Abstract: A saw bar clamping mechanism, length measuring mechanism, system for measuring the length of a tree stem, feed roller mechanism, pin retention mechanism, and a tree processor attachment comprising at least one of the mechanisms and/or system is provided. The saw bar mechanism simultaneously unclamps a saw bar and releases a saw chain for convenient removal. The length measuring mechanism comprises a measuring wheel operable to extend and retract perpendicularly to a tree stem. The feed roller mechanism includes first-side and second-side rollers, and a bearing having outer and inner sides, the bearing being rotatably coupled at its outer side to the first-side roller and rotatably coupled at its inner side to the second-side roller, the bearing making angular contact. The pin retention mechanism includes a two-part collar fitting into a groove with interference for locking a nut received by the collar.