TIPS & TECHNIQUES

A common assembly technique for large scale trains is to use a "self tapping" type of screw to make a threaded hole in a plastic part.  The high quality  plastics that our trains are made of are much softer than these screws. After a screw has been put in an removed several times, the threads in the plastic part will often be gone. Sometimes you can just use a slightly larger screw, but most of the time you have to fix the stripped threads.
 
A stripped threaded hole in a plastic part can often be repaired to be "good as new" by pouring a little plastic casting resin into the hole and letting it set. Then the newly plugged hole is drilled out with a small (#70) bit and the old screw can be driven back in. This patch seems to provide good results as the screws can be torqued very tightly and hold well.
 
Regular epoxy doesn't work very well to repair a hole with stripped threads because epoxy is usually too hard and brittle. It will either crack when the new screw is driven in or if you apply too much force the plastic will break. The casting resin is softer and takes self tapping screws much better.  Plastic casting resin can be found at Michael"s Craft Stores.
 
FIXING STRIPPED SCREW HOLES IN PLASTIC