These extra Heavy Duty trap doors are made exclusively for Zodiac. They feature extra large diameter bearings to withstand the extra load that comes with high-torque and high performance engines. Also recommended for sidecar use. Extends transmission gear and bearing life. Trap doors are CNC machined from solid billet aluminum and come with bearings installed. As they are a bolt-on replacement for the OEM trap door, installation does not require any further modification. Available for Big Twin 5 Speed transmission 1988 thru 2006, including Twin Cam models, as well as for Zodiac 6 Speed overdrive transmissions.