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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 problems!
Copyright 2020, R. Fleischer

Section 1:  Why do alternator rotors fail?

Back in the earliest Airheads days, rotor windings may have been held in place with some sort of lacquer or shellac.  After enough heating and cooling cycles, the coating might begin to crack, flake off, allowing windings move about from centrifugal (centripetal)  force.  I don't think the method was nearly as good as modern epoxies cured in a vacuum chamber, but it was pretty much standard treatment for motor, generator, and alternator windings back then.  In some instances, no coating had been applied.

I have personally seen modern rebuilt rotors fail from IMproper assembly 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' was intermittent.  The internal connection might short, or more often open at some rpm; including zero rpm.  If the rotor is properly assembled, and then vacuum impregnated with the proper type of epoxy, it should hold up nearly forever.  Should, does not mean WILL!   Rotors tend to open, rather than short, and an often-seen failure place is the hole through which the rotor wire passes.

Rotors can run quite 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 can run quite hot. Additional heating will occur on any model Airhead with the early front aluminum cover which has poorer cooling.  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 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 all these parts, and cycling heat-cold-heat-cold ...tends to bring about failures in things electrical and electronic.

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; 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 having 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 the previously mentioned wire 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 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 time before we know.  The EnDuraLast PM system has a rectifier-regulator box that is, so far, having some problems, only time will tell if that continues.  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 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, but they are comparatively young.

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 a large number of miles on Airheads; most of the mileage was on only three. I had only one rotor fail on 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 BMW /2 and later air-cooled twins is approaching 700K.  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; and, perhaps, same for your reports.

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 are KNOWN to improve reliability:
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, at least modify the older one for some sort of air flow.  I cut my initials into it, VERY LARGE with a wide cutout.  A couple of simple tests by me showed the factory louvered version is better than adding holes to the original type.

Note that aftermarket diode boards and voltage regulators are NOT proven to be better (except as against the Wehrle non-bent-over-soldered leads version of the diode board).   Yes, I know that the makers of aftermarket diode boards say otherwise.

Section 2:  How to remove and replace a rotor safely 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; and absolutely recommend against any rotor removal tool not made of hardened steel.  I also 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.

Do NOT use a spider type of conventional bearing/etc puller. You will ruin the rotor, and possibly cause other damage.

For a description and photo of acceptable and not-acceptable tools to remove the rotor, see the end of part I, in my tools article,,  where a paragraph is headed by:  Alternator rotor removal tool for the stock BMW Bosch Alternators:
Be sure to read that paragraph and look at the photo!

To replace the rotor, YOUR FIRST STEP is to obtain the factory tool 88-88-6-123-600, or an equivalent aftermarket tool.  DO NOT, as noted above, use a multiple-part tool (making or purchasing).   There are potentially VERY SERIOUS PROBLEMS if you do not follow my advice.  You can get an aftermarket tool 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.

YOU HAVE BEEN WARNED!  DO NOT FAIL TO USE A ONE PIECE HARDENED TOOL!  If you use a non-hardened tool, or one INsufficiently hardened, or a two-piece tool of improper lengths, you can BEND the tool or TRAP one piece, 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, so that is why I so strongly recommend against the wrong tools here.

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 such a tool.   Use 8.8 or higher bolt rating!   There is a great danger with a softer bolt that as you tighten it, the innards will BEND, trapping part of the tool inside the rotor.  You then have a truly nasty situation to deal with.   There is also the danger of breaking or injuring 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 for your tool; they are too hard, may be brittle enough to give a problem.  You also do NOT want any inner piece to cant on an angle and trap itself.  Either problem could cause the outer bolt to BEND.  THEN you will have BIG 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.

Next, remove the three long screws holding the stator housing to the engine.  The stator housing, with the stator attached, usually is somewhat stuck to the engine.  Pry on the stator CAREFULLY, using a piece of hardwood between the engine case and the stator will see where to do that at three places it just a bit at a time, continue evenly all around; will see why ...until the stator and housing come out as a unit.  BE CAREFUL as you do this, not only do you not want to pressure the wire windings of the stator, but you also want to be sure that the brushes do not hang up on the rotor 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). Simply pull or push the brushes outwards a bit as you work.   DO NOT pry the stator with a metal tool unless very careful. 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!

The rotor is threaded at its forward outer end, and those threads are used only when removing the rotor with the special tool.

HINT!  Have the bike in gear; rear tire on ground; hold the rear brake or otherwise keep the rear wheel from rotating.   If you don't do something, you might rotate the motor as you tighten the rotor removal tool.

It is at this point, with the stator and its housing removed, that you will insert the special tool into the rotor and begin tightening the tool, to force the rotor off the crankshaft.  The tool is inserted into and through the rotor.   The inner end of the tool is not threaded,  and has a bit of a radius (smoothed considerably).  The tool has threads near its hex bolt head (that portion 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 enough can take a fair amount of torque, ....suddenly the rotor will pop off usually with a bang noise, and it would fall; DO have your other hand ON THE ROTOR as you tighten the tool bolt.   Save the bad rotor. Rotors can be rebuilt and thus may have some value.

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.

The crankshaft nose is drilled and tapped at the factory during crankshaft production.  The inwards bottom area is where the rotor REMOVAL tool works.  The stock HOLDING or ASSEMBLY bolt has a limited length. The rotor is tightened and held onto the crankshaft by that stock hex headed limited length bolt.  Once the rotor and crankshaft matching male/female tapers have been tightened together, they are unlikely to come apart easily, even if the bolt was missing ...which is why only modest torque on the bolt is needed upon reassembly.  I tighten LESS than the book says.   I avoid doing what others often do, rotate the engine for whatever purpose by using the bolt.

The tapers must be very clean and dry when replacing the rotor. I clean the tapers with a fast evaporating solvent before assembly. 
When replacing or reinstalling the rotor, the tapers must be clean and dry (best to not even have fingerprints) ...and the stock bolt need NOT be overly tight is the TAPERS surface area that really holds the rotor in place and it is the TAPER SURFACE AREA that transmits the force ...the bolt is simply a safety fastener. My torque values article have 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 too high.  I tighten by feel, but probably ~ 14 foot-pounds.

When replacing the stator/housing after replacing the rotor, you will want to have the engine case (and stator where they fit together), clean.  I use a rag, then a bit of sandpaper or a wire brush.  It is OK to use the 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 into the engine case better with the surfaces cleaned.  Align the stator, engine case and the three long screws, and do not overtighten those long screws.  Remember to be careful with the stator as you install it, to avoid breaking the brushes or brush holder.


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.
04/10/2018:  Clean up.  Reduce excessive html, colors, fonts.  Improve some explanations and descriptions.  Better layout.  Add 10pxl margins.
07/21/2019:  Move emphasis paragraph to a bit earlier.  Add reference to tools article.  Emphasize not to use spider type pullers.

Copyright 2020, R. Fleischer

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Last check/edit: Monday, December 07, 2020