The object of this investigation was to determine the mechanical difficulties encountered with Prestone cooling at 300 degrees F. outlet temperatuer in the V-1570 type and V-1150 engines and provide means to eliminate them. The mechanical troubles encountered in the investigation consisted mainly of cylinder head failure, Prestone leakage at various points in the engine-cooling system, insufficient oil supply to cam-shaft bearings, failure of cam-shaft bearing studs and failure of the inlet manifold L and Y pipes. Changes were made which eliminated the mechanical troubles, and satisfactory flight service was obtained. The results of the investigation indicated that very satisfactory service may be had of the 1570 type engine with Prestone at 300 degrees F. outlet temperature as a cooling medium with the changes as indicated. It is therefore recommended that these changes be incorporated in all V-1570 type engines for Prestone cooling.