I'm replacing the R4 compressor on my 240D.
The bolt uses the hole that goes through the body of the compressor. It does not use the threads in the compressor body. It's as long as the compressor is wide plus just enough on the end for the nut. The bolt head and nut use a 14mm socket.
If you have a junkyard near you, you can get the bolt from any GM (Buick, pontiac, etc) from the 80s.