My older model Seloc service manual (84-96) says 5 yrs/500 hrs and even though my '91 200 probably doesn't have 500 hrs on it, the heads have never been removed, so am a bit over the 5 yr mark. FWIW when I removed the heads on the supposed low mileage '91 130(in 06), zincs had almost no pitting. I can't imagine pulling the heads yearly