For a US spec M103 it's 70Nm (~51 lbft) then 90-degrees then another 90-degrees.

Tightening sequence:

12 - 8 - 4 - 2 - 6 - 10 - 14

#1

11 - 7 - 3 - 1 - 5 - 9 - 13

Clean the head bolt bores and oil the head bolt threads before installing.

Doing this without a manual? Have fun.

Sixto
91 300SE
87 300SDL
83 300SD