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Home> Continental & Lycoming TroubleShoot
Broken rocker boss in Continental or Lycoming aircraft engine1. Stuck valve. High forces generated by trying to open a stuck valve may break the rocker bosses. A stuck exhaust valve may break the intake boss. If the exhaust valve sticks closed then combustion gas cannot exit the cylinder. The gas compress as the piston moves up to begin the intake stroke. There may be sufficient psi against the relatively large surface of the intake valve to prevent the valve from opening.2. Shipping damage. It's quite possible that the cylinder was dropped during shipment and that the rocker bosses were cracked as a result. The valve train forces during engine operation are enough to cause the crack to grow until the boss breaks. 3. Continental C85, O200, O300, GO300 aircraft engine. Rocker bosses reamed beyond allowable limits. The rocker bosses on these cylinders don't come with bushings when the cylinder is new. During repair the bosses are reamed oversize and bushings installed. If the bosses have worn excessively the hole may have enlarged past the manufacturer's limits. Minimum boss thickness before reaming is .240. Minimum center boss thickness after reaming is .18 per Continental Service Bulletin M73-13. Consult latest information from Continental for possible changes since publication of this information. Even if the bosses are reamed beyond limits a contributing factor is often valve sticking that adds the necessary extra loading to break the undersize boss. 4. Continental 360 aircraft engines - Rocker arm contact with cylinder casting.
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