valve mounting area that receive locator pins during assembly to locate and retain the flapper valve during assembly and an armed valve stop the pump uses tubular transfer members for transferring intake and or exhaust air into the crankcase and or between valve head chambers the pump has compact 180 u00b0 opposed pistons that minimize axial spacing between the pistons on the drive shaft and thereby reduces the shaking couple and noise from reciprocation each piston has its own eccentric element press fit into the connecting rods so as not to occupy space between the pistons the shaking couple can be further reduced for pistons of different masses by selecting the mass of the cup retainers to compensate for the difference in overall piston masses the pump includes an improved cylinder sealing arrangement having a circumferential groove in an angled surface at the end of the cylinder the pump also has a special cover and seal for closing the open neck of the pump crankcase and a multi lobed valve stop application_number US 712504 A description fig1 4 illustrate a pump 30 according to the present invention generally the pump 30 has a motor 32 mounted in an inverted manner in a top opening 34 of a housing or crankcase 36 containing two piston assemblies 38 and 39 two cylinders 40 and 41 are mounted to the crankcase 36 in respective side openings 42 and 43 valve plates 44 and 45 and valve heads 46 and 47 are mounted to the outer ends of the respective cylinders 40 and 41 a cover seal assembly 48 is mounted to the open neck 50 of the crankcase 36 over a bottom end opening 52 so that the interior of the crankcase is completely enclosed when the pump.