Be careful. Around here grey plastic is Poly B piping and it is not manufactured anymore, nor is it approved for new use. (has to do with leakage issues and subsequent bankruptcy of the manufacturers due to liability issues). The only fittings we can buy for them are polyB/pex couplers to allow you to cut into the line and link Pex piping in. Regardless, if it is polyB, it shouldn't realy be used in an underground use anyway.....