I referred to the 4*40 rods specificly because... they are the ones people seem to complain about the most as being subject to expansion/contraction with heat/cold.

The error I see most commonly with flexible plastic/nylon pusrods is having 1 to 4 inches of unsupported plastic rod between the fueslage exit and the control horn. (or similar unsupported sections between the TE bulkhead and the servo) The unsupported sections FLEX differently at different temperatures and are sometimes flexible enough in hot weather that you can't get any push at all...

The end sections need support either from added guides, or internal wire. I make sure that a minimum of 1/2 inch of wire is in the fuselage with the rod pushed for maximum deflection.