20mm Plain Bearing Through Deck Single. Our through deck blocks offer frictionless guidance for rope and wire control lines which are required to pass through a deck. Featuring a hard-wearing acetal body shaped perfectly to house the sheave, which results in a performance that you can rely on for adjustable rigging and other highly loaded applications.Available with one or two sheaves and in plain and ball bearing systems to suit a wide variety of applications.Plain Bearing Ð The acetal sheave spins around the central axle. This type of bearing has a long lifespan and is very low maintenance.