Technical Differential Bearing replacement

[Pugglt Auld Jock]

This is for back lash, though you also need to check for preload on the bearings via the spacers , bearing in mind what you do affects the depth of tooth engagement and so alters backlash etc.
Basically you are looking for a visible but small amount of backlash + a slight but noticeable amount of preload (enough to prevent gears moving in and out of good contact though not so much that it causes loading/overheating of the bearings), whilst at the same time the teeth should engage in the centre of the gear contact area.
Ideally you need the manufacturers original settings as a base line.
View attachment 422575

Yup, that's where I was taught to put the tip too. I think this is one of the most difficult things you can do on a vehicle, certainly one that requires the most precision. Some seem to just fall into place whilst others seem to fight you enthusiastically requiring repeated adding or subtracting of shims. A bit like shimming up the old BMC front swivels on stuff like the Mini/1100/1300/etc. When you got good at them you could do them in a few minutes but I've watched apprentices taking most of a morning to set one side up. Crush spacers on pinion bearings always used to concentrate my mind because you only get one chance with them! Embarrassing when you have to go in and ask the boss for another one - you don't do that too often.

 

[bugsymike]

Yup, that's where I was taught to put the tip too. I think this is one of the most difficult things you can do on a vehicle, certainly one that requires the most precision. Some seem to just fall into place whilst others seem to fight you enthusiastically requiring repeated adding or subtracting of shims. A bit like shimming up the old BMC front swivels on stuff like the Mini/1100/1300/etc. When you got good at them you could do them in a few minutes but I've watched apprentices taking most of a morning to set one side up. Crush spacers on pinion bearings always used to concentrate my mind because you only get one chance with them! Embarrassing when you have to go in and ask the boss for another one - you don't do that too often.

Yes Jock, always had a selection of swivel shims in my tool box, plus I have seen so called experienced mechanics try and send a car out the door with steering so tight due to badly shimmed that turning the steering actually undid the swivel nut! They probably though that, as it then became loose it had freed off and must be OK, at least until the suspension collapsed!
I didn't do many with the "crush tube/spacer" on the pinion gear bearing Jock.

 

[fiat500]

Thanks Ian, that’s encouraging to know. I’ve completed the box, so I’m now about to embark on the process. Just one stupid question: when doing the backlash, where exactly must I position the end of the DTI probe with respect to the crown gear? In other words, where must it make contact with the crown gear?
Also, I only have two inner drive shaft spring washers. I thought the setup would use four (two for each driveshaft)? Can’t see anything in the manuals.

What I worked out with those spring washers is that they seem to be there to take up any inaccuracy in the fit of the square, sliding thrust pieces against their location on the shaft. I've found them sometimes on one side only, where it would be impossible to fit two in any case. I got hold of some flat shims of the same size, and played about until I got the best fit.

 

[Pugglt Auld Jock]

Yes Jock, always had a selection of swivel shims in my tool box, plus I have seen so called experienced mechanics try and send a car out the door with steering so tight due to badly shimmed that turning the steering actually undid the swivel nut! They probably though that, as it then became loose it had freed off and must be OK, at least until the suspension collapsed!
I didn't do many with the "crush tube/spacer" on the pinion gear bearing Jock.

First shop I was appointed working foreman in was a Quick Fit type operation - anyone remember the old Firestone Tyre and Auto stores? I was responsible for hiring and firing workshop personnel - along with all the other "stuff" that comes with the position (like the longer unpaid hours). Anyway, one of the chaps I hired was an ex truck mechanic. The first set of mini ball joints he did resulted in quite an alarming incident. The workshop was all up and over roller door drive in bays - very "American" in concept. The area outside the doors was wide enough we would park vehicles opposite the doors facing out towards the main road which ran along perpendicular to the workshops. On this occasion he'd parked the mini by the kerb and facing the main road. The customer, a young female, got into the car, drove to the end of our wee road where she would have to turn left into the main road. Unfortunately my man had tightened the joints to such a degree that she found she couldn't turn the wheel so she started to pull out but stopped when she was half way across the road, completely blocking the carriageway. First we knew of it was a cacophony (good word that?) of horns blowing due to this very busy road being blocked! the poor girl was very distressed and didn't think to try just reversing back, but we quickly reversed the car out of the way and took her into the store managers office while I investigated what the problem was. I got the mechanic to sit with me whilst I reshimmed one side correctly and watched him do the other side. He admitted he'd never done one before and he did a good enough job once shown. However unfortunately his "heavy hand" - probably because of his HGV past - resulted in a trail of snapped and stripped studs and bolts amongst other problems so I had to let him go after giving him a month's trial.

