Thanks for your feedback Glen.
Which head screws moved?
On Head 1, Number 6 moved c.30 degrees.
On Head 2, bolt 2 moved 10 degrees, 3 moved 20 degrees and 6 moved 30 degrees.
On Head 3 no bolts moved.
On Head 4, 2 moved 20, 3 moved 10 and 6 moved 35 degrees.
Thus, mostly the bottom bolt, not the top three. Perhaps because the bottom of the head gets less cooling air and therefore hotter and more likely to soften?
Inside my tappet covers is not as shiny as it was at 100hrs. Since then, CHTs have exceeded 160 a couple of times (but none have ever approached the 180C often quoted as max.)Testing with two other instruments have convinced me the CHT readings are within 10 degrees of true.
What you say about backing offhead bolts makes more sense to me than the factory advice; being a 19-reg homebuilt, I have cast a wide net for maintenance ideas. Despite not having always complied with some of the factory’s earlier suggestions, the engine has given years of trouble-free service.