This pack contains a total of 8 Springs comprising 2 of each of the following sizes 20mm, 26mm, 34mm and 39mm.  These sizes are also sold individually in packs of 10.  Either 1 or 2 springs can be fitted as required (using 2 springs doubles the force required to open the flap of the letterbox).  Contrary to what many people expect a shorter coil gives a stronger restoring force when comparing coils of the same diameter made from the same gauge of wire.

The spring arms are approx. 24mm long (can be shortened with wire cutters).  All the springs are suitable for rods up to 4.5mm diameter.