Rob,

Many brands of model freight trucks have a large kingpin screw hole that will accommodate a boss on the bolster that helps keep the truck centered and acts as a stop for the truck screw.  The boss prevents the truck screw from being tightened up to the point where the truck no longer pivots. 

For my freight cars without a boss, I cut the center pivot off a Kadee coupler pocket lid and slip it over the screw.  It can also be glued onto the bolster centered on the kingpin screw hole, with CA if the bolster is resin, or plastic cement, if it is styrene.  The Kadee center pivot becomes, in effect, a shot tube which fits over the 2-56 screw, and allows the screw to be tightened up without binding the truck.

I hope this makes sense.  I'd be interested in others' solutions to the problem.

Todd Sullivan