Problems

[Al]

Couple of issues, firstly can the rear master cylinder be refurbished, new seals etc or is it sealed unit?
I also have an issue with the final drive, noticed a rumbling from back end, previously had to have the large rear bearing in the bevel box replaced twice due to a dodgy batch of bearings but this time no play whatsoever so took it to local mechanic who drained the oil and swarf he seems to think it is from the taper roller behind the large brg, anybody had a look in there and can confirm or come up with any other possibilities.

Al.

[nab 301]

No repair kit listed for the rear BMC , I replaced the complete unit. I never did get around to attempting to dismantle the old one. From memory it didn't look to be easily dismantled.
My bikes bevel box did get rough at 90 k ish miles but I had a spare . Again I haven 't looked at the old one yet..
Steptoe did some good diagnostic/ repair posts on here and on UKgser . I'm sure I can dig them up if required.
I used a screwdriver handle to the ear and reckon it was the front input bearing when spinning the rear wheel by hand.

[fatnfast]

Why not just open it up and take a look? Rear wheel off, then undo the allens around the circumfrance and pull the cover off (dont drop it!). As you say the bearing is a taper type. Its outer ring will remain in the casting, but you can get a good look at it.

There is not a lot of load placed on that bearing and whilst I replaced mine as it was apart, I didnt really need to.

I would still suspect the big bearing, the cage can break up and initially there may not be any play. For this you dont even need to take the fd apart, just carefully prise the big seal out and have a look.

[Al]

Rear end stripped down today and the inner race surface of the crownwheel bearing has started to break up, covered only 3-4000 miles since fitted. The guy doing the work is experienced on BM`s and has a good rep in town so have to assume everything was done correctly last time, the witness marks on the contact area of the gears match the description in the manual to confirm the correct settings on assembly, the only thing we can find is a very slight movement on the pinnion shaft, which when I spoke to the guys at Sherlocks is apparently normal, and the shaft spins freely with no noise so bearing appears ok. I have ordered new parts and will see what happens in about 18 months!!.

Al.

[fatnfast]

I know you are happy with your guy doing the work, but it really should not fail that quickly. He is definately heating the bearing up enough so it drops straight on? At the very most a gentle tap should finally seat it. The outer cover should ideally be warmed as well to give it an easier time when its pushed back on.

There is the question as to if its been shimmed up correctly from new at the factory, this is a lengthy and involved process and it would be proberbly more cost effective to fit a s/h rear final drive.
The shims have not gone astray at some point during previous bearing changes?

Also is the correct bearing grade deffinately being used (C). Suprisingly the cheapest supplier I could find was BMW themselves.

[Corvus]

I've no experience with stripping bmw bevel drives, but a little experience in a dim and distant past life with gearboxes.

Where heavy interference fits are involved C3 grade is normally specified. This is very common in industrial gearboxes, where a very heavy interference is typical on the shaft. Plus the main heat source is via the contacting gears and therefore the shaft.

A bearing could fail prematurely for a few reasons i could think of: contamination with debris, severe overload (which may come from incorrect grade selection or setting up), lack of lubrication, use of "re claimed" bearings, wrongly installed(as has been mentioned), brinelling (caused by prolonged shelf storage).

A look at where the race has fatigued will give strong clues. eg; in the middle or strongly to one edge etc. Look carefully for signs of indenting of the race inner face and also the sides, suggesting poor installation (as has been mentioned).

Probably not much help!

[Al]

Where is the interference when the bearing is fitted, is it the outside of the brg where it sits in the housing or the inner bore of the brg where it fits over the centre hub, depending which one it is determines what is heated up. I have plenty of experience with bearings, I fit/remove/scrap them regularly as I overhaul jet engines for a living, from what I can see of the brg it has damage to the inner race on part of the diameter only, will be able to see better when it is removed, the bearing that has failed was sourced from a bmw dealer so I assume it would be correct spec, the new one is coming from Sherlocks, a bm specialist in the south west so again assume it will be correct spec.

Al.

[Corvus]

Where is the interference when the bearing is fitted, is it the outside of the brg where it sits in the housing or the inner bore of the brg where it fits over the centre hub, depending which one it is determines what is heated up. I have plenty of experience with bearings, I fit/remove/scrap them regularly as I overhaul jet engines for a living, from what I can see of the brg it has damage to the inner race on part of the diameter only, will be able to see better when it is removed, the bearing that has failed was sourced from a bmw dealer so I assume it would be correct spec, the new one is coming from Sherlocks, a bm specialist in the south west so again assume it will be correct spec.

Al.

Can't speak for the bmw hub, but industrial gearboxes have a heavy interference on the shaft and a transitional fit (at worst) on the outer, H7 bore tolerance was typical, from memory. But with this being a single sided rear wheel there will be different ( extra) forces acting, compared to the relatively simple to determine, unidirectional force on a gear. Maybe in this design both inner and outer will be tight? Won't there be forces from the gear and also from the rear wheel? Bit more complicated to fathom.

