Below are the proper assembly instructions for a DIN 3901/3902 metric compression fitting;

1.      Cut tubing square, do not use a roll cutter.

2.      Debur inside and outside edge of tube.

3.      Slide nut and then bite ring onto tube.  Be sure to install bite ring fat side first.

4.      Lubricate all mating surfaces.  The use of system fluid is recommended.

5.      Insert tubing into fitting until it is firmly seated against the tube stop.  Hand tighten the nut as far as possible.  Be sure that the bite ring properly aligns between tube and nut.

6.      Tighten nut with wrench.  1 ½ turns for tube sizes 18mm and below and 1 ¼ turns for sizes 20mm and larger.

7.      Back nut all the way out and remove tube from fitting.  Visually inspect the bite into the tubing in front of the bite ring.  It should be a concentric bite that is about ¾ of the thickness of the face edge of the bite ring.

8.      Re-insert tubing into fitting and tighten nut with wrench until a firm resistance is felt.  Tighten nut an addition wrench flat.

http://www.youtube.com/watch?v=heZJr1iKZ1A

This technical information provided by; Mr. Chris Restorick, Tompkins Industries