The hinge pins (also the ones in the shocks) are for a 1/8" shaft. I'm not sure about the clutch one.

On the hinge pins I really don't have a problem with them coming off. I only lose them on the front hinge pins every now and then since the rear most one will be stripped off if it bottoms out on a jump. I don't really worry about that though, since that pin can't come out.

For the clutch ones, look closely at them. You'll see that one side is flat and the other rounded. When you put it on, put the rounded side facing in towards your engine. Other than that, be sure that it's shimmed properly. I've never lost one.