There are two aspects to this. Head bolts have to be tightened in the correct sequence
, and also progressively
, so that the stresses in the head bed it down completely flat.
The (numbered) tightening sequence
is shown in the first eLearn image I posted at http://www.alfaowner.com/Forum/14293193-post205.html (Swirl flaps How To and MAF clean How To anywhere? 159 1.9)
As you can see, the central bolts are tightened first, and then the others are tightened in a diagonal pattern, spreading out from the centre of the head. This minimises stresses that might distort the head and cause it not to seal properly. For the same reason you can't just torque each one to the final tightness in one go, you do it progressively.
The progression is given by the figures jon156veloce gave
What this means is
- first you follow the tightening sequence (1,2,3,4,5,6,7,8,9,10) with your torque wrench set to 35Nm
- next you go through the same sequence "" with " " " " " 65Nm
- next you go through the same sequence, tightening each bolt by 90 degrees of wrench movement
- then you do the same again
- then you do it a final time
and that is job done.
So each bolt has been tightened to 65Nm then tightened a further 270 degrees, in stages, and in the right order. Which is f-word tight, and explains why they took a 600mm breaker bar to undo. I'll be using it for those 90deg swings, my torque wrench is not long enough to get controlled leverage.
This diagonal, progressive tightening is good practice for most components that have multiple attachment points, especially those made of aluminium. Start with inner fastenings and work outwards diagonally. The inlet and exhaust manifolds on our cars especially. How tight is difficult and very much a matter of experience where there simply isn't room to use a torque wrench. Try and remember how tight they were when you undid them, and use a little less force as they're not now stuck by corrosion. I don't think any of the bolts and studs we've encountered are high tensile steel like head bolts, just normal mild steel. Use a short ratchet handle (5-6") and you won't find it as easy to overtighten/strip threads as using an 8 or 9" spanner and gorilla force. The aim is to stretch the bolt into its elastic region just enough to maintain tension that stops it coming undone. Beyond the elastic range the metal breaks.
When putting studs into components, they should only just be screwed home, not tightened against the end of the hole or thread. Tension on the stud that keeps it in place, is supplied by the nut eventually used to secure the part.
If you ever take a cylinder head off again, nb you should also undo them progressively and diagonally, starting with the outer bolts and working in. How much, it's not critical, but try and keep tightness similar and go through the diagonal sequence 2-3 times until they're loose. One of the purposes of skimming the head surface is to eliminate any lack of flatness arising from distortion, so don't worry if you didn't do that with your old head.
Meanwhile I discovered my nice clean turbo VNT vanes were still a bit sticky as I put the thing back together, due to a small amount of rust on the housing that I've now removed. I'll post some pics later.