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Everyone I've seen has a flat and a spring washer at the front bolt, plain washer on first, then flat. The flat washer is the same diameter as the step on the bolt, the spring the same as the bolts thread which is normally 8mm. As for the brake plate end, large, thick rubber washer same diameter as the step, then the torque arm goes on and that should cover the rest of the step if the rubber washer is thick enough, the idea being that the nut and plain washer on the thread should crush the rubber washer slightly so that acts like a spring washer keeping tension on the nut and also acts as an isolator for vibrations. I was going to make some of those rubber washers but as most of them are already crushed I'd need the thickness of a new one so I could source the correct thickness rubber sheet to make them with Anyone has dimensions of a new one, OD, ID and thickness I'd be obliged.
Plain or flat, same thing. How tight, well nipped it is how I'd describe it, the split pin will stop it coming off if it's too loose so not so tight that you risk stripping it and not so loose you can remove it with fingers. The step on the bolts prevents you overtightening these nuts.