When we assembled my son's GMC Topkick we tightened up the faceplate that holds the handlebars on pretty tight. However yesterday the handlebars came loose so that they would not only rotate within the clamp, but slid a bit sideways. Apparently not enough torque on those bolts.

The steel handlebar has metal ridges running parallel to the tube, also steel are the two bolts and the whatever it is called part that goes between the handlebar and front vertical tube. The front faceplate is aluminum. The ridges in the handlebar wore down part of the faceplate to powder. We don't recall if there were originally grooves in the faceplate to match the ridges in the handlebars, but if there were, they are completely gone now, as the inside is smooth except for some scratches.

After cleaning out the dust we put the handlebars back on and applied a lot more torque by hand with the Allen wrench. Should these bolts be tightened with a torque wrench, and if so, how much torque would be appropriate? At the moment they are hand tightened to the point where they no longer rotate and it feels like the hex wrench is just beginning to bend (with the short end of the L in the bolt).

Thanks.