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Why do BMW/Bosch Airhead Alternator Rotors Fail?
How to replace a BMW/Bosch rotor.
What about reliability of the Omega Alternator Rotor?
What about the reliability of the EnDuraLast PM alternator?
Warning:  Rotor removal tools potential problem!!
Copyright, 2014, R. Fleischer

Section 1:  Why do ROTORS fail?

Back in the early days, rotor windings were held in place with some sort of lacquer or shellac substance.  After enough heating and cooling cycles, it might begin to crack, flake off, allowing windings move about from centrifugal force.  I don't think the method was nearly as good as modern epoxies applied in a vacuum chamber, but it was pretty much standard treatment for motor/generator/alternator windings back then.

I have personally seen rebuilt rotors fail from not being assembled correctly was a brand-new rebuild, right out of the box.  I have seen rotors short circuit; but most have OPENED, and sometimes that 'open circuit' is intermittent, and the internal connection can close at some rpm ...or open at some rpm; including zero rpm.  If the rotor is properly assembled, and then vacuum impregnated with epoxy, it should hold up nearly forever.  Should, does not mean WILL!

Rotors can run fairly hot in their core, particularly when a high percentage of alternator maximum output is used.  Temperatures can approach 300F.  Rotors on the faired RT models (particularly the early ones with NO louvers in the front fairing piece and the early front aluminum cover) run hotter.  I think that the wider the heat-cold range the rotor is exposed to; and, the more often it is exposed to those extremes and changes, the more likely there will be a failure of some sort.   The alternator system reliability for a /5 and some later later models into the 1980's, is IMPROVED by installing a /6 or even later, better ventilated front cover.   I have not, yet, experimented with measured temperatures, using different front ALUMINUM COVERS.

I believe that higher output usage is part of the problem.   With rotors, voltage regulators, and diode boards, the higher the output, the more heat is generated in these parts, and cycling heat-cold-heat-cold ...tends to bring about failures in things electrical and electronic.   A combination of a RT or other such fairing (especially with the older front alloy engine covers that pass less air), and high output, are likely contributing factors.

Another possible factor is that the air that passes over the stator, rotor, and diode board, is not likely the same quantity, nor velocity, between some models. The air flow is not the same, for instance, between the clamshell air cleaner models and the rectangular air cleaner models.  Take a look at the air flow differences ....look at the outer timing chest metal cover which is NOT the same design on all models.  Look at how air gets inside, passes around and over the parts, and exits ....and you will see why I think this is a factor.  I've already noted the improved reliability of the /6 and later better ventilated front covers; this is not only my idea, but BMW eventually mentioned it in a Service Bulletin.

I have NOT run any sort of survey or tests to prove this one way or the other ....but my belief is that a goodly percentage of rotors fail due to how the the rider shifts gears, and how often.  Those that shift abruptly downward might be particularly prone to alternator rotor damage  because of the sudden whipping type of acceleration forces on the rotor windings (typically higher than upshifting); and, upon the rotor halves, which could even cut one of the wires, that goes through a small hole.    I have not made up my mind as to whether or not JUST high rpm is a factor, but I believe that it IS involved, perhaps more particularly on rotors that are NOT epoxy impregnated and vacuum treated during epoxy curing.

The Omega alternator rotors run very hot at their core at high output wattage, it can approach 350F; yes, it has been measured (on the earliest Omega's at least....Emerald Island products)!  That alone might detract some from their reliability.    But if the rotor is vacuum chamber impregnated with epoxy, and the rotor properly manufactured, it may be fine.     It remains to be seen how many fail over the coming years, as miles on them increases and the number sold increases.  SO FAR the reliability is pretty good.  The EnDuraLast permanent magnet alternator should be immune to rotor failures, but whether or not the system is more or less reliable than stock ...or the Omega ...will likely be some years before we know.  The EnDuraLast PM system has a rectifier-regulator box that is, so far, seeing occasional problems, only time will tell.  There have been a number of failures of these "450" watt EnDuraLast system Rectifier/Regulator boxes.  This may be due to battery problems, excessive loads, temperature problems with the RR unit ....or, a combination.  The verdict is not yet in.

I think we just do not have enough data to know if aftermarket alternator systems will be more or less reliable than the stock alternators, as the miles are accumulated.  SO FAR, I think they are fairly reliable.

It appears that the last of the stock Airheads rotors are more reliable than the earlier ones, but it is difficult to make an informed statistically good statement ....after all, they are also YOUNGER, and have had, in general, fewer heat-cold cycles and miles.

