This is a Traxxas High Stall Clutch Set for the Traxxas T-Maxx .15, 2.5, 3.3, Revo 2.5 and 3.3 trucks. A higher stall lets you rev the engine up higher before the clutch engages, for more power when you are taking off.

This is the Team Integy Lightweight 15-Tooth Clutch Bell for the Traxxas Revo 2.5 and 3.3 trucks. As you can see in the picture, this features cut-outs to reduce the rotating mass and allow for better cooling of the clutch assembly.