Surviving the 1/4-mile blast

If there’s one thing every drag racer needs besides nerves of steel, it’s gaskets that can withstand the rigors of full-throttle blasts down the quarter-mile. Stock gaskets that hold up for tens of thousands of miles on the street may not even make it a quarter-mile on the strip. Faced with this challenge, gasket suppliers have developed special high-performance gasket lines to satisfy the needs of even the most demanding racers.

Engine Set Up
Top Fuel and Funny Car racers whose cars are running thousands of horsepower typically use solid copper head gaskets with an O-ring in the head and receiver groove in the block. This setup works well and allows the racers to "fine tune" their engines to prevailing track conditions. Changing the thickness of the head gasket alters the compression ratio and power output so the engine can be tuned to changing weather conditions. Some racers will even use gaskets of slightly different thickness on opposite sides of the engine.

Copper gaskets do not have to be coated if the engine is not running any coolant. The gasket can also be reused. But for engines that do run coolant, some type of coating must be applied on both sides of the copper gasket to seal the coolant passages.

Racers who run nitrous oxide also seem to prefer to O-ring their blocks, though many have also had good success running performance head gaskets with reinforced combustion chamber seals.

Performance head gaskets designed for drag racing usually have a steel or copper fire ring inside a stainless steel combustion seal. The ring concentrates loading in the critical area around the cylinder and can triple the sealing strength. Such gaskets can withstand combustion pressures of up to 2,000 psi! Using this type of head gasket can eliminate the need for special machining that’s required to O-ring an engine.

If you do decide to O-ring the block, steel wire rings work best with cast iron heads, while copper is better for aluminum because it is softer and reduces the likelihood of brinelling (indenting) the heads.

If the cylinders have been bored to oversize, it may be necessary to use a head gasket with oversize cylinder bore openings. You may also want to vary the thickness of the head gasket to compensate for changes in compression and piston height that occur if the block has been milled.

Start to Finish
Regardless of the type of head gasket that’s used, one point that must be emphasized is the need for proper surface finish and flatness on the heads and block. No head gasket can maintain a tight seal if the surface on the heads and/or block is too smooth, too rough or too wavy.

Milling is the preferred technique for resurfacing heads and blocks today, and generally produces a high-quality smooth finish. Even so, it’s still a good idea to check the surface finish with a comparison gauge or profilometer (an electronic instrument that uses a stylus to measure surface irregularities). Flatness can be verified with a straight edge and feeler gauge.

For cast iron heads and blocks with a conventional head gasket, both surfaces should have a finish between 50 and 110 RA (roughness average), with a preferred range of 60 to 100 RA. For aluminum heads on cast-iron blocks, a smoother 30 to 60 RA finish is recommended, with a preferred range of 50 to 60 RA. With soft-faced head gaskets, you don’t want the finish to be too smooth because the gasket needs a little grip to help support the material.

For a late-model engine with MLS (multi-layer steel) head gaskets (such as a Ford 4.6L V8), smoother is better. Most of these require a finish of 30 RA or less.

Flatness is also important. On most pushrod engines with cast-iron heads, up to .003 inches (0.076 mm) out-of-flat lengthwise in V6 heads, .004 inches (0.102 mm) in four-cylinder or V8 heads, and .006 inches (0.152 mm) in straight six-cylinder heads is acceptable. The maximum allowable limit for out-of-flat sideways in any head is .002 inches (.05 mm) — with no sudden irregularities that exceed .001 inches in any direction.

Aluminum heads, on the other hand, should have no more than .002 inches (.05 mm) out-of-flat in any direction. If the clearance between the straight edge and surface exceeds the maximum limits, the head or block should be resurfaced.

Aluminum OHC heads should be checked for flatness in two places: Across the face of the head with a straightedge and down the OHC cam bores with a straightedge or bar. In most instances, both will be off if the head is warped. If the cam bores are still straight and only the face of the head is out-of-flat (a rare situation), resurfacing should be all that’s needed to make the head flat. But if the cam bores are out of alignment (much more common), the head will have to be straightened and/or align bored or honed — and then resurfaced as needed to make it flat.

