Keystone Frame

A keystone frame, the American term for an open diamond frame, has no down tubes running beneath the engine. Instead, the engine is bolted in at the front and rear and acts as a load-bearing part of the chassis, completing the frame like the keystone of an arch.

The layout saves weight and leaves space around the engine, but the crankcases must be strong enough to carry chassis loads. It has been used on many Japanese and European road bikes, and trellis and beam frames with a stressed engine follow the same principle.

Engine mounting bolts on such frames are structural, so they must be tightened to the specified torque and checked after any engine work.