Synchros on 4 speed transmission

I have a used transmission with OD and I have no idea how many miles are on it. On occasion it pops out of first and more frequently jumps out of third.

When first jumps, I just go into second using the torque of the engine to continue what was started in first gear.

When rowing through the gears or on “over run” in third this is more distracting, especially if the revs fly to red line.

How bad is it if I push slightly on the gear shifter while in third gear. I think this might cause some wear on the synchros but how much?

I don’t think it would have any effect on the synchros. Take a look at the assembly illustration. The gear rides on splines on the outside of the synchronizer. As you move the lever from neutral, it moves the whole assembly of the synchro + gear until the synchro makes contact and the assembly synchronizes. As you keep moving the lever into the gate, the gear overcomes the spring pressure holding the two parts together, and the gear slips off the synchro and the synchro moves back a bit. At this point the only way to engage the synchro again is to pull it out of the gate.

What could wear is the bronze shift fork. It sits in a groove within the gear and normally rides in the center of the groove where it isn’t rubbing. If you apply constant pressure to it, it might be in contact and wear down the part.

I’m not certain but I think the springs that prevent the gear from slipping out of the gate are in the lid. If so it should be possible to fix without removing the gearbox, if the lid can come off in place.

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Once a gear is engaged, there is no load on the synchro cones, as the contact is wholly between the engagement teeth on each gear. The synchro cones are only a clutch to match the speed of the two gears during selection. So, you can hold the gear lever to keep it in gear, but this will put extra load and therefore wear, on the selector yoke.

There can be several causes for jumping out of gear. A simple one can be insufficient adjustment on the spring-loaded detent plungers. These are accessible from the top of box, and easily adjusted. If you move the lever through its range when stationary and don’t feel much resistance going in and out of gear, then these adjustments should be checked. A small adjustment can make a big difference.

Using the premise that Murphy loves to dabble in such anomalies, the problem will be internal. If bearings are sufficiently worn, clearances between gear settings will be much greater, allowing gears to move back and forth which eventually works them out of mesh.

Just a note about the engagement of the gears. The synchro teeth are short and half the length is tapered. When engaged, the parallel portions must be fully overlapping so that no part of the lead-in tapers are overlapping the parallel sections. If this occurs long enough, the engagement teeth become tapered and the gears will not stay in mesh.

I hope this helps and others may be able to add more experience and knowledge.

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And I forgot to mention that when engaging third in a very spirited manner, which I do frequently, I sometimes get a crunch. Another press on the clutch and re engagement solves this problem.


A crunch on 3rd can be caused by worn synchro, or by weakened springs between the synchro and gear which sets the breakaway for the gear. Not enough pressure and the gear breaks away before the synchro does its job, at least that’s the way it works on my Moss box. There is no way to address either of these things without taking the box out and apart.

It can also be caused by clutch drag due to insufficient bearing throwout. This can be adjusted unless you have the hydrostatic slave.

The way to avoid it is to shift more slowly to give the input shaft time to slow down. Be sure to move the lever in a strict H pattern, not moving it diagonally.

Shift more slowly!! I sometimes drive like I stole it!!

Another note. I thought the throw out bearing was making noise and the transmission dude noted that when the clutch pedal was in or was it out? The sound was coming from inside the transmission.

So, who’s coming to Colorado for a "work on Gerard’s Jaguar vacation?!!!

My all synchro gearbox jumped out of first when backing off the throttle. I had to hold the shifter in place. Cause was worn needle rollers. Replaced on rebuild. Solved.

This happens also. Now were talking about a Engine out effort.

Hopefully It will last another 5,000 miles? I prolly would put in a Tremec T-5.