If engine designers had followed the lead of Peugeot, Leyland, Bugatti, and a few other automakers in the early days of automobiles, we wouldn’t be having a discussion about MLS vs. copper head gaskets today.
I haven’t seen an MLS gasket blown due to gasket failure. It’s always something else. – Dave Livesey, Borowski Race Engines
Many radial aircraft engines also carried over this integrated concept, but today, only a handful of engines like the Honda GXV line of industrial engines feature a unified cylinder and combustion chamber. Due to costs and manufacturing complexities, the conventional bolt-together sandwich of a cylinder block, gasket, and cylinder head is the norm in today’s production and racing engines.
The challenges of containing all the combustion pressure during the power stroke haven’t faded away, however. The head gasket is still the most stressed static seal in the engine, and “blown head gasket” is one of the most stressful adventures for engine builders, especially in the pits.
Interestingly, a solution developed in the very early days of internal combustion engines is still around to secure even the most powerful engines in racing. We’re talking about a flat, plain copper gasket. They were used in the 20-horsepower Model T, and basically the same design and material are used in 10,000-horsepower Top Fuel engines today as well as Pro Mod and other high-end powerplants.
Hottest trend in racing
The latest trend in head-gasket technology, however, is the MLS, or multi-layer-steel.
You may need a crowbar to get the heads off their massive twin-turbo motors, but Nelson Racing Engines is hell-bent on sealing up the combustion chamber. In addition to O-rings in the block, receiver grooves in the cylinder head and annealed copper gaskets between them, Tom Nelson applies a generous coating of a “secret sealant” sourced from the aerospace industry.
“It’s an A-B mix sealant that takes 48 hours to set,” says Nelson. “It smells, and you need a totally clean block. You can’t use any kind of rag that sheds.”
Nelson’s crew carefully mixes up the mysterious formula and applies it to all areas, front and back, of the copper gasket.
“It basically epoxies the head to the block,” sums up Nelson. “With copper you can always run into water issues. This step pretty much eliminated that problem.”
“Copper is old school but still has a place,” echoes Dave Livesey, engine builder at Borowski Race Enterprises. “The only thing we put copper on today is big boost, more than 25 pounds. I lean towards MLS in every engine I can.”
“Once you exceed Pro Stock, that’s kind of the break for going to copper and O-rings, adds Kevin Kistner of Cometic Gasket, a leading manufacturer of MLS gaskets. Kistner says there is no straight-up dividing line on the horsepower chart. “You have to look at cylinder pressures and overall abuse. It’s not uncommon for us to see big-block Chevys on the street making 2,000 horsepower that use our gaskets.”
In addition to straight copper, some early engines used a thin layer of embossed steel—a design better known as a steel-shim gasket. These made a brief comeback in the ‘60s performance market as Stock and Super Stock racers used the thin gaskets to boost compression ratio.
Arrival of MLS gaskets
When engines started growing in size and power during the war years, engineers realized a single layer of steel or copper would not solve all their combustion sealing problems. Enter the composite gasket. It features a strong metal core covered or sandwiched between a pliable material, like asbestos, that conformed to imperfections in the metal surfaces of the block and head.
Asbestos was replaced with graphite and other non-asbestos materials for health reasons. Composite gaskets are still popular for many factory and replacement applications today. Some performance composite gaskets are also designed with a stainless-steel ring of armor around the cylinder bore, but even with extra reinforcement they’re almost helpless when dealing with high boost levels. In the late ‘80s, sport compact performance engines were being downsized but pumping out more power with the addition of turbocharging. That’s when the industry first witnessed MLS gaskets.
“The smaller, lighter engines may have tempted engineers to use fewer or smaller head bolts,” surmises Ron Rotunno, product manager for performance products at Fel-Pro gaskets. “When you do those kinds of things, you really start getting a lot of head lift.”