One thing for sure, something quite dramatic has caused such a rapid demise. Maybe shimmed wrong for expansion?

Any evidence of "creep"?

Corrosion?

[Al]

The manual says to heat brg to 80 degrees then fit so I assume it is tight on the hub, I will have a chat to see how he fits it, if it not heated and just pressed on it will obviously cause damage, I may do it myself as we have brg heaters at work and very expensive measuring equipment so I can check everything, seems fairly straightforward in the manual.

Al.

[fatnfast]

It is a little unusual in its set up. The fact that it uses the caged crown wheel bearing and a taper bearing suggests is not a common set up.
As you say, if your mechanic has pressed it onto the crown shaft without heat then damage could have very well been done. I think you mentioned that it has been replaced more than once before, and a previous failure was put down to a dodgy batch? Hmmm....
Anyway, whats done is done. Its a pretty simple job to do yourself, hope Im not teaching anyone to suck eggs but this is how I did it:

1. Wheel and caliper off. Abs sensor off. Remove x8 allen bolts (7mm allen key).
2. Tap the black cover with a soft mallet and remove.
3. Tap out the crown wheel assembly. Dont drop it!! Remove shims, bearing seal and outer case seal.
4. Remove the 'big bearing'. Normal pullers will not get under the bearing to pull it. Use 'wedges' to start removal. I just ground a blunt taper on a bit of flat steel. Pound shop tyre levers can also be used.

Once you have some clearance you can then use a puller. This is a cheap ebay set that does the job. I did ground/file the puller legs to get under the bearing.

5. Pre heat your oven to 80 deg C. Place the new big bearing on a new bakeing tray (pound shop again) and heat for 15 mins.
6. Have your crown wheel assembly supported between two pieces of wood. Using your wifes oven gloves, fit the bearing onto the crown wheel. It should slip on easily. If it doesnt, heat it a bit more.
7. Allow the crown wheel and bearing to cool.
8. Put the black cover (without seals and shims!) into the oven, same temp, same time.
9. Remove the black cover and support with a couple of blocks of wood.
FIT THE SHIMS INTO THE COVER OR ONTO THE CROWN WHEEL. A blob of grease will hold them in place. Fit the crown wheel into the black cover.
10. While the cover is still warm, grease and fit the outer seal. Refit the black cover/crown wheel assembly onto the FD. Make sure it meshes and gently tap into place.
11. Refit the 8 allens. Torque diagonally to 26 lbft (35Nm). Fit the abs sensor (4Nm).
12. Fit a new big bearing seal. It should be flush with the outer cover.
13. Replace drain plug, refill with GL5 gear oil to bottom of fill hole threads. Replace fill plug, done!

Hope this is off some use.

[Al]

The first replacement brg only had 17 balls instead of 21 as the manufacturer/spec had changed and that started rumbling, this was probably 5-6 yrs ago and there was stuff on the BMW site about it not sure about on here and they went back to 21 again. All I can do is replace it and see what happens, if it goes again I will take it somewhere else/have a go myself( I use spanners all day, hate doing stuff at home) or buy a second hand unit, its only money I don`t smoke or gamble so I can spend a bit on the toys!!

Al.

[Blackal]

The first replacement brg only had 17 balls

Al.

You were lucky - Hitler only had one (apparently)


Al

[Corvus]

The first replacement brg only had 17 balls instead of 21 as the manufacturer/spec had changed and that started rumbling, this was probably 5-6 yrs ago and there was stuff on the BMW site about it not sure about on here and they went back to 21 again. All I can do is replace it and see what happens, if it goes again I will take it somewhere else/have a go myself( I use spanners all day, hate doing stuff at home) or buy a second hand unit, its only money I don`t smoke or gamble so I can spend a bit on the toys!!

Al.

You would think bigger balls carry more load. Fewer balls in the same circumference equals bigger balls. The brg outer and inner diameters don't change so the one with fewer balls should have a higher rating? It's probably not that simple. If they are deep groove ball type, the one with bigger balls might not have such deep grooves and therefore less axial load? Just guessing really. Where is the fatigue marking? Right in centre or on one edge?

If you have the brgs to hand and can see the number we can check the manufacturers dynamic rating for each.

I was surprised to see ball bearings used. I thought bevel gears usually produce a substantial axial load? Unless I'm getting confused and another bearing carries this burden?

[Corvus]

Wonder how many revolutions a rear wheel makes in 4000 miles?

Quite a few!

How many in, say, 50,000 miles? We'll be stacking up the zeros!

[Merecat]

Wonder how many revolutions a rear wheel makes in 4000 miles?

Quite a few!

How many in, say, 50,000 miles? We'll be stacking up the zeros!

Oh now just stop it, I'm off today and have too much time!!











Are we ignoring tyre wear?