I've taken on some experienced advice from Bleeding Knuckles and Toshi 975, combined it with the information readily available via YouTube etc. and made a start.
I haven't yet got a stand and extension tubes for my dial-gauge so all I've done so far is to assess if the pinion depth into the crown-wheel is about right. I'm told that it should be, if assembled properly before I dismantled it, as the new gearbox bearings probably don't change the settings. It wasn't right at first so I took out a shim, played with the backlash, checking it by feel and adjusting with my homemade tool. Eventually, as seen in the last image, I think I got there with a contact pattern as shown in one of my manuals. There are conflicting images of the ideal contact pattern in some websites but I'm happy with this for now.
So next thing I need to properly re-assemble the differential and bell-housing and then to check the backlash using a gauge, set the bearing preload on my home-made support frame and the final crown-wheel contact pattern, which I hope I can see through the input shaft hole.

fiat500 said:

I've taken on some experienced advice from Bleeding Knuckles and Toshi 975, combined it with the information readily available via YouTube etc. and made a start.
I haven't yet got a stand and extension tubes for my dial-gauge so all I've done so far is to assess if the pinion depth into the crown-wheel is about right. I'm told that it should be, if assembled properly before I dismantled it, as the new gearbox bearings probably don't change the settings. It wasn't right at first so I took out a shim, played with the backlash, checking it by feel and adjusting with my homemade tool. Eventually, as seen in the last image, I think I got there with a contact pattern as shown in one of my manuals. There are conflicting images of the ideal contact pattern in some websites but I'm happy with this for now.
So next thing I need to properly re-assemble the differential and bell-housing and then to check the backlash using a gauge, set the bearing preload on my home-made support frame and the final crown-wheel contact pattern, which I hope I can see through the input shaft hole.

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Now that's a pretty colour !!! Well done Peter, looks like you have good full width contact and good depth too.
Ian.

Im glad you done this first.
Its one of the jobs still to do on my gearbox as I accidently moved the positions of the bearing seatings and forgot to mark which side was which when I was cleaning them.

Franko500 said:

Im glad you done this first.
Its one of the jobs still to do on my gearbox as I accidently moved the positions of the bearing seatings and forgot to mark which side was which when I was cleaning them.

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Easily done.:bang: Having messed about and got my head round this over the last week, I suspect that there might be some sense in ignoring the commonly stated advice that if you don't disturb the locking-ring the pre-load/backlash etc won't need re-setting. Anyone who likes the mechanical side of things will easily get the hang of how to set it. The settings are so easily affected by small adjustments and (I'm sure) slight wear in bearings, that it probably would have saved me this grief if I had re-set everything when I rebuilt my other gearbox.
Things stalled whilst I waited for some of those small shims for the drive-shaft (bag of 10 off eBay for about £2) so I'm now nearly ready to put it together properly.
I had to make a Mk.II version of the adjusting tool as the forces involved twisted the Mk.I.
I'm a bit mixed up about what the gear-pattern should look like. Intuitively I think mine looks good and agrees with my "Motorbooks" Fiat 500 manual. But there is conflicting information on the Internet and on various bits of literature I have amassed.:bang:

fiat500 said:

I'm a bit mixed up about what the gear-pattern should look like. Intuitively I think mine looks good and agrees with my "Motorbooks" Fiat 500 manual. But there is conflicting information on the Internet and on various bits of literature I have amassed.:bang:

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Hello Peter, When I did my box I always remembered something I heard in school MANY years ago in the engineering class " with gears, no matter what type, if you can achieve a full width - full depth pattern without the gears bottoming out you are about there"!!!! That's old school teaching !!! If I may say it looks like you have good contact all round
Ian.

Bleeding Knuckles said:

Hello Peter, When I did my box I always remembered something I heard in school MANY years ago in the engineering class " with gears, no matter what type, if you can achieve a full width - full depth pattern without the gears bottoming out you are about there"!!!! That's old school teaching !!! If I may say it looks like you have good contact all round
Ian.

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Thanks again for the reassurance Ian. Apparently most cars now use hypoid diffs which means the pinion engages below or above the centerline of the crownwheel. So I'm assuming that the simpler and less stressed geometry of the 500/126 setup allows a more uniform contact pattern. I've got my sealant so it goes today.

I've mounted the semi-assembled gearbox on my home-made jig/stand because the backlash measurement is so small that I felt the setup needed to be really rigid if the adjustments were to be accurate and to mean anything. It's just a piece of angle-iron long enough to be clamped in the vice with a couple of holes to bolt through the bellhousing and two outriggers to support the driveshafts horizontally. A couple of blocks of wood support it at the back and I've levelled it accurately. Soft-jawed clamps are used to prevent movement of shafts as required.

