Chris-PA
Where the Wild Things Are
I've used 5mm x .8 helicioils on plastic with good results - recoil, ignition coils, carb support screws.If the thread is into plastic a good repair can be made using the threaded inserts for wood in furniture making they have a tapped hole in the centre with an allen socket to enable you to screw them in & a spiky coarse thread on the outside that will bite into the plastic a dab of superglue on fitting the insert keeps them in place they come in various sizes & names looking on E bay under threaded inserts for wood will give you an idea of what they are like.
With helicoils it is important to make sure the screw goes all the way through the coil, or at least most of the way through. If it only snags a few threads the coil can be pulled apart.