Compensating for head lift is the key selling point for MLS. Although the actual gap can be slight, numerous problems will develop with any breach of the seal between block and head. Fel-Pro engineers developed a test rig that feeds over 2,000 psi of compressed air into a cylinder head’s combustion chamber, then linear displacement sensors measure how far the head lifts or moves away from a base that resembles the cylinder block. The data acquisition compares not only sensor to sensor within a cylinder, but also cylinder-to-cylinder numbers.
“Think of it as a mini earthquake on the deck. It could be only .001 or .002,” says Rotunno. “It’s not a tremendous amount, but if the composite gasket doesn’t recover, that’s going to be a big deal. When the head lifts is when you really run the most risk of having combustion gasses escape from once cylinder to the other or to the outside. With an MLS head gasket, you’re essentially building a spring that maintains the sealing pressure between the head and deck when you’re experiencing head lift.”
Don’t over engineer
MLS manufacturers share a basic common strategy when designing gaskets. Three to five layers of 300 series stainless steel are sandwiched together, with two or more of those layers embossed around the cylinder bore and coolant passages. The dimensions and shape of these embossing beads, along with how they’re positioned against each other when stacked, determine the gasket’s “spring rate.” The non-embossed layer provides a strong foundation and is used to help achieve the overall desired thickness. The outside layers are then treated to a thin Viton or polymer coating that helps the gasket conform or “cold seal” to any irregularities in the surfaces of the block and head. Some manufacturers may heat treat the steel before applying these coatings.
“We don’t try to over-engineer the embossments,” says Kistner.
In addition, engine builders shouldn’t try to over-analyze each manufacturer’s embossment tactics. There are many different factors that go into the embossment design, including bore-center distances, along with number and location of fasteners. One manufacturer may use an aggressive arc profile in one area of the gasket, and a “lazy S” in another.
“You can design a gasket to divert more clamping force toward the combustion seal,” says Hunter.
“That’s something you simply can’t do with a composite gasket,” adds Rotunno, “that is, moving the stress around to maximize the ability of the gasket to seal.”
Some MLS gaskets are also designed with a “stopper” feature that helps prevent over compressing the gasket, which could affect the spring function of the embossments.
Engine builders shouldn’t concern themselves with the number of layers in a gasket. Four is not necessarily better than three, and so on.
“It’s mostly to achieve the desired thickness,” assures Kistner.
How does a gasket blow?
Engine builders may be quick to blame the product when there is blown gasket; however, the fasteners, torque procedure and even block/head designs can prevent the gasket from doing its job—especially where there is a serious spike in cylinder pressure.
“Most of the things that are bad, you can’t see,” says Rotunno. “It’s pretty darn hard to blow a head gasket. It’s usually the result of something other than the integrity of the gasket.”
“I haven’t seen an MLS gasket blown due to gasket failure,” adds Livesey. “It’s always something else.”
Bench racing the engine conditions leading to blown head gaskets would take longer than trying to understand a NASCAR rule book. Many of the causes can be traced to an ignition issue, such as pre-ignition or detonation, where the cylinder is severely shocked and cylinder pressures increase dramatically. Another big issue is proper installation of the cylinder head. Improperly calibrated torque wrenches, weak or incorrect fasteners and other factors can allow a loose or uneven clamping force that will spring a leak. Even a cooling system that allows hot spots or an overheating engine will contribute to head gasket failure.
“A lot of drag racers use electric water pumps, and some are not up to the task as much as we had hoped,” says Rotunno.
On the downside, MLS gaskets do require strict attention to surface finishes on the head and block, so working with a quality machine shop or having access to a profilometer is essential.
“You’re dealing with more finite materials in the coatings,” explains Kistner. “You’re compressing the embossments for a mechanical seal, but the rest of the material is not going to conform to surface imperfections.”
Should you reuse MLS?
A suggested goal for surface finish is 30 Ra for MLS head gaskets, but manufacturers may say their gaskets are good for up to 60 Ra. Another installation requirement is dry fitting. Do not apply any type of supplementary sealer or coating to MLS gaskets.
“Some people like to put sealer on everything,” warns Rotunno. “That’s a really bad idea for MLS. Sealant can have an adverse effect on the rubber we use. The problem is overspray getting between the layers, and that can form a leak path.”
