Sealing ring, Copper, 13.16 x 10.4 x 0.8 mm, Becker, U Series, U4.20 Part# 300090

This is a copper sealing ring (Part # 300090). It is a flat ring made of soft copper. The size is 13.16 mm across the outside, 10.4 mm through the hole, and only 0.8 mm thick U4.20 pump, this small copper ring sits between threaded connections. Think of it as a crush washer. Every time you screw two metal parts together, this ring goes in between. When you tighten it, the copper compresses and stops any air or oil from leaking past.

You can see this in the Seal & Gasket Kit for the U4.20 (Part# 54900040300). That kit includes two of these 300090 rings . The Major Overhaul Kit for the U4.20 (Part# 33805600000) also includes two of them .

So before you order, check how many you need. If you are doing a full rebuild, you will likely need two.