The stainless steel tubing in the SF-28 and SH-27B in-line heaters may be lined with PE-90 tubing (1.27 mm OD and 0.86 mm ID). 

Slightly stretch the PE-90 tubing and thread it through the stainless steel tubing. The PE tubing will relax into it's normal shape over a few minutes and should provide a tight coupling between stainless steel tube and the PE tubing..