[cooperhw]

You probably already know the thread diameter (more than likely 3/8" N.C.T.) by having removed the original bolt. To know how long a bolt you need I would put the bracket up against the hole and then put a pencil in the hole till it bottoms out. Mark the part of the pencil (or hold your fingernail on it) sticking out. You need a bolt 1/4" less in length than that.