43inches,
Quite so, and the later amendments to FAR 43 do require fatigue lives.
If I recall correctly, the PA-38 was the first FAR 43 aircraft with a design fatigue life.
However, even recent Cessna aircraft have been certified under grandfather clauses, FAA does not generally apply standards retrospectively. And speaking of Cessna, there is the SID program, if it applies to your operation in US --- it does not apply to private operations.
Tootle pip!!