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Alternator rotor, stator, brushes, etc.
For BMW Airhead Motorcycles


© Copyright 2018, R. Fleischer

Skill level required for most any work on these items:
Non-ham-fisted beginner or better.

Rotors and Stators:

This has been a confusing subject to many. Anton Largiader's website article with text & photos will explain explain some things; or, some differently. I have minor disagreements with his article, does have some useful information & has some photos you should look at. I have the most detailed and pertinent information here in my own article. Read Anton's article here:

The original /5 BMW Airhead motorcycles had a 180 watt alternator with the end that fits into the motor timing chest cavity being 105 mm in diameter. SOME 1974-1975 /6 bikes, & possibly a few barely into 1976, were made with the 105 mm cavity. These /6 bikes with 105 mm stators had 280 watt alternators, so it is possible to upgrade a /5 from the 180 watt alternator to a 105 mm 280 watt alternator with the appropriate parts changes and the parts WILL FIT. These changes are stator & diode board, at a minimum. If your /5 rotor measures ~7 ohms, which was the original value, then I recommend it be changed to a next version rotor of ~3.4 to 4 ohms; the electronics VR is optional, but recommended. ALL /6 and later stators had a center tap on the stator windings, & some small diodes were added to the diode board. If you do not use the /6 or later diode board, the output will be less. I suggest NOT using the last version of the Airhead rotors, which were ~2.8 ohms. For all other /6 and all later Airhead motorcycles, the cavity and stator were 107mm.

Technical information, mostly nerdy:

The original rotors of the /5 were 73.4 mm in diameter. Those were approximately 6.9 ohms and are usually referred to as the 7 ohm rotors. In 1974, the center tap was added to the stator windings, the rotors now had heavier wire and 3.4 ohms, but still 73.4 mm. The R90S, ONLY, had a larger inside diameter for the STATOR. That stator assembly had a BMW part number of 12-31-1-243-003. NOTE the -003 number. The -002 was the stator for all other bikes (not the R90S). If you use the -003 stator, your maximum output is lower, & the curve of rpm at which output occurs is less steep too.

In 1975 or 1976, the ROTOR size was made slightly smaller, at 73.0...probably some bikes had enough crankshaft whipping to allow rotor to scrape against the stator....., which the R90S STATOR was supposed to be the fix-for, originally. My guess is that BMW changed things so as to not confuse the rotors/stators, as with a stator inside diameter change, if used with a smaller rotor, would greatly decrease output; yes, confusing. The STATOR HOUSING has the alternator number on it, and the Bosch number, for these 1975 or 1976 changes was 0 120 340 005. These have slightly lower low rpm output. No books nor commonly available literature describes these things, except in this article you are reading.

Most of these things are a bit moot. The more important things (??) are that the stator part number on it & the fact that as you INcrease the air gap between stator & rotor, by means of changing the stator to a larger inside diameter, or the rotor by decreasing its diameter, or both, then the lower rpm charging suffers a bit, & the high end output could suffer some too. That is why the original R90S parts had fewer total watts deliverable at any rpm.

The final change came in the early 1990's. The rotor became 2.8 ohms, the stator had a change in resistance, now was 0.8 ohm sections, & the stator assembly is 12-31-1-244-641. The maximum output was a bit lower, but output started at a lower rpm.

The result is that STOCK versions of all the alternators from the introduction of the /5 in December of 1969, to the end of Airhead production in 1995-6, was that output wattages could be 180, 234 or 238, 240, 250, 260, 280....and perhaps a few others, or some corrections are needed...there is confusion on the real outputs...and my values here include the Authorities (Police) versions.

I have data on my website from REAL testing, on a known perfect system, a 1983 bike that I personally owned & prepared. The article is: That article will be placed in the Airheads website.  There is a LOT of information besides the Bosch measurements; including extensive expansion of information/output-versus-rpm, etc. I suggest that you read it after you read the present article.

Probably more important than any of the above:

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