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So you have to remove your alternator rotor, eh? Whether it's because you have a dead rotor or need to remove the timing chain cover plate, it's a painless job, with the right tools. Getting To The Rotor: Disconnect the battery's ground wire. Remove the 2-3 bolts holding on the front engine cover. Note their location first, then remove the alternator's wires (clockwise from noon on most models: blue, yellow, green, red, black and brown). (Note: this is a good time to clean your wiring connections.) Remove the three stator housing bolts at 2, 6 and 10 o'clock. If the stator does not come loose on its own, gently use a large wrench, plier end or BMW steering nut tool to pry between the engine case and the stator winding holder, BUT NOT THE WINDINGS THEMSELVES. Rotate pries 4, 8 and 12 o'clock to ensure that it is removed squarely. Upon reinstallation, tighten stator housing bolts in a similar fashion to ensure a square installation. This is also a good time to inspect/replace alternator brushes if they're 8mm or shorter.
Rotor Assembly Overview: The rotor fits on the conical end of the crankshaft at the front of the engine. Both the crank's tip and the rotor are threaded. The BMW rotor bolt is only threaded at the end, but UNLIKE most bolts, the smooth portion between the bolt head and the threads is smaller in diameter so as not to get caught up on the rotor's threads. The rotor puller has these switched as seen below: 
(Note how the alternator bolt clamps the rotor onto the crank where the puller pries the two apart.) So now you need a rotor puller, right? Our links page (Use ctrl-f to find text in page) has links to many businesses like Cycleworks, Motorrad Elektrik, Bench Mark Works, Eurotech and other good sources for one-piece rotor pullers that cost anywhere from $4-10 before shipping. If you want a one-piece, check 'em out. But as you can see above, all you really need is bolt and a piece of metal (shank) of the right sizes and strengths so that it won't deform in use which can prove costly. So you need two parts: The first one's easy. Go to your local hardware store and buy a M8 (8mm shaft diameter) 1.25 (thread pitch) grade 8.8 (strength rating stamped on the bolt head, higher numbers are fine, but NOT lower) 40mm long (the length of the THREADED section of the bolt) HEX head (not allen) bolt. DON'T buy a bolt with less than 40mm of thread or it might not work. DO NOT use the BMW alternator bolt as a substitute since they cost +/- $8 to replace and a M8 1.25 40mm grade 8.8 hex-head bolt costs less than 50-cents. The next part (the shank) is just as cheap, but takes a little sweat to make. Though I've heard of some cutting up a allen wrench or drill bit in a pinch (but NEVER a common nail), this is a tried and proven way. Buy a 1/4-inch diameter, grade 8 (six marks on the bolt head that make a "*") bolt that is at least 4 inches long (what we're really after is the 2-inch unthreaded section in the middle). If you can only find metric bolts, get the largest one whose unthreaded portion is at least 50mm long, will slide inside a M8 1.25 nut and is at least grade 8.8. As with the bolt mentioned in the previous paragraph, do not use a lesser rated bolt. 
Use the store's bolt/chain/cable cutter to snip off the head and threaded ends of the 1/4-inch bolt leaving a 2-inch unthreaded section. Grind the ends flat, bevel the right (90-degree) edges and deburr. Chucking the shank in a brace (manual hand drill) makes it easier to hold and spin for an even grind. There are two important reasons to bevel/round-off the ends besides removing the sharp edge: the shank will tend to center itself in use and it makes the best advantage of the bolts strength in use (elasticity, force direction, etc.) while resisting mushrooming or other deformations. 
Using The Puller: Put the bike in fifth gear and have someone step on the rear brake; this keeps the crank from spinning. Remove the BMW alternator bolt and install the puller shank followed by the puller bolt and spin it in finger tight. Use a socket wrech to tighten the bolt and it should pop off into your waiting hand. If it proves stubborn, leave the bolt tightened and give its head a sharp rap. The shock should cause it to come free. A good rotor will show no more than a couple ohms resistence between the slip rings and no continuity to ground with either slip ring. All mating surfaces must be totally free of dirt, dust, lint or petroleum products before reassembly. Occasionally, a rotor's short only occurs when it's spinning in which case it must be replaced with a known-good rotor to check if that is the problem's source. Do NOT run the bike without a rotor as its base mates with crankshaft/timing-chain-cover seal. Many businesses like Motorrad Elektrik sell rebuilt rotors for $90 USD with core exchange. Trick: If you think about it, all you really need to carry or be able to source is the shank. There are many M8 bolts on an Airhead that you can make do with in a pinch like the rear axle pinch bolt on twin-shock models, some fork pinch bolts, fender/fork mount bolt, etc. as long as the threaded part is a minimum of 40mm long. You might be able to use shorter thread length bolt, but it might get pretty tight. Using a shank longer than 50mm is ill-advised as you want the puller bolt to have maximum engagement with the rotor's threads. Hope it helps. Joe 'Cuda' #3335
'78 R80/7 |
