Lauren,
OK, here goes:-Firstly, you need a gas tight seal, and an oil tight seal, and a water tight seal, so torquing the head down will help the head gasket achieve that. Secondly, you need to reduce the effects of expansion/contraction as the engine heats up and cools down - and ensure the mating surfaces remain flat (and therefore gas tight) during these times. Thirdly, with a side-valve engine, the push rods to the tappets come up through the block to the head, and operate the valves. Torquing the head down will keep relative distances the same, so the valve operating mechanisms will work efficiently and at the right time. Three other parts of the engine that need torquing:- Main bearing caps, big-end bearing caps, and the bolts that hold the flywheel to the crankshaft. There are lots more. Did you know that when you torque up a bolt, you are stretching it? When you stretch steel it goes through an elastic deformation phase (remove the load and it springs back), a plastic deformation phase (remove the load and it stays in the same position), and then suffers terminal failure and breaks. Doing a bolt up is more interesting than it may first appear (or maybe I just need to get out more) ;-) Good luck in your assignment.
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