Originally Posted by
HeliComparator
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.
FYI: While the Lycoming SI covering the valve checks for cylinders with the improved guide material states it does not apply to those engines installed in rotorcraft, a call to Lycoming Support may solve that discrepancy for you. Of note, the latest Lycoming helicopter engine model, the HIO-390-A1A, provides a 1000 hour valve check (vs a 300 hr) as standard.