The oil pump on the 75 RD250 motor has this strangely shaped cover over the large washer behind the nut. I don't see it on the parts fiche. I can only think it just stops the knock pin from falling out. Is this correct?
correct. it was on the later bikes.
I know of a few people who could have used that little doohickey on their pump
Arrow mentioned this and said it was really a stop-gap that didn't actually work. If the pin walked out, the protector would simply bind the pump or knock it out the rest of the way anyways.
There is no substitute for a correctly fitted, tight fitting roll pin. The idea of the roll pin retainer didn't really work very well at all, hence why it was phased out and two subsequent methods (to retain the pulley) added later. The addition of the plate also effectively shortens the length of the threads on the plunger shaft. This can be seen in the OP's photo.
Also, and I've seen it a few times, if that bracket gets knocked at all it can prevent the minimum stroke.
I fit the roll pin in a special way. With a tapered end at first (for ease of fitting and to protect the soft aluminium hole), but when completely fitted there is no tapered end, so full diameter all the way through the hole.
It wasn't fitted like this in the factory.
I also greatly prefer to press the roll pin in, as opposed to hammering it in.
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