You plan on heli-coiling a head bolt? Probably not all that reliable for that application, going up a size would be more reliable , been a long time for me since I did any s2 stroke engine work but it seems to me about 18 ft lbs is the torque, also be sure the head is surfaced on glass with sand paper glued to it so it is true, I can say I have never found one that was not warped to some degree when I checked them, I have a special machine I designed about 35 years ago that surfaces all those aluminum engine parts so when they are put together they mate up perfectly. For a one time thing some 80grit on a piece of heavy glass will work it just takes some time doing it that way.