BMW K bikes (Bricks)

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daveyson

daveyson
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How to understand the K100 engine output shaft. Bmw_k110  


THE GOOD
The Drawing.
Details of a 1986 model, it has been drawn to be easy to look at, not to scale, so some parts are not shown in the lower half of the drawing.

The bearing is fitted into the groove of the engine casing, so is fixed in location. The circlip also acts as a spring and clamps the absorber gear to the bearing, so they become like one solid unit, including the absorber housing which is riveted to the absorber gear. The absorber and its housing have five vanes each, with rubber dampers in between, to cushion heavy loads.

Looking at the bottom of the drawing, tightening the clutch nut clamps together the compression ring, clutch basket, spring housing, absorber, and the end cover plate. They then also become clamped to the fixed in location absorber gear. This also eliminates output shaft endfloat. The absorber housing riveted to its gear, places a preload on the cush drive (absorber assembly)

THE BAD
The Power Flow.
The crankshaft gear turns the absorber gear. Since it is riveted to the absorber housing, its vanes (pink dotted lines) transfer the force to the rubber dampers. The rubber dampers transfer the force to the absorber. The absorber transfers power to the output shaft, because they are splined together. The output shaft transfers power to the clutch basket, because they are splined together.

AND THE UGLY
How It Works.
The thrust washer clearance should be less than 1mm. To measure, hold the output shaft vertical with the bigger bearing uppermost, also with the thrust washer and rubber dampers removed. 

Let's say the clearance is 0.7mm. If the parts are replaced, the clutch basket cannot return to its previous location using a one millimeter thrust washer, it will be 0.3mm (0.3+0.7=1) to the left on the drawing, so there's an 0.3mm gap between the clutch basket and the spring housing.

When the clutch nut is turned with a torque wrench, lets say after a quarter of a turn, the output shaft is pulled about 0.3mm to the left, closing the gap, and the diaphragm spring is compressed by 0.3mm. At this point, lets say its 15 ft/lb. As the torque wrench continues turning to its destination, the pressure on the diaphragm spring does not increase since the gap is closed, but the pressure on the output shaft components is, until the torque wrench setting is met.

Under heavy acceleration for example, the rubber dampers compress under the load. This means the absorber gear has turned more than the output shaft. If not for the thrust washers, this would mean sliding steel to steel contact on both ends of the absorber gear.

As the thrust washers wear, the reduced thickness of the absorbers thrust washer increases the thrust washer clearance. If the clearance is excessive, let's say 0.9mm, rather than dismantle the engine to replace the absorbers thrust washer, a 1.25mm thrust washer could be used on the clutch basket instead, as an option to consider.

If the thickness of the clutch basket thrust washer is less than the thrust washer clearance, the output shaft would have endfloat, causing noise and vibration.

To measure the thrust washer clearance with the engine assembled (in theory,  using a clutch nut that's not also a lock nut, hand tight) if an 0.5mm washer is installed, and the endfloat measures 0.2mm, that would mean the thrust washer clearance is 0.7mm (0.5 + 0.2)

So what should the thrust washer clearance be? Dunno. If anyone does, or has measured it, that would be a welcomed post, I'll guess somewhere between 0.4mm to 0.8mm. I only know of two measured examples, 0.45mm and 0.66mm.


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11/1985 bmw k100rt (late model)  Vin. 0090567
    

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