LC4-640: main bearing spinning?

Discussion in 'Thumpers' started by DaBit, Sep 5, 2010.

  1. DaBit

    DaBit Been here awhile

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    Today I noticed something strange. Had the engine on the workbench and decided to pull the clutch, measure the main shaft axial clearance, and eventually shim it out due to the unavailability of the 'graduated bearing retainer'.

    Well, axial clearance was way too much. It soon became clear why: the main bearing worked itself outwards.

    [​IMG]

    Huh, strange. That bearing is held in by a bearing retainer. Unscrewed the retainer, which displayed a nice groove where the bearing touches the retainer (top retainer in the image below).

    [​IMG]

    'Well, let's see if there should be a groove of any kind. Let's check the retainer from the spare engine. Removing it is only a 5 min job'. Surprise: same damage but to a lesser extent (bottom retainer in the picture above).

    As far as I know only a spinning outer race of the main bearing can cause these grooves. Not good.

    2 LC4 engines, one 2003, one 2005, same issue. Both with the roller bearings. Can't be a coincidence. The roller bearings seem fine in both engines; running surfaces are smooth and undamaged, and so are the rollers. There are marks visible on the rollers from axial loading, but they have no depth.

    Anyone seen this before? Is there a hidden problem somewhere?

    I used some acetone to clean the area as well as possible, applied some Loctite 603 retaining compound (capillary action draws it between bearing and casing), and tapped the main bearing in until it was flush with the casing. Have to order a new retainer, though.
    #1
  2. Tseta

    Tseta Lost

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    :eek1

    So the symptom, "inconsistent clutch engagement point", was therefore not due to unfavorable stacking of tolerances, but due to actual wear inside the engine?

    It is difficult to tell from the pictures that how deep the worn section actually is? Could you measure it?

    Maybe a comparison should also be done with a known new retainer.

    A bearing spinning in its seat is never good...

    Sorry, my post is probably not much help. Maybe others will chime in...

    Cheers,

    Tseta
    #2
  3. Ironwood

    Ironwood Friday Harbor, WA

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    So how did the bearing go back into it's bore? Was it nice and tight or did it just fall in there?

    Joe
    #3
  4. DaBit

    DaBit Been here awhile

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    Exactly my reaction!

    It seems so. With the bearing flush with the casing main shaft clearance is approximately 0.3mm.

    'running engine': 1.9mm (5/64")
    'spare engine': 1.2mm (3/64")

    I will order a new one. These are unuseable.

    No, but I have no idea what else can cause the damage to the retainer.
    However: No damage was visible to the small bit (which was once inside) of visible outer bearing race.

    Driving the bearing in again also took the regular amount of effort; a lot of small blows with a metal hammer and a driver around the bearing race. Not less than I would expect, and certainly not 'it slipped right in'.


    The fact that the retainer was damaged, oh well. Just one more annoying issue. I had some more annoying issues such as a damaged cylinder bore, a broken oil scraper ring, a cylinder head with a crack somewhere which leaks coolant to the exhaust port and an exhaust header with holes rotten in it. A couple of ex Paris-Dakar pistes in Morocco fully loaded is not something you should do if you want to keep your LC4 in showroom state. ;)

    The fact that I have two engines with the same issue bothers me.
    #4
  5. Tseta

    Tseta Lost

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    I believe there must be many other LC4 owners out there who are reading this and are equally bothered by your issue.

    I thought about the possibility of the bearing spinning in its seat. Your findings so far seem to be ruling this possibility out: no damage was found on the little bit of visible outer surface, the bearing still turns smoothly, no "crunchiness" or other damage is apparent and the bearing took "regular" effort to drive back in. If the bearing had spun, one would think that the seat would now be a loose fit to the bearing and that the bearing itself would have some apparent damage to it.

    So, perhaps there is something totally else going on? (Pure speculation here...) Micro-movement or creeping of the bearing due to engine vibrations/resonances? This could also explain the wear on the retainer, which in turn would allow the bearing to move even further out.