And the girl? Our boss was a very kindly and generally nice man who got her calmed down and generally chatted to her while we sorted the problem out. He followed it up by giving her a written "voucher" for free services and repairs for the next 12 months and I went out on road test with her before handing the vehicle back to her. She was very nice and luckily decided it was a genuine "mistake" on our part and we parted good "friends" In fact she did rather well out of it as she got her next service and MOT completely free including matterials! However I was very aware it could have had a very different outcome and put in place a system of random checks on work done - in addition to my usual practice of always road testing safety critical repairs myself. However I was only too aware of how differently it could have all turned out and was a very real wakeup call for me.

 

[bugsymike]

First shop I was appointed working foreman in was a Quick Fit type operation - anyone remember the old Firestone Tyre and Auto stores? I was responsible for hiring and firing workshop personnel - along with all the other "stuff" that comes with the position (like the longer unpaid hours). Anyway, one of the chaps I hired was an ex truck mechanic. The first set of mini ball joints he did resulted in quite an alarming incident. The workshop was all up and over roller door drive in bays - very "American" in concept. The area outside the doors was wide enough we would park vehicles opposite the doors facing out towards the main road which ran along perpendicular to the workshops. On this occasion he'd parked the mini by the kerb and facing the main road. The customer, a young female, got into the car, drove to the end of our wee road where she would have to turn left into the main road. Unfortunately my man had tightened the joints to such a degree that she found she couldn't turn the wheel so she started to pull out but stopped when she was half way across the road, completely blocking the carriageway. First we knew of it was a cacophony (good word that?) of horns blowing due to this very busy road being blocked! the poor girl was very distressed and didn't think to try just reversing back, but we quickly reversed the car out of the way and took her into the store managers office while I investigated what the problem was. I got the mechanic to sit with me whilst I reshimmed one side correctly and watched him do the other side. He admitted he'd never done one before and he did a good enough job once shown. However unfortunately his "heavy hand" - probably because of his HGV past - resulted in a trail of snapped and stripped studs and bolts amongst other problems so I had to let him go after giving him a month's trial.

And the girl? Our boss was a very kindly and generally nice man who got her calmed down and generally chatted to her while we sorted the problem out. He followed it up by giving her a written "voucher" for free services and repairs for the next 12 months and I went out on road test with her before handing the vehicle back to her. She was very nice and luckily decided it was a genuine "mistake" on our part and we parted good "friends" In fact she did rather well out of it as she got her next service and MOT completely free including matterials! However I was very aware it could have had a very different outcome and put in place a system of random checks on work done - in addition to my usual practice of always road testing safety critical repairs myself. However I was only too aware of how differently it could have all turned out and was a very real wakeup call for me.

In theory when you are the foreman/manager it is simply a case of telling the staff what to do and ensuring the smooth running of the business, similar to you, I found I was having to show the staff what to do and sort out their c*ck ups. It finally got so stressful I was grateful when a friend offered me to go self employed with him at a garage and sales business, it worked out very well and I found I was turning out more work in the garage side on my own than the previous site where we had four mechanics, a storesman and myself and this with no stress!
It's the same as any job, it's a good as the bloke doing it.

 

[Toshi 975]

What I worked out with those spring washers is that they seem to be there to take up any inaccuracy in the fit of the square, sliding thrust pieces against their location on the shaft. I've found them sometimes on one side only, where it would be impossible to fit two in any case. I got hold of some flat shims of the same size, and played about until I got the best fit.

I worked on about 10 gearboxes and it was a lottery as to wether there were any of those thin spring washers behind the drive blocks, one each side seemed the norm.

