High quality made stainless steel cylinder with an inner diameter of 23.75mm.  Gives an excellent seal with all cylinder heads I tested.  There's a single vent that's well machined. 

Specifications
Material: Stainless steel
Internal Diameter: 23.75mm
Length: 72mm
Hole: 1/2 (40.5mm distance of the vent to the cylinder head end)
Recommended Barrel Length: 94mm - 208mm

Cylinder Size Guide