Traxxas backplate/ 20x1. 4mm o-ring (for engines w/o starter) (trx 2. 5, 2. 5r, 3. 3) 38-5274r description: the aluminum backplate replaces the ez-start drive unit on the trx 2. 5, trx 2. 5r, and trx 3. 3 racing engines. It allows more efficient fuel/air mixture flow through the crankcase and significantly reduces overall engine weight. Using the backplate requires an electric bump starter and should only be used on models with unobstructed flywheel access. If the engine was originally equipped with a starter, then a different crankshaft is also required. Use part 5288r (ips) or 5289r5289r (multi-shaft) crankshaft. Accessory part for these models: trx 2. 5r trx 3. 3