Derbi Crankshaft SKF C4 Bearing And Crank Seal Kit (6204 Steel Cage)

The FAG 6204 C4 Steel Cage crankshaft bearing and seal kit is a complete crankshaft bearing and seal kit for bottom-end rebuilds for Derbi engines. It includes two precision crankshaft bearings and a pair of crank seals.

Crankshaft bearings are one of the most critical components in any engine. They support the crankshaft at extremely high RPM while maintaining perfect alignment under constant load, heat, and vibration. When worn, they can cause crank play, loss of compression stability, increased friction, and in severe cases, catastrophic engine failure. For this reason, they should always be replaced during every full engine rebuild.

FAG is a German bearing manufacturer and part of the Schaeffler Group, widely used in OEM automotive production, industrial machinery, and motorsport applications. Their bearings are known for extremely high precision, durable steel construction, and consistent performance under extreme conditions, making them a trusted choice for both OEM and performance engine builders.

C4 Tolerance

The C4 internal clearance is a high-clearance specification designed for engines operating at elevated temperatures and sustained high RPM. In comparison, standard Derbi crank bearings are typically C3 specification, which is suitable for stock or mildly tuned engines. C4 bearings are used in more extreme, high-performance, or racing builds where increased heat expansion occurs. While they may produce slightly more mechanical noise at idle or low load, they provide improved stability, reduced risk of binding at high temperatures, and greater durability in heavily modified engines.

Crank seals are equally important as the bearings themselves. They maintain crankcase pressure integrity by sealing both ends of the crankshaft, ensuring proper fuel-air mixture behavior in a 2-stroke engine. Worn or reused seals can lead to air leaks, unstable tuning, poor starting, lean running conditions, and potential engine damage.

Both crankshaft bearings and seals must always be replaced during a full engine rebuild. Reusing old components compromises the entire bottom end, even if other engine parts are renewed or upgraded.

This kit provides a complete, reliable foundation for rebuilding the engine's bottom end with OEM-level fitment and performance-grade durability for high-stress applications.