Beginning the Motor Reassembly. Redux…

The Bottom End…

I’ve had a little time to get my mind back into a positive outlook on this project. All my needed parts have arrived, so it’s time to begin putting it back together. On Project Red, I never removed the crankshaft, so this would be my first time managing that process. This is one of the airhead tasks that comes with a somewhat higher degree of technical ability and should not be attempted without the right tools and comfort using them. I am far from an expert. This is just my record of how I did this work. I am not saying it is the best way to do this.

The Infamous Thrust Washers (stop disks)

The crankshaft in these motors is installed with a required allowance for forward and backward movement. When installed it is held in place in relation to the rear of the motor housing by the flywheel assembly. When the flywheel is removed, the crank can move fore and aft quite a bit. That movement is what can cause major problems with the internals of the motor and is the reason you must secure the crankshaft when you remove the flywheel.

Fully assembled, the range of motion that the crankshaft can move is limited by two thrust washers, called stop disks by BMW. One of these is installed on the inside of the motor housing between the rear of the housing and a rear journal on the crankshaft. It limits the total aft movement potential of the crankshaft. On the outside of the motor housing, a second thrust washer is fitted between the guide ring and the motor housing, beneath the rear main oil seal. These thrust washers are held in place by two pins that are pressed through the housing with each pin supporting both the inside and outside thrust washer. Additionally, the guide ring and flywheel prevent forward movement of the crankshaft.

The assembly needs proper end play to allow oil to do its job protecting the moving parts. The acceptable range of movement is .08mm to .10mm. Because BMW’s factory machining for this fitment is not quite as precise as would be ideal, there are several sizes of thrust washer. The trick is selecting a combination that results in the final assembly allowing motion within the acceptable range. To do this, you have to make a bit of an educated guess about the correct size of the internal thrust washer. It is not a part that can be easily swapped out since it requires removing the crankshaft to access. The outer thrust washer can be swapped by simply removing the flywheel.

Calculating the combinations is one of the airhead tasks that takes patience and organization. I used a good micrometer and recorded my measurements for each one. Then, I created a table to clearly see what I could expect from each combination.

Measuring and Selecting the Thrust Washers

I purchased four thrust washers for this job so I would have options. I also had the internal thrust washer that was already on the bike and still in good condition, so I had five to work with.

I started by measuring my old internal thrust washer and my damaged external thrust washer. To do this, I took 12 measurements around the circumference of each one, noting the measurement. For the external thrust washer, I only had 8 usable measurements, since there was damage from my previous adventures. Of the usable measurements, I added the maximum measurements from each one to get the total thickness of the factory installed units.That gave me a total of 5.2019 mm in thickness.

Next, I measured all four of my new thrust washers, using the same method. I then compared all combinations of thrust washers to see what options I had to land at something close the original total thickness. I selected the thrust washer that appeard to provide the most compatibility for the inside. In my case, this allowed for three possible outside thrust washers to get in the neighborhood of the total thickness.

Measuring End Play

With the inside thrust washer picked, I installed it along with the crankshaft and bearing housing. Then I selected one of the three possible partners for the outside and installed it along with the flywheel. Using a high-precision dial caliper, I then measured the Total movement of the crankshaft. My guess was close, but not quite. It measured at .06mm, which is at least .2mm too low. Swapping out for a different outside thrust washer a reinstalling the flywheel, I landed at .10mm. This is a good measurement and will allow the oiling system to do its job.

At this point, everything looks good, so I proceeded to install the cam shaft, oil pump, flywheel, timing system, and timing system cover. That’s when I ran into an issue.

Binding Crankshaft

After every step in this process, I checked the free rotation of the crankshaft. It just felt like a good precaution and since this is my first go at this specific task, I wanted to be extra sure it was going well. With the timing system installed I noticed the crankshaft felt a little tighter than I expected. Not much, but enough that I was surprised at the resistance. At this point, I double-checked everything including the alignment of the two timing chain sprockets. Finding no issues, I decided to proceed with the timing cover installation.

The cover installed normally and I had the same, slightly tight rotation of the crankshaft until I torqued the timing cover fasteners. Once they were all properly torqued, the crankshaft would not rotate. Hmm. I think this could only be caused by one of two things. Possibly, the inside thrust washer could be too thick and the pressure put on the nose bearing by the timing cover could be enough to press the crankshaft into the thrust washer enough to prevent rotation. The second possibility is that the nose bearing was not fully seated on the crankshaft, even a little cocked to one side and the added pressure of the cover might be causing axial alignment issues if the components were not able to remain in concentricity. Either way, I knew I had to remove some bits to examine more fully.

Ultimately, I opted to remove the crankshaft and begin the whole job again. This time, I ordered a red thrust washer to use on the inside of the motor. This would allow the crankshaft to have slightly more movement towards the rear of the motor and would solve for the first possibility above. I also fully cleaned all of the contact surfaces for the parts in case there was any debris or lubricant that was contributing to the poor fitment.

Installing the Bottom End. Again…

Reinstalling the bottom end went smoothly. This time with a slightly thinner red inner thrust washer, I did not notice any unusual tightening of the crankshaft and free rotation was as I expected for the whole process.. For the final installation, I landed at .12mm of crankshaft end play and no detectible rounout.

I did run into an issue with the nose sprocket this time around. It did not want to slip right on the crankshaft as it did before. This could be a very slight alignment issue with the key, or possibly a misread on the temperature. I wound up having to press the sprocket on, which created no issues.

I also used the Cycleworks installation tools to keep pressure on the nose bearing for the entire cool-down period, rather than simply keeping some pressure on the assembly until the parts bind. I did the same to keep pressure on the timing cover and ensure it had fully seating over the nose bearing. That is not usually needed, but would ensure there was no question about the assembly. This all paid off and I retained expected free rotation of the crankshaft.

To complete the job, I installed the oil pump and reinstalled the flywheel. I’ve had to purchase a bit of an inventory of flywheel bolts since I wound up torquing two sets that I thought would be final. Anyhow, now the bottom end is basically done and I can move on the the top end and stator.

Video of the Bottom End Install