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I am removing the input seal carrier. The tool in my hand is essential to unscrew this part of the rear drive.


Here is a rear drive, taken down as far as needed to renew all four seals. I refer to them as the "inner" and "outer", the "input" and the "spine" seals. The first two seal the ends of the large crown gear, the input goes behind the input splines and the spline seal goes under the 24mm nut that secures the spline piece. All but the spline seal require a special tool installer. It would be almost impossible to install the seals correctly without these tools. Counting the holding fixture, there are five of the tools shown. I assembled this collection in the Butler and Smith days.


The tool here immobilizes the input spline piece so that I can unscrew the 24mm nut that secures it to the input shaft.


for lighting, one for everything else. I stuck them behind the left-hand battery cover so they’d be invisible but handy. After three and a half decades, perhaps it was time to take all the connectors off and clean things up. After a pleasant hour or so of replacing a couple of connectors, sanding all the con- tact surfaces, replacing everything, treating a bit of rust and repainting the mounts I had welded up back then; everything worked and has not presented any further problems. Usually with the fuseless Slash Fives, I simply cut the power (red) wire at the battery and install an inline 25 Amp fuse to protect the entire wiring system. I haven’t worked on a rear drive for a


number of years. This month, I’m now starting on the third one. The first two involved leaking seals, the last one is an eBay purchase. I bought it (for not much) knowing I’d be renewing the drive splines. The various models of airhead BMWs use different drive ratios. The “ratio” refers to the number of gear teeth of the pinon gear as related to the number of teeth on the larger crown gear. The crown gear has the drive splines welded to it to power the rear wheel. The rear (or final) drive requires a


number of steps to get things apart and back together. You can fabricate a number of the BMW


shop tools (or figure out a work-around) to work on many parts of the airhead bikes. Many of the tasks involved in the repair and maintenance of the airheads can be done with reasonable skill levels and basic tools. The rear drives however, really do require the use of specific shop tools for several of the steps of disassembly, seal installation and reassembly. Fortunately, I managed to collect a set of the needed tools a long time ago. The rear drive also requires heat to tear


down and rebuild for several parts of the job. This really IS “one to leave to a knowl- edgeable shop.” There is also a rather fiddly procedure to check the shimming (on two axes) when it goes back together. If the rear drive needs new splines, it can be rebuilt by several vendors. I have used Hansen’s BMW in Oregon for several decades now. If you tear the rear drive down and send them the one part, the cost will run around $300. If you just send in the entire rear drive, it’ll run over $450. While the drive is apart, it is an excellent time to repair the


The torches come out three times during the disassembly of the rear drive.


Here, I am


expanding the area around the input seal carrier, prior to unscrewing it.


This particular rear drive had a buggered up driveshaft fill plug. With hard sharp steel threads in a small soft aluminum hole, this is a commonly stripped fastener.


I did the initial drilling using a


drill press and fixture to get a good 90-degree hole.


Now, I'm cutting the threads prior to


installing a 12mm Heli-Coil to restore the fill threads.


September 2016 BMW OWNERS NEWS


51


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