I suspect that the only way to determine to any great extent what is going on would be to do a rather detailed survey of miles, number of heat-cool cycles, rpm and shifting types (smooth, match rpm? ...or tends to shift a bit jerkily ....).... average trips being very short, short, or long ....RT? type of front outer cover?  Clamshell model?....etc. Many things could contribute ...making it difficult to say what is going on ....and making it somewhat meaningless (not totally, perhaps) on how long rotors last, or do not, for any one particular rider.

I have had ONE rotor fail on one of MY Airheads; it was an improperly made aftermarket rotor. I've also had ONE diode board fail (standard non-bent leads on as-delivered Wehrle board, 1983 R100RT).  For me, the mileage on my BMW air-cooled twins is approaching 700K (/2 and later models).  I had a LOT of miles on /5 bikes that had the non-epoxy-impregnated rotors; ...with NO alternator problems.   Can we conclude anything much from my statement about MY rotor experiences on RT's?   MAYBE ...maybe NOT.  I have always done FEW short jaunts; never overtaxed my alternators in my street bikes (But, no problem with the alternators in the race bikes either); don't often shift down with sudden sharp rpm rises.  On the other hand, maybe as much as half of that mileage has been on RT's with rectangular airboxes ...which are definitely harder on the alternators, due to heat effects (one of my three RT's came with the NON-louvered front fairing and I DID soon after purchase modify it).  I am not so sure ANYthing can be concluded from my anecdotal report here in THIS paragraph.

I think it would take quite an in-depth survey to determine what was causing the various failures. Even if we could determine trends, etc. ...what would you DO about it?  Increase cooling to the parts? ...HOW?? ...yes, there ARE a few changes that help ...some I am not sure about. 

THREE things that are KNOWN to improve reliability are:
1.  Use a later front aluminum alloy engine cover, it has increased ventilation due to changes in the air passageways.
2.  Use solid metal diode board mounts (and NOT a stack of metal washers) on those models having the original lousy rubber mounts.
3.  If you have a faired-model, be sure the front fairing cover is a louvered version, or, modify the older one for some sort of air flow.  I cut my initials into it, VERY LARGE with a wide cutout.

Section 2:  How to remove and replace a rotor MUST HAVE THE CORRECT TOOL!    You can buy the proper tool from BMW; or, an aftermarket source.  I DO NOT APPROVE the use of a two-piece tool for removing the rotor.  I cannot be sure that the stock rotor removal tool, or aftermarket one piece similar tool, is correct for all aftermarket alternator rotors, such as the Omega and EnDuraLast.  YOU must either KNOW ...or ...make measurements, easily done with flashlight, and a piece of stiff solid wire of large gauge (or piece of old welding rod, or a straightened coat-hangar) and an accurate length measuring tool of most any type.

To replace rotor, your FIRST STEP is to obtain the factory tool 88-88-6-123-600, or an equivalent aftermarket one.  DO NOT, as noted above, use a multiple-part tool.   There are potentially VERY SERIOUS PROBLEMS.  You can get an aftermarket version from many places, including some dealerships!  Here are two places that have their own tool:      NOT .com!     Cycle Works LLC, located at 5805 Haskins Street, Shawnee, KS, 66216   (913) 871-6740.  Contact the owner, Dan Neiner, at: ((NOT .com!!)).   Dan bought out the tools+ section of Ed Korn's well-known Cycleworks business.

WARNING!!!  DO NOT FAIL TO USE A ONE PIECE HARDENED TOOL!!!  If you use a non-hardened tool, or one INsufficiently hardened, you can BEND the tool INSIDE THE ROTOR.  This can cause $$$$ damage to the crankshaft, not just the rotor.  If you think about it, how will you remove the bent bolt, or the rotor, or both?    Usually properly home-made multiple part tools work fine for rotor removal, but if you goof, this can lead to very serious problems.

Disconnect all battery negative leads at the battery negative terminal.  Unfastening the large battery negative cable at the hollow speedometer cable bolt may not be adequate, if you have other wires at the battery terminal itself.  With the battery disconnected you may now safely remove the front alloy cover.  If you have an oil cooler at the top area of the front cover, you may have to remove the two 10mm hex head bolts holding the radiator to its bracket.  Put the cooler on a rag, or otherwise protect the fender paint. 