 

[Toshi 975]

And preload on the bearings .
When the diff bearings were removed were there any shims either side and if so did you keep them in order for each side so that you have a starting point.
Too many will make the diff bearings whine and seize due to too much preload on them, too few will give excessive backlash in the drive, etc. etc.
By the way did you disturb the pinion gear as well, as that is another that requires precise setting up if you don't want the car to sound like it is being attacked by a kamikaze dive bomber.
A fair bit of care and dial gauges, measurement of preload is etc. involved. Ideally some here on forum has a gearbox manual describing any specialist tools required and the settings etc.
If not it may be worth reading up about the principles of setting diffs in general.
There do seem to be a lot these days online. This is one though not Fiat, but basic principle the same:-

12 Tech Tips For Differential Assembly & Setup

This quick primer of tech tips is designed to be informational for less experienced installers and a refresher course for seasoned differential technicians.

www.yukongear.com

There are no shims associated with setting the backlash. The taper roller bearings are backed up by large threaded adjusters ( part 11) which can push the bearings to set their position within the housing (part 7) thus determining the poison of the crownwheel in relation to the pinion and the backlash. There are shims behind the pinion on the mainshaft which can adjust the contact. There are bronze thrust washers inside the diff (part 2) which are said to be available in different thicknesses but I have never seen them for sale.

 

[Bleeding Knuckles]

Hi, I don't know which Haynes Manual you have, but if you can use the 126 book (download on here) and refer to page 62 figure 6.10 it shows that the box, bell housing and shafts MUST be complete and assembled before you start. The dial gauge tip feeds in through the input shaft hole, it's fiddle but can be done. The bearing adjusters can also be rotated (if required) when you have a good setting mark the location of the adjusters with a punch after the turning torque is done. The 126 book gives the correct procedure for the Backlash and the roller bearing turning torque which should be completed after the backlash - see page 61 para 23 and 24
Ian.

 

[dapedza]

Hi, I don't know which Haynes Manual you have, but if you can use the 126 book (download on here) and refer to page 62 figure 6.10 it shows that the box, bell housing and shafts MUST be complete and assembled before you start. The dial gauge tip feeds in through the input shaft hole, it's fiddle but can be done. The bearing adjusters can also be rotated (if required) when you have a good setting mark the location of the adjusters with a punch after the turning torque is done. The 126 book gives the correct procedure for the Backlash and the roller bearing turning torque which should be completed after the backlash - see page 61 para 23 and 24
Ian.

Thanks Ian, I’ve got both the 500 & 126 Haynes manuals, which are good, but still manage to either miss out the odd step, or gloss over it. I’ve also got the 400 page FIAT Factory Workshop Manual, also good, but littered with mentions of factory tools!
Will let you know how I get on. Feeling positive as the box looks and feels good, just need to crack this next challenge. Now I’m starting to understand why Ricambio want £1,200 for an overhauled unit!

 

[bugsymike]

View attachment 422576View attachment 422577

There are no shims associated with setting the backlash. The taper roller bearings are backed up by large threaded adjusters ( part 11) which can push the bearings to set their position within the housing (part 7) thus determining the poison of the crownwheel in relation to the pinion and the backlash. There are shims behind the pinion on the mainshaft which can adjust the contact. There are bronze thrust washers inside the diff (part 2) which are said to be available in different thicknesses but I have never seen them for sale.

That makes life a lot simpler having threaded adjustment rather than shims.
Was anything altered on the pinion side?

 

[Toshi 975]

That makes life a lot simpler having threaded adjustment rather than shims.
Was anything altered on the pinion side?

I tended to leave the pinion shims alone when I could, did not want to push my luck

 

[Pugglt Auld Jock]

I tended to leave the pinion shims alone when I could, did not want to push my luck

On the principle of "if it ain't broke, don't fix it!"?

 

[Toshi 975]

Thanks Ian, I’ve got both the 500 & 126 Haynes manuals, which are good, but still manage to either miss out the odd step, or gloss over it. I’ve also got the 400 page FIAT Factory Workshop Manual, also good, but littered with mentions of factory tools!
Will let you know how I get on. Feeling positive as the box looks and feels good, just need to crack this next challenge. Now I’m starting to understand why Ricambio want £1,200 for an overhauled unit!

That Ricambio price would have worked at nearer to £1700. £1295 + vat + shipping + £150 pending the return of your gearbox which you would need to pay for yourself. So going DIY you learn a bit more about your car and save a good sum of money.

 

[bugsymike]

That Ricambio price would have worked at nearer to £1700. £1295 + vat + shipping + £150 pending the return of your gearbox which you would need to pay for yourself. So going DIY you learn a bit more about your car and save a good sum of money.

Just to put it all in perspective the Fiat 500 I owned in the late 1970s cost £40 with Mot on the road in a private sale, though I did have to put a clutch in it and do the push rod seals.