While once again peering wistfully inside my more or less orphaned DPY crankcase I found myself pondering, not for the first time, the purpose of the 3 small screws associated with the main bearing outer tracks. Are they there just to prevent the outer bearing tracks rotating in the crankcase casting or is there some more subtle purpose that has escaped me?
Surely if they were simply to prevent movement then one would be sufficient, three begins to look like the proverbial committee’s horse and if nothing else triples the number of things that might get loose in a place where loose things ‘aint wanted!
Also does anyone know what thread was for these little blighters?
The intention was to be sure that the cup does not revolve in the case bore and by doing so shut off the oil supply. The bore in the case is 2.625″ and the outside diameter of the cup is 0.6295″ so the case must be quite hot to install the cup. The shrink ring has a 0.009″ interference so must be at least dull red heat on installation. I have seen many later engines without these screws and have not (so far) seen any cups that have rotated. To be safe, the interference fit should hold the cup and it should not rely on small screws to prevent it rotating. An anology is the crank fitting, where the drive should be entirely by the fit of the taper and the key just ensures the timing. My guess is that in the dim and distant past, they had problems with the cup coming loose and rotating, cutting off oil supply as the oil hole in the case no longer coincided with the oil hole groove in the back of the cup and so that side would eventually seize. They would have altered the interference fit and added the screw holes and this has been perpetuated. Once they got the fitting sizes correct, the screws were not necessary but were never re evaluated.
So many things on the Scott are like this, so that even Birmingham models had the unique and more expensive three hole carb flange originally designed for single down tube two speeders. Dogma defies logic at times. Kind Regards Roger