Squirrelly 2.5" x 4 inch Exhaust Flex Pipe 304 Stainless Steel Interlocking Liner Coupler For High Performance Applications We have introduced Stainless Steel Flex Couplers that have bellows with interlocking lining to help increased exhaust flow through the coupler, and prevent excessive back pressure to build. Made of 304 stainless steel throughout, including weld rings. Will never rust. Made to handle forced induction turbo and supercharged vehicles. Interlocking lining. Nonmagnetic. 2.5" x 4" long.