Originally Posted by
ShyTorque
I noticed that SB was dated 2004 and it was obvious from the wording that Lycoming intended to fit valve guides of "an improved material", to negate the requirement. Seems strange that after 17 years the requirement is still there, at least on helicopter engines.
Hi-Chrome valves were installed on engines built after around 1998, that negate the need for that SB. Unfortunately for some reason, you can't fit Hi-Chrome valves to engines installed in rotary wing, so the SB still stands.