Thanks everyone, for the many suggestions.
I visited the BMW dealer, and had no trouble finding the part.
When I mentioned flex disk, the parts man said "what size". He said
they come in small, med, and large. He than walked about 5 ft, pulled
out a couple of drawers, and brought a 4 inch disk, and a 7 inch disk
for my inspection.
I selected the small one. The price was $63 with tax. A little more
than the $25 mentioned in the by Leeson.
The actual size is 4.25 in X 1 inch thick, with 6, 1/16 inch wall
inserts for 3/8 inch bolts. Stamped with the BMW logo, which probably
added $30 to the price.
A couple of weeks ago, I spent a few hours looking for off the shelf
Most of the flexible couplers I found by searching the internet are made
for marine applications, and expensive.
I spent a lot of time looking at couplers in the McMaster-Carr and
Graniger catalogs. There are several types available that might do the
job and the ones for 2 inch shafts were also a little pricey. You need
to buy 3 pieces. Two, 2 inch shaft couplers, and a rubber insert. As I
recall, Boston Gear has a nice one that would be about $150 for each
coupler, and $40 for the insert.
So, I saved a little money, and have the joy of trying something new,
and making my own part. The part will be a lot easier to make than the
custom top plate and rotor shelf I just made, for the non Rohn tower I'm
Now that I have the BMW part number, I will look for a cheaper
substitute for the next one I need.