Gearboxes

["Slack Alice" Simon]

John D, in the "What have you done today....." thread, says his gearbox "imploded"

If you read this, John, what happened?


And as a general question, what is most likely to go wrong with a gearbox?

I'm asking because I would like to know what trouble I might expect on the Russia - Japan trip.

Generally, gearboxes in normal use seem to go on forever.

I'm asking about 4 speed boxes in particular.   

I think I know that selector springs can weaken or break, and gear levers snap.

What else?

Thanks

Simon

[Chris KC]

Weak selectors are a nuisance but won't stop you driving Simon.

Gear levers do snap - this happened to me in the wilds of Sweden. I drove 26Km in 3rd gear to the nearest village and found a friendly welder - sorted.

The 'problem' with 4 speed synchro gearboxes is that if enough end play is allowed to develop the gears can literally drop off the end of the shaft and seize the box. If you are lucky this may not cause collateral damage, if you are very lucky you might be able to 'coax' it back on by removing the box top and carefully levering. It happened to me in Switzerland and I spent 3 days removing the engine and gearbox, stripping and re-assembling the box, and putting it all back together again before driving on. My advice then is that if you plan to use a 4 speed synchro, have the top off and check for end float and if necessary address it with new bearings or shims. Crash boxes are slower but tougher.

[Terrytuned]

Simon one of the problem with the 3syncro 4spd box is the 1/2nd syncro hub can over throw and the balls and springs jump out and jam the
         hub and you cannot select or deselect 1st or 2nd gears the 2syncro box does not have this problem.
Terry.

[Chris KC]

Simon one of the problem with the 3syncro 4spd box is the 1/2nd syncro hub can over throw and the balls and springs jump out and jam the
         hub and you cannot select or deselect 1st or 2nd gears the 2syncro box does not have this problem.
Terry.

Yes you're right Terry, weak / stuck detent springs are part of the equation too, that's more or less the same thing that happened to mine only the hub also tilted off the end of the shaft and locked in place. It took some dislodging even with the box out of the car. I don't think this can happen if the detent balls and springs are good and the end play is within reasonable limits. My box hadn't seen maintenance in a long while. I wrote a brief account of the incident in A7CA mag 1997D if perchance you have a copy Simon.

[jansens]

Simon one of the problem with the 3syncro 4spd box is the 1/2nd syncro hub can over throw and the balls and springs jump out and jam the
         hub and you cannot select or deselect 1st or 2nd gears the 2syncro box does not have this problem.
Terry.

Been there, done that! You do have to be careful assembling things to avoid that happening. 

I think the question is if you have a working box is it best to leave it untouched (maybe just give it clean oil) because I am sure once you open it up to start replacing things like balls and springs you'll find other things that need doing too (layshaft end float, inter shaft bush and so on).

Simon

[Chris KC]

Simon one of the problem with the 3syncro 4spd box is the 1/2nd syncro hub can over throw and the balls and springs jump out and jam the
         hub and you cannot select or deselect 1st or 2nd gears the 2syncro box does not have this problem.
Terry.

Been there, done that! You do have to be careful assembling things to avoid that happening. 

I think the question is if you have a working box is it best to leave it untouched (maybe just give it clean oil) because I am sure once you open it up to start replacing things like balls and springs you'll find other things that need doing too (layshaft end float, inter shaft bush and so on).

Simon

It's the end float that's the killer. If you have a load of axial slop, fix it.

["Slack Alice" Simon]

Thank you.

I will learn about endfloat and check it out on my boxes.

Simon

[Dave Mann]

Yes after my 3 synchro box locked up in Cornwall I prefer the 2 synchro box.

[Bob Culver]

I have heard of related problem in other cars. I recall a workmate driving 40 miles in 1st in an A40 Farina. But I presume with the Seven the 2nd synchro cone is wedged also, preventing use.

But just what endfloat is the problem? 1st gear within selectors? Or is it selector maladjustment or mainshaft position? 1st gear too rearward relative mainshaft. Is it excessive 1st/2nd hub movement due worn synchro cone? Is there a cure? I suppose cones can be prised off and spaced. Does anyone make new synchro cones? Or is it 2nd gear on bush float, or bush on3rd, or 3rd hub on bush, or 3rd bush on mainshaft?? i have seen none of the latter seriously worn.
With that type of basic synchro gearbox it is always a mystery what stops the adjacent synchro cone from working into contact. But many makes had the same arrangement.

Of the relatively few gearboxes I have worked on I have been impressed by the general quality. Ridged 1st/3rd motion bearing and loose synchro cones seem main failings. 1st/reverse pit eventually but function for years after. As I have mentioned before I have had a lot to do with Javelin cars; despite semi luxury price tag the gearboxeas are relatively crude. Many had major replacements well before 100,000. 1st gear seems more frail than a Seven!

[Chris KC]

Bob, it's basically anything which permits the output shaft to move in the opposite direction to the gear thus increasing its travel relative to the hub - in '97 I concluded it was a combo of worn synchro cones (not replaceable at reasonable cost so far as I'm aware, only by picking from another box) and excessive slop in the end bearings (which are easily replaced and shimmed as necessary).

Worn/ stuck detent balls and springs in the hub are certainly a factor; I'm not sure though whether it's possible to expose the balls if axial free play is under proper control, I suspect not. I do recall I selected 1st gear fairly briskly which probably didn't help. One thing I (or rather the Bosnian mechanic who helped me) did at the time was to put a spot of weld on the selector gate, thus restricting the travel of the selector to the necessary amount.

I wouldn't want anyone reading this to draw the impression that 4 speed boxes are unreliable - I've done nearly all my A7 motoring on them. They are however perhaps less tolerant of neglect than the crash boxes - maintain them once every few decades and they're fine. If you are methodical they are not unduly difficult to work on.