Karl Norris showed me how to tighten head bolts on my old rst. Basically do them in stages and leave 10 mins between. Also put a drop of oil on the threads and either side of the washer to help.
Stage 1 = 20nm then rest
stage 2 = 40nm then rest
Stage 3 = 45-50nm then rest
Stage 4 = 180 degrees
Job done.
Paul.