The present invention pertains to pump jacks, and particularly to a new and improved form thereof in which the frame of the jack serves as a supporting means for the jack mechanism, and includes in itself the crank case for the engine, suitable casing for the reduction gearing, and the bearings and supports fort the crank shaft and the gearing. The frame of the jack consists of but three pieces, a base-plate, a combined crank and gear case, and a combined cover-plate and crank shaft bearing.