The cap (7) contains a sealing pad (16) and has a cylinder lock (10) with a bolt (14) to engage in an aperture (22) in the exhaust pipe (3).

It consists of a flat capsule (1) of absorbent air-permeable material surrounded by a rim (1a) and sealed by a closure disk (2) bonded to the rim (1a).

The central region of the outside of the closure disk (2) is coated with a swab-like adhesive layer (5) which in turn is covered by a tear-off siliconized protective film (6).