Technical Gearbox locked solid

[cinque500]

Last weekend I took my ‘75 500R with Nanni 5sp up the test hill at Brooklands for the first time. If you were there you would have seen my little red car come to a stop just past the flags, but we didn’t quite make it to the summit and had to be pushed the last 6 feet. My gearbox locked solid whilst in second gear. I couldn’t select any gears, not even neutral. Got the thing apart this week and much to my surprise not a single piece of visual damage, nothing in the oil, no teeth damaged, no pins in the case where they shouldn’t be etc. Tonight I slackened off the two big nuts on the shaft ends. When I slackened off the castellated nut everything freed up, re-tightened it and it’s still spinning freely, what could’ve happened?

 

[bugsymike]

Last weekend I took my ‘75 500R with Nanni 5sp up the test hill at Brooklands for the first time. If you were there you would have seen my little red car come to a stop just past the flags, but we didn’t quite make it to the summit and had to be pushed the last 6 feet. My gearbox locked solid whilst in second gear. I couldn’t select any gears, not even neutral. Got the thing apart this week and much to my surprise not a single piece of visual damage, nothing in the oil, no teeth damaged, no pins in the case where they shouldn’t be etc. Tonight I slackened off the two big nuts on the shaft ends. When I slackened off the castellated nut everything freed up, re-tightened it and it’s still spinning freely, what could’ve happened?
View attachment 475325

Not enough end float on thrust washers when under extreme load.?

 

[the hobbler]

Not enough end float on thrust washers when under extreme load.?

Be careful with these 5-speed conversions---I have been strongly advised by Tony Castle-Miller (Middle Barton Garage) that the 5th gear must only be used as a "cruising" gear and NOT and "accelerating" gear. The reason?---the 5th gear shaft is only supported at one end.

 

[cinque500]

Not enough end float on thrust washers when under extreme load.?

I don’t know enough about it. Do you mean there should be a thinner shim? If indeed there is one …. I’ll need to dismantle and have a look.

 

[cinque500]

Be careful with these 5-speed conversions---I have been strongly advised by Tony Castle-Miller (Middle Barton Garage) that the 5th gear must only be used as a "cruising" gear and NOT and "accelerating" gear. The reason?---the 5th gear shaft is only supported at one end.

I was is 2nd gear but are you saying it can affect all gears? Could the shaft have flexed then? Ironically 5th was the only gear I could select. 5th (and 4th for that matter) are long overdrives so not much acceleration going on, but I guess that puts more strain especially with the torque which always surprises me (740cc).

 

[bugsymike]

I don’t know enough about it. Do you mean there should be a thinner shim? If indeed there is one …. I’ll need to dismantle and have a look.

No personal experience on that gearbox, just trying to think of a cause of the problem.
Presumably if you take a look at where it thrusts when under load there may be some signs of where it was locking/seizing up, heat/scoring etc.?

 

[cinque500]

So I reassembled the bottom shaft so I could see if there was excess play but actually it’s tight. No play and the bearings don’t sound happy. Presumably I need one or more extra shims so I have a tiny bit of play? But even with this issue I don’t see the connection with the lockup?

 

[bugsymike]

So I reassembled the bottom shaft so I could see if there was excess play but actually it’s tight. No play and the bearings don’t sound happy. Presumably I need one or more extra shims so I have a tiny bit of play? But even with this issue I don’t see the connection with the lockup?

With it stripped down is there no sign of marking or heat damage etc.?
Do the suppliers of the gearbox have a measurement of permitted/correct end float at that shaft?
When under load the box will heat up and expand the metals so what is just a bit tight would become seized as in your case.
When you examine the bearings is there signs of over heating on the balls/rollerbearings /races etc?
Is there enough oil getting around to this area?

