Start with the center bolts...
Start with the center bolts and work your way out in a sequence. Three or four steps are required as you start lightly and work your way up to the final torque measurements. Starting in the middle and working out with a three- or four-step process ensures an even and consistent clamp load on the cylinder heads.
"Head studs offer better clamping than head bolts due to a more consistent clamping force. Head studs only have one force when you are tightening them, whereas a bolt has twist and stretch while it is being torqued. I recommend studs when you build an engine with nitrous, a supercharger, or a turbocharger. It is also good for when you are removing the heads frequently," noted Sink. Fel-Pro's Rotunno also suggests a switch to studs because the more times you remove a bolt, the greater the chances of wearing out the threads in the block.
Some believe that adding more torque to the bolts will yield better clamping force, but it is just the opposite. "If you over-torque the fastener, you can take it past its yield point. The yield point is the stress level to which a given material exhibits a permanent deformation. Once that occurs, you've compromised the material," stated Sink.
One of the most popular causes...
One of the most popular causes of a torched head gasket is due to a lean air/fuel mixture (read: detonation). No amount of clamping force or style of head gasket can prevent this disaster.
Adding larger head studs is one way to achieve more clamping force. The stock 5.0L utilizes 7/16-inch head bolts or studs, but it can be modified to accept larger 1/2-inch studs or bolts. However, if you need to modify a stock block for 1/2-inch hardware you're better off going with an aftermarket block (which will have provisions for 1/2-inch hardware). The larger hardware allows for a greater torque range when bolting down the heads-75 ft-lb for the 7/16 versus 105 ft-lb for the 1/2-inch studs. The 4.6L engines usually run just the 7/16 inch.
One of the most important things to know when dealing with bolts or studs is that some sort of lubricant must be used on the threads and under the head of the bolt during the torquing process in order to get accurate torque readings. Assembly lube or special lubricant from ARP is the best stuff to use, but we've seen motor oil used in a pinch. The torque ratings vary depending on the lubrication used, so we suggest checking with the manufacturer prior to any installation.
Improper installation can...
Improper installation can also lead to a blown head gasket. Double-check all of your torque measurements to ensure the proper thickness, bore size, and torque ratings are accurate. This will help lead to a long, healthy life span for your newly installed head gaskets.
- 5.0L stock head-gasket thickness: 0.041 inch (compressed)
- 4.6L stock head-gasket thickness: 0.036 inch (compressed)
- 4.6L stock head-gasket thickness: 0.041 inch ('03-'04 Cobra engine, compressed)
Varying the thickness of the head gasket will change the compression ratio; a thicker gasket will create a larger combustion area, effectively lowering the compression. A thinner one will have the opposite affect. Altering gasket thickness is a good way to make slight adjustments, but it can have adverse effects. Gasket variances will change the height of the cylinder head, forcing a possible change in pushrod length. On a modular engine, it can change the cam timing, so we recommend sticking with traditional-sized gaskets.
Cometic Phuzion head gaskets...
Cometic Phuzion head gaskets are designed for severe applications, and we have seen them used in some of the 2,000-plus-horsepower supercharged and turbocharged entries in NMRA competition. Phuzion is a traditional MLS gasket with a gas-filled O-ring. When the engine heats up the gas-filled ring expands forming a tight seal.
- Torque rating for 7/16-inch head bolt: 75 lb-ft
- Torque rating for 1/2-inch head bolt: 105 lb-ft
Torque-to-yield bolt for modular engines:
Two-Valve
- Step 1: Tighten using 30 ft-lb
- Step 2: Turn an additional 90 degrees
- Step 3: Loosen one complete turn
- Step 4: Tighten using 30 ft-lb
- Step 5: Turn an additional 90 degrees
- Step 6: Turn an additional 90 degrees
Three-Valve
- Step 1: Tighten using 30 ft-lbs
- Step 2: Turn an additional 90 degrees
- Step 3: Turn an additional 90 degrees
Four-Valve
- Step 1: Tighten using 30 ft-lb
- Step 2: Turn an additional 90 degrees
- Step 3: Turn an additional 90 degrees