lapping hubs

[steve davidson]

Due to an increasing quantity of oil escaping from the rear hubs I took the decisions to pull out my differential and strip it down.  The quantity of oil was not massive, but it was annoying as it ended up on the rims and dangerous as it was coating the brake drums. 
 
A few things;
 
Firstly, I was surprised to find no evidence of the inner seals.  The cups and washers were all there but no sign of a felt seal or a more modern lip seal, the adjusters are the basic type with no oil return spiral.  Interestingly the axel tube was oily but certainly not swimming in oil as I thought would be the case. The car has done at least 750km since I last checked the diff oil level and it still seemed pretty good when pulled it down.  I’ll be putting new lip seals back in but my takeaway lesson is that in my diff at least the oil does not readily want to move from the differential to the axel tubes. 
 
Upon stripping the unit down, I found that the axels were in poor condition with cracks and flogged keyways.  Lesson two, keep an eye on the retaining bolts and make sure they are nipped up tight, mine had obviously had some movement. 
 
Next I found missing paper seals between the inner and outer hubs but a good glob of silicone.  When re-installing I’ll be putting in a paper seal with a smear of Permatex or similar. I had considered taking the wheel nuts off and lapping the inner hub to the outer hub but not sure if that’s really necessary if the inner seals are working as planned.  Any thoughts from those experienced?
 
The tapers inside the hubs were extensively pitted and thus needed a lot of lapping to get a sound fit of the hub to the axle.  Using a coarse lapping paste I spent about 30 minutes trying to get a decent surface but the pitting was so widespread and deep that I was a long way from getting the homogenous result I was looking for.  To speed things up I attached the hub removal tool and spun it with a drill (see below).  I used a small screw driver to provide a linkage between the drill and the hub, if I’d had a spare ½ UNC and lathe I’d have made a more professional connection.  I lapped in both directions and continually stopped to remove paste from the keyway and deposit it back on the shaft.
 
My question is this; will lapping in the above manner cause issues later?  I have heard from people a lot more experienced than me that the lapping should be done with small side to side movements keeping the keyways semi-aligned, i.e, when lapping the movement should be no more than say 45 degrees from the final location.  But I’ve also seen tapered joints lapped on lathes.
 
Any thoughts would be well received.

[squeak]

Hi Steve,
 try to settle for not removing ALL the pits and score marks, as removal of too much metal will draw the axle too far into the taper. Two things result, the taper can protrude beyond the hub causing the axle nut to bottom against the axle and more importantly the inner spur gear on the axle will bind against the diff carrier.  Difficult to then correct you would have to buy a new hub.   regards  Russell

[Stuart Giles]

As said, you just need to lap until you get a good contact area, it's not usually practical to remove every pit from the hub. One thing to watch for is that a step often forms in the small end of the hub taper after lapping. If this is left in place it will stop the hub pulling up onto the halfshaft taper when the nut is tightened. I file the step off leaving a very short parallel section at the outer end of the hub.

You can usually get an idea of whether the sunwheel gear at the end of the halfshaft is going to bind on the inside of the diff if you do a loose dry assembly and try to pull the hub outer away from the inner when its assembled on the axle case, if you can't, you just need to grind back the halfshaft inner edge of the sunwheel gear, these gears are really generously sized for the job they have to do, and losing a 1/16" of width hasn't caused any issues when I've done this. This grinding really needs to be done with a toolpost grinder in a lathe, or in a proper grinding machine.

[steve davidson]

Hi Steve,
 try to settle for not removing ALL the pits and score marks, as removal of too much metal will draw the axle too far into the taper. Two things result, the taper can protrude beyond the hub causing the axle nut to bottom against the axle and more importantly the inner spur gear on the axle will bind against the diff carrier.  Difficult to then correct you would have to buy a new hub.   regards  Russell

Thanks mate, these ones were so old that there is a very real risk of the risks noted above eventuating.  Our mate Trev has found me a few others that look in better shape so I'll be checking them out this week.

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As said, you just need to lap until you get a good contact area, it's not usually practical to remove every pit from the hub. One thing to watch for is that a step often forms in the small end of the hub taper after lapping. If this is left in place it will  stop the hub pulling up onto the halfshaft taper when the nut is tightened. I file the step off leaving a very short parallel section at the outer end of the hub.

You can usually get an idea of whether the sunwheel gear at the end of the halfshaft is going to bind on the inside of the diff if you do a loose dry assembly and try to pull the hub outer away from the inner when its assembled on the axle case, if you can't, you just need to grind back the halfshaft inner edge of the sunwheel gear, these gears are really generously sized for the job they have to do, and losing a 1/16" of width hasn't caused any issues when I've  done this. This grinding really needs to be done with a toolpost grinder in a lathe, or in a proper grinding machine.

Thanks Stuart, that's a great tip