In the case of Lycomings at least these valve guides have been hardened on cylinders produced since 1998, without having to certify a new type. On the Lycoming

In the case of Lycomings at least these valve guides have been hardened on cylinders produced since 1998, without having to certify a new type. On the Lycoming engines approved for 94UL I'm not aware of a restriction that these new guides must be installed for unleaded use.