HKS plugs should be .026-.028 straight from the factory. Stock plugs are as much as .036. If you measure all of the HKS plugs and they are all between .026-.028 then you have a choice to make. You can either use them as is or GAP them down to the .022 recommendation that BMS gives. If any of them are outside of the .026-.028 range to start you might as well regap them all to .022. If you are going to regap one then you might as well regap them all. The biggest thing is to keep them consistent.