FMF Factory 4.1 End Caps | 040641. A Now offering complete end cap rebuild kits. Whether it be for a Powercore 4, Q4, Factory 4.1 or the new RCT we have you handled. The kit includes Stainless Steel, Titanium, or Carbon Fiber (RCT Only) end cap, 3/16 stai... - FMF Factory 4.1 End Caps | 040641.