A performance engine is only as reliable as the gaskets and surface finish used to seal it
By Larry Carley
The most critical engine gaskets are the head gaskets since they have to withstand the heat and pressure of combustion. And unless the engine is a drag motor with a solid billet or filled block, the head gaskets also have to keep coolant out of the cylinders.
Many performance engines are capable of making several times as much horsepower as a stock engine of comparable displacement, so the head gaskets must be durable enough to withstand the higher pressures. Head lift is probably the most common cause of head gasket failure in a performance engine, and some engines are worse than others (such as Ford small blocks). The amount of head lift an engine experiences depends on combustion pressures as well as the number and pattern of the head bolts, the size of the head bolts, and the rigidity of the block and heads.
When a gasket manufacturer is developing a new gasket for a high power application, they will typically map the areas on the head and block sealing surfaces to determine where clamping loads are highest and lowest, and where head lift may be affecting the seal. They then devise a gasket that may have extra embossment, shims, reinforcing layers or raised sealing beads in problem areas so the gasket doesn’t lose its seal or fail.
Crushing is one of the toughest challenges to overcome, and is a common cause of head gasket failure. Hot spots such as the area between the siamese exhaust ports in small block Chevy heads cause the head to swell more in this area. Aluminum heads can make the crushing problem even worse because aluminum expands at a higher rate than cast iron. If the gasket is not designed to withstand the crushing forces in the hot spot areas, it may permanently deform, lose its seal and fail.
Heat is the enemy, so the hotter the engine runs the greater the crushing loads on the head gasket. It’s not unusual for a NASCAR engine to run as hot as 260 to 270 degrees F or higher during a race. Partially covering the grille opening with duct tape is a common trick pit crews use for improving aerodynamics, but the trade-off is less airflow to the radiator and higher engine temperatures.
These guys are running right on the ragged edge, so anything that causes the engine to overheat can cause big problems very quickly. A piece of track debris such as a discarded hot dog wrapper that ends up clogging the grille may obstruct airflow enough to cause the engine to overheat and blow. It’s happened more than once.
Even in a street performance application, overheating can cause a head gasket to fail. Most stock gaskets can safely handle a modest increase in horsepower without any ill effects. But with big power adders such as a blower, turbo or nitrous oxide, or a significant increase in displacement with a long stroker crank, the resulting power levels may be more than a stock gasket can handle.
Detonation and pre-ignition can also create problems in a performance engine if the fuel mixture goes lean.
Most performance engines are running a relatively high compression ratio compared to a stock engine, and boosted engines create even more pressure in the cylinders. Consequently, if the fuel octane isn’t high enough or the fuel mixture goes lean, the engine can go into detonation/pre-ignition and self-destruct rather quickly. Usually the engine will burn a piston before it blows a head gasket, but mild detonation over time can crack the combustion armor on a head gasket.
Contributing factors that can cause detonation/pre-ignition and head gasket failure include too much spark advance and any cooling problems that cause the engine to run too hot.
When to Upgrade to a Performance Head Gasket
A rule of thumb for performance engine building is that if you are doubling an engine’s power output, or are going much over 550 to 600 hp, you should upgrade to some type of aftermarket performance gaskets. Upgrades usually require no additional modifications. You just swap a performance MLS gasket for the stock head gasket.
Composition gaskets can usually handle up to 600 hp or so. Stock MLS gaskets can usually handle more, depending on the applications. But for serious increases in power, you need to upgrade to a purpose-built performance MLS head gasket.
Performance MLS gaskets often feature special embossments, higher temperature coatings and other design innovations for specific engine applications that help them handle higher temperatures and loads. Some of these MLS gaskets use a more gradual radius on their embossments to reduce stress and improve conformability so the gasket will evenly distribute load across the contact area.
Some MLS gaskets destined for performance applications may also undergo a special stress relieving treatment to improve their durability and ability to retain torque.
Most aftermarket gasket suppliers offer some type of MLS head gasket for popular performance applications such as SB/BB Chevy, SB/BB Ford, SB/BB Chrysler, and even some sport compact engines. MLS gaskets have become the “go to” gaskets for racing and street performance engines because they can handle just about anything.
The only exception today is Top Fuel and Blown Alcohol dragsters and funny cars which are still running copper shims and O-rings. But who knows? MLS gaskets may even find a niche here someday.
One manufacturer told us their latest performance MLS gasket (which has four layers and a special laser welded stopper layer) has been successfully used in a 3,300 hp big block Chevy.
Surface Finish Requirements
MLS head gaskets are made of several layers of embossed stainless steel (most are 3 or 4 layers thick, but some have more). A thin coating (.001˝ to .0015˝) of nitrile rubber or Viton is used on the external surfaces as well as between the layers to provide maximum sealing.
Most aftermarket MLS gaskets can handle surface finishes as rough as 60 to 70 Ra microinches, but some specify a smoother finish of 30 to 50 Ra. Smoother is always better, and if you can get the finish down to the low teens or even single digit, great! But for most applications, a surface finish in the 20 to 30 Ra range is more than smooth enough for a performance MLS gasket.
