my two piece tester had one schrader at the couple for pressure in and another schrader at the gauge for pressure out(when depressed of coarse). The first tube that screws into the head didn't have a schrader valve in it nor did it have threads for one. Therefore that first tube's volume between the head and first schrader(in the coupler) has to be added to the volume of the combustion chamber. Therefore what the gauge shows is lower than actual pressure with the sparkplug in.