The RCM3297 Eccentric Idler set is one of a few timing components which is often overlooked.  When you rebuild your Subaru engine, one practice which is common to almost all builds is a block and cylinder head face skim.  This skim can and does change the dimensions of the engine, which can have an effect on cam timing.  RCM eccentric idlers allow the builder to correctly align the right-hand inlet and exhaust pulleys, and therefore correctly time the engine.