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.
If "wording" you reference is the term "mandatory" you'll find that terminology was added by the legal department instead of the engineering department. What Lycoming has begun to do is to certify some of their new engine variants/models under Part 33 instead of the old CAR 13. While Part 33 requires additional documentation it also provides the path for Lycoming to create Airworthiness Limitations Sections where they have moved some of the more "popular" mandatory SB requirements. Being compliance with the ALS is a regulatory requirement owners for these new models, like the Lyco HIO-390-A1A for the Entrom TH-180, the valve/guide check and a few other checks will now truly be mandatory per the FARs.