These alloy sleeves fit all 18 mm motors.
In motors without delays (A8-0, B4-0, B6-0, etc) used in cars or gliders the last bit of thrust is wasted and the hot gases can sometimes cause structural issues. This otherwise lost energy can be directed back through the nozzle and a bit more useful thrust is produced.
These sleeves can also be used in clustered models to prevent an excessive amount of recovery ejection.
In essence, the sleeve converts an 18mm A/B/C/Dx-0 motor into a A/B/C/Dx-P
Length: 60 mm
Inner diameter: 17.8 mm
Outer Diameter: 18.5 mm
Weight: about 2 grams