that did allow the gears to separate a little bit.

It's possible the bearings are also worn. Once I saw the damage I just decided to clean up and install my backup transmission.

This transmission was always hard to shift. I figured it was just because it was an SM420 and I did not know how it should feel.

I'll report back how the backup transmission works once I get it all back together.

 

Be sure to inspect the other transmission for the same issues.

On a side note,What little I can see of the trans, a new set of bearings and syncro's should put you back in action, barring any other issues, of course.

 

The syncro blocking ring is spinning on the hub where it is supposed to be “pinned” to the hub by way of three dogs that are missing. That notch in the photo is supposed to have a flat-ish plate, or dogs (3 total?) that locks the blocking ring to the hub its riding on. There should be corresponding notches in the hub where the dogs fit in, and spiral retainers inside the hub that hold the dogs in place.
With the blocking ring spinning on the hub, you effectively have no syncro action. The blocking ring is supposed to be pinned to the hub/mainshaft, and the taper is supposed to slow/brake the corresponding gear to assist with matching gear changes. With it spinning, there is no braking action.
-Donny

 

One of the missing "dogs" is clearly seen in the photo. It is to the right of the syncro slot and under the sliding ring.