51-24-01 LYCOMING: Applies to All Lycoming GO-435-C2 Engines Serially Numbered 1815-11 and Below Not Having the Letter "P" Stamped on the Upper Right-hand Corner of Each Cylinder Rocker Box and Installed in Navion Model B Aircraft.
To be accomplished by June 1, 1952 or next overhaul, whichever occurs first.
To prevent loosening of the exhaust valve seats in Lycoming GO-435-C2 engines installed in Navion Model B airplanes, the exhaust valve seats are to be peened in the cylinder head. The cylinders must be removed from the engine for this operation. When peening is accomplished, stamp "P" 1/8 inch high on upper right-hand corner of cylinder rocker box flange face near exhaust push rod.
(Lycoming Service Bulletin No. 145 also covers this subject.