Remember watching and thoroughly enjoying the race where Helmut Dähne & Hans-Otto Butenuth raced their BMW against a 750 Triumph known as "Slippery Sam" ridden by (I think) Percy Tait and either Mick Grant or maybe Ray Pickrell (someone will put me right I guess) Again if memory serves me right they swapped riders every two laps.

Helmut D set a blazing pace and after two laps Hans-Otto took over..
Cannot remember the lap it happened but, as someone else mentioned earlier, Helmut and Hans had been slinging the big BMW around so much that they had ground a hole in the rockerbox cover and eventually ran out of oil..
Slippery Sam went on to win but if the BMW hadn't packed in they would never have caught it.
I'm sure I have a pic or two of Helmut on a BMW... must look for it.