As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ANOTHER OPERATOR OF CESSNA 421'S ADVISED THE NEED TO REMOVE VALVE SPRINGS TO CHECK THE SPRING RETAINER (PN 642343) FOR BREAKAGE DUE TO VALVE GUIDE NOT BEING INSTALLED ALL THE WAY - THEREBY CAUSED 'ROCKING' AS SEAT IS HELD OFF OF CYLINDER. THIS OPERATOR DISCOVERED THIS PROBLEM AFTER INSTALLATION OF MAGNETIC PICK-UP DRAIN PLUGS (AND LENGTHY SEARCHING).