It's quite fiddly to set up the dial gauge but the steel framework helps with the magnetic stand.

As you can see in the video it didn't take long to verify that I've achieved about 0.10mm backlash at present and this is about right; more fiddling and rechecking before this is set permanently.

[ame]https://youtu.be/PPY4UajvX-I[/ame]

fiat500 said:

It's quite fiddly to set up the dial gauge but the steel framework helps with the magnetic stand.
https://youtu.be/PPY4UajvX-I

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Well done Peter, I couldn't agree more about finding a good position for the magnetic stand

Ian.

I'm now on the Mk.III version of my bearing adjuster tool. As you tighten the serrated rings to put on the pre-load it takes a great amount of force to turn them the fractional amount which applies the pressure which creates the rotational torque. Consequently I bent the handle and had to weld on a stronger one.
You do really need the level of control that a proper tool gives because as you approach the correct backlash setting and then the torque setting, very minor over adjustments can easily upset things. You really have to follow the Haynes advice which suggests rotating the bearings after each adjustment to allow the bearings to centralise and give a true reading. So I've been messing about for ages to get this right, which involves constantly removing and replacing the dial gauge, clamping and un-clamping one of the driveshafts or the end of the pinion shaft depending on what I am adjusting. But finally I think I have it about right.

I used the Ian Bleeding Knuckles method of a 1lb bag of gravel on a stick. My stick is a piece of IKEA cabinet-mounting bracketry which helpfully had the holes to secure it to the driving flange in just the right place. I called it a day when the gravel was just trying to move the driveshaft. As you have to clamp one driveshaft so that the weight turns the crownwheel and hence the bearings and doesn't just shortcut by turning the diff, the torque being measured also includes the resistance of the pinion shaft and bearings as the crownwheel can't turn if these are clamped up. But it all feels right and has improved the contact pattern on the crownwheel even more and therefore allows me to proceed with the easy job of re-assembling the gearbox.

Once the the thing is back together I will be doing a final check on preload/backlash by leaving off the input shaft seal assembly and trying to poke my dial-gauge extension through the gap. :sucks:

If anyone lands on this thread when they are considering doing this job then I recommend that they first verify that they have surplus supplies of patience in stock.
After re-assembling the gearbox innards and re-fitting the bell-housing I needed to check that the settings had stayed true. There is still enough room to get a long extension probe from the dial-gauge through the hole where the first-motion shaft enters the gearbox with the shaft fitted, but it's fiddly to set up and in my case I had to remove and reset it many times because in order to get a true reading after any adjustment of the bearings you need to rotate the shafts etc., which means removing the gauge and any clamps etc.
Ultimately I started to appreciate the relationship between backlash and pre-load and in this case, once one was set correctly the other was almost there as well. But it's very subtle and needs some fine control to get it right. I must have been adjusting and measuring for an hour and a half before I was happy with it. I really can't imagine that a worn gearbox which has been dismantled and rebuilt will retain the specified adjustment values in the way that is always described. In fact, if I slackened and then re-tightened the nuts that hold the bearing housings to the gearbox I suspect that it would be awry once again.:bang:
With the end-cover of the box fitted it was hard to lock up the pinion-shaft for the backlash readings so if I did this again I would leave it off for now. So I put it in reverse gear and used a soft-jawed clamp to resist any gear movement. When checking the preload the way I did it you need the pinion free but one shaft locked up.
So there's few ancillaries to finish and then I need to fit the box in the car. I regret not fitting a new gearset for the worn first gear, because it did look manky in the clear light of day, but I'm hoping to get away with it for a while at least and I get the benefit of (hopefully) quiet, nicely meshed, original gear cogs in all the other gears.

fiat500 said:

I really can't imagine that a worn gearbox which has been dismantled and rebuilt will retain the specified adjustment values in the way that is always described. In fact, if I slackened and then re-tightened the nuts that hold the bearing housings to the gearbox I suspect that it would be awry once again..

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Hi Peter, It's so good to hear someone else complain about what a Faff it all is - no wonder the job is not completed often I get what you mean about a worn gearbox and specified clearances and I wonder how bad things must be in a hard worked hi milage bargain from Italy??? I did also find a slight problem with the fitting of the bearing retainers in that if I tightened one nut too much the adjustment went out, so it was a case of little by little on opposite nuts until tight You must also have the patience of a saint :worship::worship::worship:

Ian.