The old one had the bolt installed from the bottom and a castle nut/cotter pin on top.
The new one has a regular bolt/nut with no provision for a cotter pin.
First, it seems like it would be better to put the bolt in from the top, so it can't fall out, if the nut were to come loose
Second, the castle nut/cotter pin setup seems more secure in general.
So, my thought is to reuse the old bolt/nut...
Idler Arm bolt
