3 rail, 4 synchro gearbox assembly question

While waiting for my Healey to come back from the panel shop I have started to reassemble a Dolomite gearbox/OD that has been under the bench in bits from when I had a Vitesse.

When all the bits are assembled onto the mainshaft there is 1/8" fore-aft free movement in the 1-2 synchro hub on its spline. The synchro hub assemblies are both NOS as are the gears so wear is not a consideration and everything works correctly apart from the hub moving on its spline. When pushed by hand into either 1st or 2nd gear, the three detent balls are visible and are very close to escaping. This is how it came apart (was a badly rebuilt box prior) and I'm guessing there are some washers or spacers missing to hold the hub from moving.

Has anyone rebuilt one of these boxes and found a similar situation? 160018, 137834 and 137775 shown in the pic below are present yet 156911 can move freely back and forth.



Andy.

You're saying that the center hub (the part of 156911 outside the blue box in the illustration above) is moving on the mainshaft, right? I believe that is not just normal, but necessary. As the shift fork starts to move to select a gear, the hub slides on the shaft until the side comes up against the 3 fingers on the synchro ring; which is what forces the synchro ring against the matching cone on the gear being selected. Only then does the outer ring start to move on the inner hub, to mesh with the dog teeth on the synchro ring and then on the gear.

But I should hasten to add that my experience is only with the TR4-6 box, which although very similar is not the same.

The clearance might be less with everything new, but as I recall 1/8" is pretty typical for the 'big' TR box. It goes up as the synchro rings and matching cones wear.

I believe having the balls partly exposed when in gear is also normal.

I had the same question about both a TR box and a Spitfire box. They both have about the same amount of play and both work great after the rebuild.

Thanks for the replies. I had a bit more of a look at it this afternoon and have a better description of what is happening.

The concern I have is that if I pull the synchro ring fully into (say) 1st and then pull the hub back towards 2nd, the balls partially come out which wedges the hub forward into 2nd synchro ring. The balls are more than half exposed which converts the radial ball/spring force into axial force. The synchro ring is then rubbing continuously against its friction face, held there by the three balls which are almost, but not quite able to pop out of their holes.

The above situation will not automatically occur as normally the hub is pulled and held in the direction of the selected gear. I am concerned that there is nothing other than the friction of the balls pressing outward on the synchro ring to stop the hub jiggling about and getting into the situation described above which will drastically shorten the life of the synchro cone.

After completing the mainshaft assembly I now see that the 3-4 selector can doe exactly the same thing.





Andy.

When together, I don't think the shift forks allow that much movement to allow the balls to pop up.

I just went downstairs in the basement where I have a TR3 gearbox that came out of the TR3 I'm now driving with a Toyota Supra box. I took this box apart and only refreshed it with new bearings and synchro rings with intentions of using it. It hasn't had any other parts replaced. The movement of both synchro hubs are little more then a 1/16" as nea ras I could measure with a ruler. Below is a picture of the hub while engaged with the synchro ring showing the exposure of the detent ball.

CJD said:

When together, I don't think the shift forks allow that much movement to allow the balls to pop up.

Click to expand...

Ditto. The stops are in the top cover.

Thanks again for the replies.

Re the shift forks limiting the movement- the 1-2 shift definitely goes all the way and is evidenced by a wear mark on the side of the old 1st gear matching the shape of the engagement ring.

It seems like this is perfectly normal behaviour and perhaps a design that wasn't quite as well thought through as it could have been so I'll continue to reassemble the box and od once I hunt out a couple more gaskets and a tail shaft bearing.

Appreciate your input

Andy.

You said it was a poor rebuild ... any chance the stop is missing from the top cover ?

It never ran after the rebuild. I got it off one of those older gents that tinkers with brit gearboxes and tries to make a buck out of it. He sells 'rebuilt' trans/od's which I discovered afterwards have a reputation for leaking like sieves and so on. I smelt a rat when I took out the drain plug to see if it had oil inside and found a lot of metal fuzz on it. To be fair he did offer a refund and was very decent about it but I really wanted an OD box at the time and didn't care so much for the trouble or cost.

I'd be fairly certain the top cover/selectors have never been apart since new and they otherwise seem to work perfectly. If I apply torque to the gears so the dovetail engagement teeth find their home position, the engagement ring is all the way up to 1st gear so this is definitely correct.