“They believe if a little is good, then a whole bunch must be better,” adds Kistner. “It can run out of places to flow out, then gets trapped. You end up hydraulic’ng the head, and it shows up as a false positive on the torque wrench.”
There remains debate over reusing MLS gaskets. Manufacturers officially recommend against it but admit many race teams that tear down engines frequently, including Pro Stock, have not confessed any problem with limited reuse.
“It’s not a money thing with some of these race teams,” says Rotunno. “If you pull the head and there’s still good rubber on both sides, it most likely will seal.”
Reusing copper gaskets isn’t as critical an issue, and that’s not the least of its benefits. Copper gaskets work great in dry-deck applications; that is, engines that don’t run coolant or lubricant through the gasket. As already noted, they’re especially strong in engines with multiple stages of nitrous or boosted applications. Aside from being a fairly strong but pliable metal, copper is an excellent heat conductor, which helps prevent localized hot spots through more efficient heat dispersion. Copper is more elastic than other metals, so it will stretch a little before splitting. Nitro crew chiefs also appreciate the tuning aspects copper gaskets offer, as they can easily switch between thicknesses during routine teardowns to change the compression ratio. Copper gaskets, however, have a tough time sealing fluids.
Why NASCAR doesn’t use copper
“Copper was never a solution for an endurance engine,” says Hunter, noting that even when NASCAR was “killing” composite head gaskets in the late ‘80s, copper was never considered. “But on high-cylinder-pressure drag racing engines, there still is no better solution.”
Copper gaskets are most efficient when supported by an O-ring and receiver groove arrangement. A stainless-steel or copper O-ring is installed into the cylinder block for each cylinder. A mirror-image “receiver” groove is cut into the cylinder head. When the cylinder head is torqued down, the O-ring pushes the copper gasket into the receiver groove, effectively creating a lock around the cylinder. Some engine builders prefer to O-ring the head and machine the receiver groove into the iron block or the iron cylinder liners in an aluminum block.
“That way you’re shoving the copper into a hard iron surface, not aluminum,” says Hunter.
Copper will continue to be the preferred choice for many engine builders who have developed an installation and sealing procedure that works for their applications. Copper spray should be applied on both sides of a copper gasket before installation. Some builders also apply thin beads of silicone around coolant holes on both the block and head, although SCE has diversified its copper gasket line with versions that offer integrated coolant and oil seals on both sides of the gasket. Another requisite for installing copper gaskets is retorquing the heads.
“It’s a pain in the ass to pull the headers or rockers off,” admits Livesey. “We run it on the dyno and go through a heat cycle. Then let it cool down overnight and retorque in the morning. Also, you do have to break loose the nuts or bolts and then torque it back. You can’t just tug on the torque wrench again.
MLS or Copper?
Another caution is that MLS gaskets can’t be used with blocks or heads that have still have O-rings or receiver grooves. On engines that have been O-ringed in the block, the O-ring can be machined down flush and the deck resurfaced.
“We’ve done that with blocks on our CNC machine without any problem,” says Livesey. “If you pull the ring, then you have to deck the block way down and that could change your whole program.”
Finally, one engine builder says copper is more forgiving in the event of a catastrophic gasket failure.
“One thing people should know about MLS gaskets,” says Nelson,” is that when you really blow a gasket it can do major damage to the head. With copper gaskets it’s usually repairable. I don’t know, something turns MLS into a blow torch, and you’re either welding up the head or throwing it away.”
History has demonstrated that many high-performance and racing engines won’t survive with composite gaskets, and MLS is proven to be a worthy alternative if the surface-finish issues are properly addressed. And it appears that MLS gaskets will support even more boost and demanding operating conditions in the future as fasteners and head-installation procedures improve. Conventional wisdom for now, however, suggests that copper gaskets are best suited for non-endurance engines making more than 25 pounds of boost, or when running heavy doses of nitrous or nitro.
“In the big picture,” sums up Kistner, “the head gasket is only part of keeping that cylinder sealed. You still have to pay attention to everything else around it.”