An intake manifold brings the fuel/air mixture into the combustion chamber, where it can be ignited. It attaches to the throttle body on one end and the cylinder head on the other. The cylinder head contains inlet and exhaust valves and the intake manifold opens directly over the intake valve. The seal between the intake manifold and the cylinder head must be strong and reliable. The bolts that hold the two pieces together must be the correct tightness or the gasket will be damaged and coolant will leak.
Torque is a measure of twisting force and is measured in foot-pounds. When a homeowner screws in a light bulb, the force his or her fingers put on the bulb is torque. Mechanics measure torque with a special tool called a torque wrench, which shows the mechanic how much force he or she is exerting. Proper torque will keep the bolts that hold the intake manifold and cylinder head together from stretching or sustaining damage to the threads and ensure that the fit between the two components is tight.
There is a space between the intake manifold and the cylinder head containing a metal or composite gasket. The softer gasket molds to the two pieces and prevents leaks. In order to create a proper seal, the bolts along the intake manifold must be tightened in stages: usually finger tight, half of final torque and then final torque. The inside bolts must be tightened first, followed by the outside bolts. The intake manifold manufacturer will include specifications for the proper torque and special instructions for the particular manifold being installed.
The website Tork Specs recommends using a final force of 16 foot-pounds of torque to attach the throttle body to the intake manifold and a final force of 19 foot-pounds of torque to bind the intake manifold to the cylinder head. The site suggests that the bolts be lubricated with engine oil or an equivalent before tightening.
