If it's for the L36 Ecotec (el naturale)
Head bolts - Tighten them all to 34nm in correct sequence. Then repeat to 90nm
Goop is not recommended.
Brand new head bolts is recommended because the factory bolts are one use (they stretch)
The service manual i have actually states tighten to 34nm then set your wrench to 90nm and turn the bolts 90 degrees (if it clicks at 90nm before reaching 90 degrees then stop). If they didn't get to 90nm then turn another 90 degrees and stop at 90nm) - which is a complicated way of just torquing them to 90nm.
I have just done this job so i have a tip which you didn't ask for but might save a drama.
When your loosening the header stud nuts, crack one, loosen a bit, then tighten it back up. Do this until you've cracked em all then loosen them, obviously using some penetrant first.
If you don't do this, then you will most likely snap the last stud on each bank, because of the angle and pressure of the header on the last stud.
Snapping a header stud is a bit of a pain, although you are taking the heads off so not too hard to 'easy out'.
If you do have stuffed header studs, the thread on the head side is 5/16 unc and on the nut side is 5/16 unf.
Also... Loosen the exhaust flange nuts too, so you can get the headers out of the way. I thought i could just bend the headers out of the way, and i did, got the heads off, but couldn't get them back on like that. Wished i lifted the car and loosened the flange nuts before i pulled the motor apart, but you can reach the bolts on the flange reaching up from the side. A 13mm long shank on a few extensions was the go for that i believe.
Good luck
