Changing out a crankshaft

I am chasing down a resonance and I suspect the crankshaft. I have another engine.

I want to keep this as simple as possible, so can I just swap the crankshaft and try it?

Will I need to use the other shells as well, and do I need to replace the bolts? Can I do up the big ends without using the special two-in-one tool?

I have changed the alternator rubbers and have removed the heat shield. This is much bigger than that.

I replaced the output shaft, this didn't fix the problem. When I did, I found the clutch plates had the paint marks in the wrong place. I fixed this and it made only a small difference.

My resonance happens whether I am in gear or neutral. Whether the clutch is engaged or not. Mainly between 2.5k and 5k.

I see your bike is an '85. How many kilometers does it have? If it is a high mileage '85 my suspicion would not be the crankshaft but the output shaft. The output shaft is geared to the crank. The output shaft uses a split gear with a spring between the two halves to apply tension to the gear on the crank and reduce gear lash and noise. The gear is on the end of a cush drive, attached to the cush drive housing with six rivets. This design has two failure points. The six rivets will work against the cush drive housing and, over time, oval out the holes. Eventually the rivets loosen in the holes enough to make a noticeable rattle when cold at idle, and eventually the rivets break off, one by one, and leave you with this nice running engine that will not spin the output shaft.
The spring in between the two gear halves, over time, chews away the pins it attaches to, and allows a bunch of play in the gear as well.
BMW updated both of these items for the 1986 model year, switching to a new, better attached spring and twelve larger rivets to attach the gear to the cush drive. You can install the later model parts in your '85, but doing so requires removal of the gearbox and intermediate housing, as well as the oil/coolant pump. Because the labor charge will be so high, it is usually less costly to buy a used engine from a wreck and simply swap engines. I have been down this road with my own '84 K-RS.
The cranks of these bikes almost never wear out, but there is one thing they occasionally do, and that is to spin the number four bearing. I have seen that too. They put the thrust bearing in a less than ideal place. The bummer with this is the crank is not damaged, but the bearing journal is ruined, so you have to throw the block away. It's not a happy thing to see, especially on a low mile block with perfect Nikasil.

I did replace the output shaft earlier this year. It was a 6 rivet shaft and I put in a 12 rivet shaft. I was trying to fix the vibration. The old shaft was good. Nothing changed although the new cush drive may have had a small effect.

I got a new backlash/pretensioner spring but it didn't fit. I remember having to drill the hole in the spring to 5.5 or 6mm so it would fit on to the shaft. I remember thinking if it is the wrong spring, it may not control the lash. And it doesn't seem to. When I take off at a low/medium power, it rattles a little in a predictable fashion.

This may affect damping of the resonance but it isn't the problem. The resonance was there before the shaft change.

It seems to be greatest at 3.5k. There is a very small range at 4k where it seems to cancel and go quiet, but it picks up again at 4.5k.

I also have an exhaust system issue at 2.5k but not sure about this one. It seems to rattle at 2.5k and in 5th gear at 2.5k under accelleration, the exhaust burbles in a regular way, like there is an issue with one cylinder. At idle, I also have an exhaust note that prefers one cylinder but I know I didn't do one of the clamps up last time I removed them, and I guess the inside of the exhaust is rusted out anyway.

But it makes me kind of suspect an upper engine issue. When I last did a compression check I got 140-160 all round, but I remember I had an issue getting a consistent reading. I wonder if the tapping noise I mentioned in another thread might indicate a burned valve? BTW, the bike has 100,000mi on it.

Zitat von "JoelB"

I did replace the output shaft earlier this year. It was a 6 rivet shaft and I put in a 12 rivet shaft. I was trying to fix the vibration. The old shaft was good. Nothing changed although the new cush drive may have had a small effect.

I got a new backlash/pretensioner spring but it didn't fit. I remember having to drill the hole in the spring to 5.5 or 6mm so it would fit on to the shaft. I remember thinking if it is the wrong spring, it may not control the lash. And it doesn't seem to. When I take off at a low/medium power, it rattles a little in a predictable fashion.

This may affect damping of the resonance but it isn't the problem. The resonance was there before the shaft change.

It seems to be greatest at 3.5k. There is a very small range at 4k where it seems to cancel and go quiet, but it picks up again at 4.5k.

I also have an exhaust system issue at 2.5k but not sure about this one. It seems to rattle at 2.5k and in 5th gear at 2.5k under accelleration, the exhaust burbles in a regular way, like there is an issue with one cylinder. At idle, I also have an exhaust note that prefers one cylinder but I know I didn't do one of the clamps up last time I removed them, and I guess the inside of the exhaust is rusted out anyway.

But it makes me kind of suspect an upper engine issue. When I last did a compression check I got 140-160 all round, but I remember I had an issue getting a consistent reading. I wonder if the tapping noise I mentioned in another thread might indicate a burned valve? BTW, the bike has 100,000mi on it.

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If accurate, the compression is low. I don't know the details of how you did your compression check. Try a leak down check. You will know what is and is not holding compression immediately. Unless you let an exhaust valve clearance get too tight I doubt you burned a valve. More likely you have a small intake leak. Finding and curing those has cured the various ticks that my bikes have developed over the years.

If my engine is any yardstick (meter stick???) they have some resonances. There is a lot of monkey motion inside the intermediate housing. Those gears in there are noisy and that is normal. There are some intake resonances as well. Does pulling the clutch in change the sound? You might have a rough bearing on the input shaft making some noise. It could be a rough bearing anywhere in the engine if you have enough miles on it to wear out an output shaft.