Snowbum's BMW Motorcycle Repair & Information Website

I have personally seen modern rebuilt rotors fail from IMproper assembly ...one 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 300°F.  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 350°F; 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