O-Rings
An o-ring is the part that fails when a fitting that used to seal starts weeping, and it is also the part most likely to walk off during a teardown.
Alphacool covers the compression fitting sizes: 11 x 3mm and 12.5 x 1.5mm for 13mm hard tube fittings, 14.8 x 3.1mm and 15.2 x 2.4mm for 16mm hard tube fittings, a 12 x 2mm NBR70, and the 12.6 x 0.9mm replacement for the Eisblock HDX-3. Barrow supplies G1/4 o-rings in black, 10 to a pack.
Measure the old one rather than guessing. An o-ring that is close but wrong will seal on the bench and weep once the loop is warm and under pressure.