If you are building a street performance engine that has a cast iron block and aluminum heads, and are using conventional steel/fiber composite head gaskets or expanded graphite head gaskets, the surface finish should ideally be 60 to 80 Ra (360 to 480 Rz).
Don’t go smoother than 40 Ra (240 Rz) or rougher than 100 Ra (600 Rz) with a composition gasket. Rougher surfaces limit gasket conformance, while smoother surfaces increase the tendency for gaskets to flow, reducing the gaskets blow out resistance.
If you are using a stock MLS head gasket, the surface finish should be 30 Ra (180 Rz) or less unless the MLS gasket has a thicker coating that can seal a rougher surface. Note: Never apply any kind of sealer to a MLS gasket! It may react with the coating and cause a problem.
Though Ra (Roughness Average) has traditionally been used to describe surface finish, most gasket engineers today say a more accurate perimeter is Rz, which is the average difference between the peak height and valley depth. Ra can have a wide variance across a given surface profile, so Rz gives a better indication of the actual texture across the surface.
To measure Ra or Rz, you need a profilometer. If you don’t have one, you are shooting in the dark and assuming the surface finish you’re getting is in the ball park. Maybe it is and maybe it isn’t. The only way to know for sure is to actually measure it. Most dry milling machines with the proper CBN or PCD cutting bits can achieve an extremely smooth surface finish.
Waviness across the surface is also important. The less waviness the better: no more than .0004˝ with MLS head gaskets. Trouble is, you can’t measure waviness with a profilometer. It takes special (expensive!) lab equipment. Waviness problems can be caused by vibrations and a lack of rigidity in milling equipment.
The flatness of the head and block surfaces is also critical in a performance engine. The maximum amount of out-of-flat should not exceed .001˝ within three inches in any direction in a stock engine, so even less is best for a performance build.
For a V8 engine, the maximum allowable out-of-flat specification for stock cylinder head and block deck surfaces is .004˝ lengthwise and .002˝ sideways. Aim for .002˝ if you’re building a performance engine. For a V6, the recommendation for a stock engine is less than .003˝ out-of-flat lengthwise and .002˝ sideways. Again, shoot for half that with a performance build.
As for an in-line six cylinder engine, the stock recommendation is no more than .006˝ lengthwise and .002˝ sideways out of flat. If you have .003˝ or less out-of-flat lengthwise and .001˝ sideways, you should be in good shape to seal up the head gasket.
Gasket Installation Tips
The surface of BOTH the head and block must be clean, smooth, flat and free from scratches, pits or corrosion for a good head gasket seal. That’s true for any engine, and absolutely critical for a performance engine. If the flatness of a head or block is not within specifications, it needs to be resurfaced.
When installing the head gasket, follow the recommended procedure for tightening the head bolts (angle gauge or torque-to-yield). Use motor oil or specially formulated head bolt lubricant on the bolt threads and underside of the bolt heads to obtain the correct loading on the bolts and head gasket.
Dirty or damaged threads, dry threads or threads that have been lubricated with the wrong type of lubricant can all create misleading torque readings that can prevent a head gasket from sealing.
Do not reuse torque-to-yield (TTY) head bolts. They are one-time use bolts that permanently stretch when tightened. Reusing them is risky because they can break or fail to hold torque. Toss the old TTY head bolts in your scrap metal bin and replace them with new ones.
If you are reusing conventional head bolts, make sure they have not stretched, are not damaged (no nicks or gouges, no distorted threads) and are not corroded. If any bolt appears questionable, replace it. For high horsepower engines, scrap the stock head bolts and upgrade to stronger aftermarket head bolts.
Whether the engine is a brand new casting or a seasoned block, always clean and tap the cylinder head bolt holes in the block. This will assure proper loading and accurate torque readings when the head bolts are tightened.
Another tip is to brush off the black oxide coating on new cylinder heads bolts before the are installed. The oxide coating increases drag on the bolt threads, and may thus reduce loading on the head gasket when the bolts are torqued to specifications.
Use an accurately calibrated torque wrench. Beam style torque wrenches typically stay in range better than adjustable or dial style torque wrenches, but are harder to read. If you haven’t had your torque wrench calibrated within the last year, have it checked to make sure it is accurate.
Tighten each cylinder head bolt to specifications in the recommended sequence. Some NASCAR engine builders use about 20% less torque on the head bolts when they put the engine together to reduce bore distortion. For a street performance engine, use the factory recommended bolt specs.
Tighten the bolts to full torque in three even steps. Some performance engine builders let the engine sit overnight, then loosen and retorque each head bolt the next day to assure even loading of the head gaskets.
With intake manifold bolts, torque to specifications in a crisscross pattern to make sure the gaskets are evenly loaded and seal tightly. With valve cover and pan gaskets, don’t overtighten cork or rubber gaskets. Installing gaskets that have compression-limiting grommets can prevent crushing and gasket damage. Use a sealer and/or adhesive on cork and composite gaskets, but do not use sealer on rubber or coated gaskets.
For more information on surface finishes and gaskets, check our website. For a complete list of gasket suppliers, visit our online Buyers Guide.