    I'll be keeping an eye on this thread. I'm very interested in what you will find out.

    Cheers,

    Tseta
    #5
  6. DaBit

    DaBit Been here awhile

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    I don't think I will find out much more. My Adv is in bits and pieces for about 2 months now (other jobs and chores taking my time, and a couple of 'surprises' :( ), and that crotch rocket I'm riding now is not good for my knees, wrists, neck and drivers license.

    I'll order a new retainer, use a little Loctite 603 between the retainer-bearing contact patch also, and call things 'good to go'. Unless someone points me to a hidden problem I'm overlooking.


    My theory:

    When the clutch is pulled an axial load equal to the force of the 6 clutch springs is placed on the main bearing shaft, and that load is not exactly insignificant.

    When replacing bearings in an engine casing, all one has to do is heat it to 100-150 degrees C (212-300F) and the bearings just fall out without any external force.
    Now, when tackling a stretch of deep mud for example, the engine becomes hot and the clutch action puts an axial force on the bearing. There is only the retainer to keep it in place, which has only a tiny contact patch with the outer race.

    Then we have the roller bearing. The rollers have a flat surface on both ends which is jammed to the bearing races when an axial load is placed on the bearing. This must cause some friction.

    Now, due to engine temperature we have a fairly loose fitting main bearing. We have friction which puts a rotational force on the outer bearing race. The outer race might spin a little and there's still the vibration. And at last we only have a tiny contact patch of the retainer with the main bearing outer race, which will wear 'fairly quickly' (several 10000 km's).

    Anyway: If I was asked to install a new main shaft bearing in an LC4 engine, I'd put some retaining compound between the bearing and casing. And if I were KTM, I would consider another bearing type. I don't think the currently used roller bearing is the best solution.
    #6
  7. trailrider383

    trailrider383 867-5309

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    Yes, but the whole assembly is warmed up, including the bearing. So I don't think you have a "fairly loose fitting" main bearing when the engine is warmed up. I doubt they would engineer something like that.
    #7
  8. anotherguy

    anotherguy unsympathetic

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    The bearing doesn't need to spin to have that type of wear. A bearing sliding back and forth will beat the retainer up. You said the bearing land in the case was good w/o scarring and the outer race of the bearing as well? That really eliminates spinning as the reason for retainer wear.

    When reinstalling the bearing I'd apply LocTite 609 stud and bearing mount to a clean bearing race and install it into a clean land. Just what I'd do to try and stop a repeat of the issue. I hate doing shit twice.

    If you're curious about 609's ability to hold in such and environment I've been using it for valve guides in old aluminum heads for a couple decades without failure. Takes some heat to remove when the time comes and there's no burnt oil present when removed.
    Here's a chart on strength in various materials after cure.

    [​IMG]
    #8
  9. DaBit

    DaBit Been here awhile

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    Yeah, well, it's a theory. Theories of similar validity told us that the oil supply depleted last year. So much for the theory...

    But anyway: steel has about half the coefficient of expansion of aluminium. So instead of becoming so loose that the bearing can slide out effortless, it only becomes so loose that it takes little effort.

    Still the same diagnosis: bearing fitting too loose, or retainer too flimsy.

    I have not taken the bearing out since that would have taken a lot of effort, it would have destroyed the bearing, and the bearing was OK so it isn't needed. Thus, I didn't inspect the bearing land.

    All I inspected was the few mm of bearing race which was above the casing. This looked perfectly fine. Also, when a bearing starts spinning and eating the bearing land I guess that would be visible from the outside also.

    Since the original bearing retainer lasted 50.000km or so, I suppose that just replacing the retainer solves the problem for quite some time.

