The early version of the K100 ( to 04/1985 ) has the problematic round 4 pin wiring connector going into the tank to the fuel gauge and fuel pump. I'm in the process of fitting an improved plug connector. The connector is bolted into the left front underside of the tank using a large diameter plastic threaded nut. I found that when I undid the nut, the sealing rubber washer (Part #: 61 31 1 459 448 described as a "gasket ring") had turned into a soft gooey mess. The dimensions of the washer are 27.5mm OD; 21mm ID, 2.4mm thick. BMW replacements of this part are inexpensive, but, I would like the peace of mind of a better rubber compound - fuel leaks can lead to fire. I've purchased several 100mm x 100mm 3mm thick Viton (FKM) sheeting to make my own with.

Have you searched for a Viton O-ring or circular gasket that would fit?

Hi Duck - You must be psychic - I have just returned from a bearing specialist and been informed they do not stock Viton washers but they did provide me with 2 Viton o-rings (always good to have a spare) matching the ID and OD of the flat rubber washer (gasket). This should work well. It'll b a few months before I trial the o'rings, but I'll post the results. The Viton sheeting I earlier ordered will no doubt in time serve well for other gasoline sealing purposes.