To make a geardiff for the RB5/RB6,you need the:

Kyosho UM605 Diff Gear Set

Kyosho UM610 Diff Bevel Gear Set

Kyosho UM611 Diff Case Packing

Kyosho UM612 Diff Shaft

Kyosho UM613 Differential Bevel Shaft

Kyosho BRG014TS Teflon Shield Bearing set (items part of the RB6 kit)

Kyosho ORG045S Silicone O-Ring

If you want you can use steel bevel gear set:

Kyosho VZW424 V-OneR4 Steel Diff Bevel Gear Set