This is our big bore/stroker heavy duty clutch kit for the Raptor 660. This kit was designed to handle the power of a heavily modified engine or extreme riding conditions. The friction and steel plates are the same as our std kit, but the clutch springs are stiffer. We use only cork style friction plates in our clutch kits, this compound is proven to be the most durable of all friction materials. Our steel plates have smooth surfaces, most clutches use "dimpled" or "textured" steel plates. These steel plates DO NOT last as long and can not handle as much H.P. as smooth steel plates. The "dimples" are concave in shape which reduces surface material causing less contact area and easier clutch slippage. We also add an extra full size friction plate to our kits so you can remove the larger I.D. friction plate and wave washer and replace it with a full size friction plate for a stronger clutch kit. This option gives more surface material and can handle harsher riding conditions and higher H.P. engines.