    As a security measure I 'flushed' the area between bearing race and land with acetone. Then I put a few drops of Loctite 603 between race and land. Loctite 603 is an anaerobic cylindrical retaining compound which is desiged to work OK on less than perfectly clean surfaces. Very strong and almost more fluid than water, so the best part of it wicked between land and race. Whatever didn't wick inbetween can be removed with a shop towel (anaerobic, doesn't harden in presence of air...), making this a very clean solution with minimal effort.

    Removing, degreasing, apply loctite, reinstall would be far stronger and better, I agree.

    Getting to the main bearing and it's retainer is almost less work than removing the front wheel. So a 'see how this works, do the full monty when required'-approach is quite acceptable.

    Oh, I do believe you. It's amazing what modern glues and derivatives can accomplish these days.
    #9
  10. bmwktmbill

    bmwktmbill Traveler

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    This is about the most interesting thread in a year.
    Thanks!!
    bill
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  11. komatias

    komatias Been here awhile

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    Thermal expansion coefficient of aluminium is double that of steel. So the hotter the engine casing gets the looser the bearing will get too.

    I have seen this kind of damage on other assemblies. The hardend steel race of the bearing is fretting the retaining plates. If the motion was continuous, you would see small deposits of the retaining plate on the bearing race.

    I would simply measure the bearing and the bore to see what tolerance band they machined it to and see what can be done. What you might have is that friday afternoon bearing that is just a tad on the low side of the tolerances when the OD was ground..
    #11
  12. Tseta

    Tseta Lost

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    Thanks Komatias, I knew there was a "proper" English term for the micro-movement that I speculated.

    Fretting on Wikipedia.

    Sounds all very plausible and would certainly explain why the bearing retainer wears even though the bearing does not spin in its seat.

    Most interesting...

    -T
    #12
  13. DaBit

    DaBit Been here awhile

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    That 'fretting' stuff sounds very plausible indeed!
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  14. gunnerbuck

    gunnerbuck Island Hopper

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    I plan on doing some engine maintenance in the next little while which includes pulling the clutch... I'll have a look at what state my bearing retainer is in after 124,000 + KMs and I'll check the one on my spare engine now...
    #14
  15. anotherguy

    anotherguy unsympathetic

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    It will be interesting to see if his is an isolated case or not.
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  16. DaBit

    DaBit Been here awhile

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    gunnerbuck: how's the retainer on the spare engine?

    Also interesting to know: roller main shaft bearing or ball bearing?
    #16
  17. gunnerbuck

    gunnerbuck Island Hopper

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    Retainer is good but it's a ball bearing engine...
    #17
  18. skibootdog

    skibootdog Been here awhile

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    from 1996, rxc 620. to 625 lc4 and 05, 06 625 smc, that I rented out by the day for riding on gravel roads, at 12000 to 15000 teardowns the retainers for the bearing in your pic only showed contact wear,no depth. however when any of the the 19 motors with this type bearing would cut the retainer in less than 3000mi after a rebuild. I think the heat transfer from bearing to case expands and weakins the case alloy. the fix is add alloy and rebore case. the dakar motors ktm used at that time were a diffrent alloy than sold to retail buyers.
    #18
  19. DaBit

    DaBit Been here awhile

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    So basically you had:

    - 19 bikes.
    - With 12000-15000 miles (or km's?) on the odometer they were torn apart. Only contact wear visible.
    - With 15000-18000 on the odo (3000 after rebuild), the retainers were shot.

    Correct?

    Questions:
    - All 19 bikes? Thus 19 out of 19?
    - Did you replace the main shaft bearing during rebuild?
    #19
  20. komatias

    komatias Been here awhile

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    Am I correct in thinking that the bearing is not a press fit? (Don't have a LC4 am a mechanical engineer with years of experience)

    If it is not a press fit then of course this bearing will slip or move around. As I said, the expansion of an aluminium alloy is 2 time that of the hardened steel bearing race.

    The solution by ski boot dog is what I would do but then you will have to drop the engine to remove the bearing.
    #20