 

[fiat500]

Do you know the history of the gearbox as regards any replacement parts or those that might have been taken from another box? Was it noisy on the diff?
It is possible that this is related to the location of the pinion shaft. Even in the Haynes manual the descriptions that apply to this are complex. At the factory each shaft was measured to within great accuracy so that it was known how far short of the exact distance it should, that the pinion would protrude in the direction of the crownwheel. This was so that the crownwheel and pinion meshed exactly on the centreline of the diff. If any gears on that shaft are changed there is a possibilty that this precision is lost.
I'm too much out of being hands-on with these to be sure that this might cause the box to tighten or lock up, but I recommend reading the relevant Haynes chapter, and fully dismantling to verify the correct assembly to the best of your ability.
Between shims and the fifth gear additions, I thing @bugsymike and @the hobbler are on the money.

 

[cinque500]

I have made I think a significant discovery which I’ll get to in a mo. But on some of your questions re. the history of this gearbox, I bought it with this box already fitted. And the diff was a bit noisy.
So onto the ‘discovery’ - it concerns the bearing on the pinion gear shaft which is rather sloppy. This isn’t a taper bearing so am I right to assume it should have minimal play?

 

[bugsymike]

I have made I think a significant discovery which I’ll get to in a mo. But on some of your questions re. the history of this gearbox, I bought it with this box already fitted. And the diff was a bit noisy.
So onto the ‘discovery’ - it concerns the bearing on the pinion gear shaft which is rather sloppy. This isn’t a taper bearing so am I right to assume it should have minimal play?

I know it is for demonstration, but don't like bearing in vice like that.
Does seem excessive movement.
If you put two fingers in centre and give the bearing a spin , does it sound harsh/noisy?

 

[cinque500]

I know it is for demonstration, but don't like bearing in vice like that.
Does seem excessive movement.
If you put two fingers in centre and give the bearing a spin , does it sound harsh/noisy?

it was literally just to hold it for the vid as it didn’t have three hands. I’ve since discovered it’s got CBK etched on it which is a type of type of taper bearing but when I apply force in a longitudinal direction, it doesn’t really seem to remove the slack so I think it is shot. It certainly is rough and noisy too.

 

[bugsymike]

it was literally just to hold it for the vid as it didn’t have three hands. I’ve since discovered it’s got CBK etched on it which is a type of type of taper bearing but when I apply force in a longitudinal direction, it doesn’t really seem to remove the slack so I think it is shot. It certainly is rough and noisy too.

I was just remembering in the past the trick of grinding a bit into a half shaft bearing to weaken the outer race, squeezing it in a vice, looking the other way as you smack it with a hammer and bits go flying everywhere, it was common practice in the 70s on rear wheel drive Fords. then a benelli clamp around the inner, so two tyre levers could prise it off after doing the grinding trick again on the shrink collar, followed up by pressing the new bearing onto the shaft then heating the shrink collar to drop down to hold it all into place. There was a similar set up on Ladas (Fiat 124) but as dealers for those we had a proper tool, two C shapes that fitted in a drum over the bearing and then you used the press to do the job neatly.

Any chance on your gearbox that the bearing was fitted the wrong way around if it is a thrust/taper bearing?

 

[cinque500]

I was just remembering in the past the trick of grinding a bit into a half shaft bearing to weaken the outer race, squeezing it in a vice, looking the other way as you smack it with a hammer and bits go flying everywhere, it was common practice in the 70s on rear wheel drive Fords. then a benelli clamp around the inner, so two tyre levers could prise it off after doing the grinding trick again on the shrink collar, followed up by pressing the new bearing onto the shaft then heating the shrink collar to drop down to hold it all into place. There was a similar set up on Ladas (Fiat 124) but as dealers for those we had a proper tool, two C shapes that fitted in a drum over the bearing and then you used the press to do the job neatly.

Any chance on your gearbox that the bearing was fitted the wrong way around if it is a thrust/taper bearing?

“Any chance on your gearbox that the bearing was fitted the wrong way around if it is a thrust/taper bearing?”
Yeah possibly. I don’t see why this bearing has worn out so badly compared to the others, albeit the others are ball bearings and don’t have to deal with radial and axial loads.