I'll be editing this.....
Okay Matt and John,
Here we go again
(Sugaring season well behind us, clean up mostly done. Last year I made 148 gal of Maple Syrup, this year only 86. Not a very good year for me. Weather was too cold.)
Now back to posting about final drives. Yikes!
I have alluded to my theories before in past posts.
First, has anyone ever seen or used BMW special tool No 33 2 601 ? If so, I'd like to talk to you about it.
I am assuming that the BMW Special Tool, or something like it was used during initial factory assembly.
This is the tool pictured in the BMW Service manual that is used for measurements for calculation FD crown wheel bearing shim thickness. I posted a pic of this tool in a previous post (first pic in the next linked post). Note that the artist's rendition of the BMW Special tool suggests that there is a gap between the tool and the outer race of the bearing. That suggest to me that the outer race of the bearing is not being securely or evenly stabilized with respect to the inner race of the bearing. In the Service Manual method, this measurement is to determine the distance from the outer race of the crown wheel bearing to the mating surface of the final drive case:
Don (1dbweldor) fabricated some very nice looking tools. The one on the lower right side looks like a facsimile of the BMW special tool No. 33 2 601:
It seems to me that this tool is designed to hold down the crown gear securely while making measurements specified in the BMW Service Manual. When this tool is securely holding down the crown gear, the inner race of the crown wheel bearing is also secured since it is pressed on to the crown gear. HOWEVER, since the crown wheel bearing is a Class "C" (sloppy) bearing, the outer race can tilt with respect to the inner race. Very light pressure will cause the outer race to cant a little with respect to the inner race.
If the BMW Special Tool fails to secure the outer race exactly parallel to the inner race, then measurements will be off. Note that there is a cutout in only one location of the BMW Special Tool for making the micrometer measurements. Other points around the outer race of the bearing are not accessable for measurement and comparison. If the outer race is tilted, there is no way to determine it.
I fabricated my own "special tool" for making measurements as described in the BMW Service Manual by cutting up an old FD cover. See this tool pictured in the second picture in this thread:
Unlike the BMW Special Tool, my setup allows for measurements 180 degrees opposite each other. With experience I realized that these two locations could yield two very different measurements due to tilting or canting of the outer race. I learned to adjust the cover fasteners alternately until I achieved equal measurements at opposite points of the outer race. Only then was the outer race parallel to the inner race and the bearing not tilted. Once I achieved this, I found that I could get identical results from both the BMW Service Manual method and DMAN's dial indicator method (shown in my video).
It seems to me that the BMW Special tool is designed to hold down the crown gear but DOES NOT secure the outer race of the bearing.
IF the crown wheel bearing was a "tight" bearing, and no tilting of the outer race with respect to the inner race was possible, the no error would be introduced using the BMW Special Tool.
But, if my theory is correct, the BMW Special Tool does not secure the outer race and the pressure placed on the outer race by the micrometer when taking measurements will tilt the outer race. This results in an inaccurate reading and results in a THICKER SHIM when the calculations are completed. AND, if this is true, AND BMW service departments are still using this tool for FD rebuilts, even if they make a good faith effort to measure for shim thickness, they could still be getting an error resulting in excess shim thickness.
My theory goes like this: The engineers that designed the FD and specified the class "C" crown wheel bearing, didn't communicate all that well with the people that wrote the assembly manual. The assembly people came up with the special tool for making the measurements for pre-load shim thickness. They failed to account for the possibility of the tilt of the outer race during measurements which resulted in a bunch of FDs with excess shim thickness.
The beauty of DMAN's dial indicator method as shown in my video is that it eliminates the chance of bearing outer race tilt introducing a measurement error. We owe him a great debt for introducing his method to us.
Just to give credit where credit is due, here is the original post by DMAN describing his method.