Next, pull off the push-on connections at the stator and brush holder area. Remove the three long screws holding the stator to the engine.  The stator housing, with the stator attached, usually is somewhat stuck to the engine.  Pry on it CAREFULLY, using a piece of hardwood, between the engine case and the stator will see where to do that at three places it a bit at a time, evenly all around; will see why ...until the stator and housing come out as a unit.  Be CAREFUL as you do this, that the brushes do not hang up on the slip rings, as you can break the brushes ...or break the plastic brush holder if not cautious (expensive from BMW, much less so from Euromotoelectrics).  This is hardly the only caution. Here is the next caution:

DO NOT!! pry with a metal tool (unless very careful) ...AND, no matter WHAT you pry with do NOT want to nick nor scratch the stator laminations nor nick/crush the stator copper wiring.  Be careful where you pry.

When replacing the stator/housing after replacing the rotor, you will want to have the engine case (and stator where they fit together), cleaned up.  I use a rag, then a bit of sandpaper or a wire brush.  It is OK to use sandpaper on the stator metal where it fits into the engine ....and same for the engine cavity where the stator fits.  The stator will fit better with the surfaces clean.

The rotor is threaded at the forward outer end, and those threads are used only when removing the rotor.  Otherwise, the rotor is held onto the crankshaft by the hex headed bolt you see when you look at the rotor.  That hex headed bolt tightens the rotor to the crankshaft, as the nose of the crankshaft is internally threaded (and has a limited depth, and the inwards bottom area is where the rotor REMOVAL tool works).  Once the tapers have been tightened, they are unlikely to come apart easily, even if the bolt was missing ...which is why only light torque on the bolt is really needed upon reassembly.  I tighten LESS than the book says.

A bit deeper into this:
The crankshaft nose is drilled and threaded to accept the long stock bolt you see.  The interface between rotor and crank nose are TAPERS.  The tapers must be VERY clean and DRY when replacing the rotor.  In order to remove the rotor, a special tool is needed.   The tool is inserted into and through the rotor, and the inner end of the tool is not threaded!!,  and has a bit of a radius.  The tool has threads near its hex bolt head (that screws into the rotor threads).   The far end (tip) of the tool, as noted it is not threaded, 'bottoms' in the drilled/threaded hole of the crankshaft which is not drilled very deep.  Thus, as the tool is tightened, it forces the rotor off the crankshaft nose.

Tightening the removal tool can take a fair amount of torque, ....suddenly the rotor will pop off usually with a bang noise, and fall; DO have your other hand ON THE ROTOR as you tighten the tool.   Save the bad rotor. Rotors can be rebuilt and thus may have some value.

HINT!  Have the bike in gear; rear tire on ground; hold the rear brake or otherwise have the rear wheel kept from rotating ...otherwise you might rotate the motor as you tighten the removal tool.

I usually keep a rotor removal tool in my on-bike tool tray on my Airheads ...and even carry one in my K bike tool tray ...never can tell when I will need to help someone.

WARNING...I am aware that article(s) have been written on how to make a simple two piece tool to remove the rotor. Under NO CIRCUMSTANCES should you use a common low grade bolt when making the tool.   Use 8.8 or higher!   There is a great danger with a softer bolt that as you tighten it, the innards will BEND, trapping the tool part inside the rotor.  THEN you have a truly nasty situation to deal with.   There is also the danger of breaking the second piece, that fits into the rotor before the bolt is affixed, if the second piece is of wrong length, or diameter, or hardness.   Do NOT! use a piece of drill shank; they are too hard, may be brittle enough to give a problem.  You also do NOT want the inner piece to cant on an angle.  That could cause the outer bolt to BEND.  THEN you will have BIG PROBLEMS!

When replacing the rotor the tapers must be clean and dry (not even fingerprints!) ...and the stock bolt need NOT be overly is the TAPER that really holds the rotor in place and it is the TAPER that transmits the force ...the bolt is simply a safety fastener. My torque values article has the settings, but here they are:  Factory specifications are 16.6 to 19.5 foot pounds (23-27 Nm).  I suggest that these values are much too high.  I tighten by feel, but probably not over 14 foot-pounds.

Don't over-tighten the stator housing long screws.


09/14/2010:  add section 2 on how to replace a rotor
06/24/2011:  Clean up and clarify minor points.
10/13/2012:  Add QR code; add language button; update Google Ad-Sense code remove language button later, due to problems on some browsers
09/12/2014:  Clean up more.
02/12/2015:  Remove information on using a homemade tool, but emphasize cautions if someone insists. Emphasis that I do NOT approve of use of the two-piece homemade tool, and why.
03/22/2016:  Update metacodes, format, fonts, colors, layout.
10/21/2016:  Metas, scripts, slight layout and other HTML improvements.

Copyright, 2014, R. Fleischer

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Last check/edit: Friday, October 21, 2016