KK4 Series SMC S Couplers KK Series has a unique connection method. A slim body design and large effective area are achieved with a construction that does not use steel balls and therefore does not restrict the flow path. Blocks are avoided due to no valve spring located within the flow path. This is a lightweight coupler smaller in body size. Note Do not use the S couplers with a leak tester or for vacuum retention because they are not guaranteed for zero leakage