Michael Galimi
May 5, 2010
Photos By: Steve Baur

It's common knowledge that head gaskets are a necessary evil in our world of internal combustion. "The purpose of a head gasket is to seal the cylinder head and engine block," explains Kevyn Kistner of Cometic Gaskets. And while sealing the head to the block seems like a simple task, the amount of stress, and the environment those gaskets live in, is as harsh as it gets in the internal combustion process.

That thin gasket is an extremely important piece whether you're talking in terms of horsepower or reliability.

Ron Rotunno of Fel-Pro also added, "From a sealing perspective, they [head gaskets] have the toughest job in the engine, as they must seal hot combustion gases, oil (pressure and drain back), as well as coolant-all the while making positive contact between a dynamic head and block assembly. This is not a static assembly-the head lifts as each cylinder fires and also exhibits scrubbing (east-west movement), for which the head gasket must accommodate." Technological advancements have helped produce stronger and better head gaskets for today's high-performance engines though.

Mike Dezotell of Dez Racing said it best when he pointed to street-driven Stangs in his shop that are approaching 1,000 hp. "Better head gasket technology has allowed us to put serious boost into these engines without fear of blowing the head gaskets. Boost and cylinder pressures are much higher than what we saw 10 years ago," he said.

To get you up to speed, let's start with the basics. There are four styles of head gaskets in the marketplace: MLS (multi-layer steel), composite, copper, and elastomeric. In the Mustang world, composite and MLS gaskets are the most commonly used. Copper head gaskets are quite popular in high-end, low-production, racing engines.

"Modular Ford, or any OHC-style engine, should always use an MLS gasket (naturally aspirated or power adder) to maintain cam-bore centerline and minimize line-bore distortion," explained Brian Carruth of Trick Flow Specialties. This is due to the sensitive nature of the camshaft location in the heads. If thicker or thinner gaskets are used, the cam timing will be altered. MLS gaskets are not actually required for modular applications, but the Trick Flow guys do recommend them. On the 5.0L side, Mike Dezotell said both graphite and MLS work, but leans towards the MLS for superior sealing. Some precautions need to be taken when utilizing this style gasket though, as they require extremely smooth block deck and head surfaces for a proper seal.

Graphite head gaskets are popular, but are deemed for one-time use. A switch to graphite gaskets is suggested when swapping cylinder heads, especially if you convert to aluminum heads. Since iron and aluminum have different expansion rates, graphite head gaskets can draw heat away from hot spots and help keep the heads and block sealed under adverse conditions.

"The key difference between traditional passenger car head gaskets and Fel-Pro Wire Ring-style performance head gaskets is the wire-ring combustion seal. It is inserted within a 321-series stainless steel armor surrounding each cylinder, and the wire-ring design provides highly concentrated sealing around the combustion chambers. The pre-flattened wire-ring raises combustion sealing force to roughly three times that of standard head gaskets. These gaskets are available with either copper (for early aluminum head applications) or steel (all others) wire-rings," shared Rotunno.

As horsepower is increased, better seal is required. Carruth said it best: "Everything has a limit, and if you stress something beyond the material's properties it will fail." The same can be said for graphite gaskets when the cylinder pressure increases due to the use of nitrous or forced induction. There isn't a given power range when graphite gaskets will fail, but most people should consider stepping up to an O-ring or MLS gasket when they add a power adder, especially in 5.0L applications when the heads, cam, and intake have been upgraded.

The O-ring head gaskets incorporate a special wire around the cylinder bore openings of the gasket, and the cylinder head must have an O-ring receiver groove machined in. As the head is torqued, the O-ring is pressed into the receiver groove, forming a stronger seal. When it comes to O-ring-style gaskets, Fel-Pro is a popular brand.

"The Fel-Pro Loc-Wire Performance gaskets work on a similar concept as the Wire Ring Performance gaskets using an oversize wire-ring, which requires a matching receiver groove. This design uses both clamp load as well as the mechanical gasket/wire/deck interface to maintain sealing integrity," said Rotunno.

The latest craze is to use MLS gaskets. "The head surface and block deck have to be perfectly straight and smooth to use MLS gaskets. I use a graphite gasket on stock 5.0L blocks because the deck's RA [RA stands for roughness average and is measured using a profile-meter] is usually too rough for MLS gaskets," commented Dezotell. Cometic recommends a minimum RA of 50 micro-inches for use with MLS gaskets, anything rougher will affect the sealing properties. According to Dezotell, the RA of a stock 5.0L block, despite it being too rough for MLS applications, is just right for the graphite ones.

MLS gasket technology is truly remarkable. Carruth stated, "Because of different materials and the thermal expansion rates, a cylinder head gasket must move and adjust with the block/heads to maintain a seal. A MLS gasket kind of works like a spring."

MLS gaskets often utilize between three and seven layers of steel with the outer layer/layers made from stainless spring steel. The outer layer is also coated with a thin rubber coating. The center layers can vary in thickness and provide the support and seal. Another positive is that they can be reused over and over.

Serious engines with high compression, big nitrous oxide hits, and massive boost will be best suited to the use of copper head gaskets or Cometic Phuzion MLS gaskets. Copper head gaskets are found in the NHRA Nitro ranks, proving durable under the harshest conditions. Ken Sink of Milodon said, "I recommend running copper gaskets on all high-compression and supercharged/turbocharged applications."

The copper gaskets are built using a solid piece of copper and a CNC machine. Milodon says they are acceptable for street use as well. Sink recommends good machine work (Ed. note: on the block and heads to ensure a flat and smooth surface) and a dab of silicone on each side of the gasket around the water ports to help seal them.

Sink made the suggestion to add stainless steel 0.030-inch O-rings in the block, which would require a machine shop to add O-ring receiver-grooves around each bore. The O-ring acts in a similar fashion to the graphite O-ring head gaskets by forming an extra strong seal. Copper head gaskets receive the same torque sequence and final torque settings as composite or MLS gaskets.

Bolt 'em Down
We'd be remiss if we didn't discuss head bolts and/or studs when talking about head gaskets. Having proper head fasteners are an integral part of head gasket sealing and they are available from several companies like ARP, Milodon, Mr. Gasket, Manley, Ford OEM, and others. Essentially, there are three kinds of head fasteners when dealing with Ford engines-torque-to-yield bolts, head studs, and regular head bolts.

For modular engines, most people rely on OEM Ford replacement bolts that are torque-to-yield, or they use traditional head studs. "Bolt stretch will occur when a bolt is installed properly. Torque-to-yield bolts require a specific installation process to properly stretch the bolt according to the manufacturer's recommendations. This ensures that the proper clamp load is being achieved. It usually consists of tightening each bolt to a specific setting, and then turning each head bolt another 90 degrees; then you follow up with another 90-degree turn. These bolts are for one-time use. The stock 5.0L head bolts have the same result-once removed, they must be discarded.

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"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.

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.

Specs You Should Know

  • 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.

Torque Rating

  • 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