With pan and cover gaskets, both mating surfaces must also be flat. Dents in pan flanges should be care fully straightened prior to installation. Nicks, gouges or waviness on the sealing surface of a machined cover should be removed by remachining or sanding the cover. The same goes for intake and exhaust manifolds.

• Performance exhaust manifold gaskets are typically designed to accommodate aftermarket headers rather than stock cast-iron exhaust manifolds. Since aftermarket headers are less rigid than cast iron headers, flange warpage can sometimes be a problem. To maintain a tight seal, performance exhaust gaskets are sometimes designed with more built-in compressibility than their stock counterparts. Some are made of copper or graphite to allow more movement between the head and headers as the pipes heat up.
• Determining the "right" exhaust header gasket set can often be a challenge because of the variation in port configurations (round, square, D-shaped, raised), size and spacing that are possible with different heads and headers.
• Make sure the intake gaskets match the heads and manifold. Check port dimensions and match accordingly. It may be necessary to trim the gaskets if the heads and manifold have been ported or modified. It’s also important to determine if the engine is going to be used for everyday driving, drag racing or both. The intake manifold gaskets for a typical street V8 application will have open exhaust crossover passageways so the intake manifold will receive the necessary preheating during warm-up. If these passageways are blocked, as they are on most racing intake manifold gaskets to keep heat away from the manifold, the engine may stall and stumble when cold and/or during cold weather operation.
• Performance valve cover gaskets may be comprised of premium cork/rubber or molded silicone. Premium cork/rubber materials typically have higher temperature resistance (300° F versus around 225° F for standard materials), and are denser for better torque retention and sealing.

• Performance oil pan and timing cover gaskets may have special features that make them easier to remove and reuse. Again, some have metal cores to help the gasket retain its shape.

411 on Sealants
RTV silicone and anaerobic sealants have been around for a long time and are often used in place of certain gaskets. For performance applications, special high-temperature formulations of RTV silicone are available that can withstand temperatures of up to 700° F. Some of these contain metal oxides or copper particles to aid heat dissipation.

The main advantages of RTV silicone are: (1) it can seal almost anything; (2) it can be used in place of a cut gasket if no gasket is available; (3) it remains extremely flexible; and (4) it won’t shrink (it swells slightly when exposed to oil).

Anaerobics are usually recommended to seal castings with machined surfaces, to lock fastener threads (thread-locking compounds) or to hold closely fitting parts in place.

RTV silicone should not be used on cut gaskets as a sealer because doing so makes the gaskets "slippery," which may cause them to slide out of position during installation. But it is sometimes used in conjunction with cut gaskets to seal the seam where gaskets join together (such as the corners under the intake manifold on a V8).

Most racers prefer cut or molded gaskets over RTV silicone sealer for valve covers, oil pans and manifolds simply to facilitate quicker removal and installation. RTV silicone does take about 30 minutes to set, and up to 24 hours to reach full cure. Even so, it is still used in many performance engines to seal such things as cam covers, cam plugs, water pumps, etc.

Contact adhesive is another product that can be used with cut gaskets to position gaskets for easier installation. Gluing the valve cover or oil pan gasket in place, for example, can save time during assembly.

Hardening and non-hardening gasket sealers can be used to improve the seal around various components when cut gaskets are used. Always follow the gasket manufacturer’s recommendations when using these products because sealers should never be used with coated gaskets, such as head gaskets.

For disassembly, an aerosol gasket remover/stripper product is recommended. This can save time and eliminate the risk of damage that can be caused by scraping parts to remove old gaskets.

Sealing Caution
Under no circumstances should any type of chemical sealer, shellac or adhesive be used on a conventional nonasbestos or graphite head gasket. Such head gaskets have a solid or perforated steel core faced on both sides with a soft material such as nonasbestos aramid fiber or expanded graphite. The steel core provides strength and rigidity while the soft facing material allows the gasket to conform to minor irregularities in the head and engine deck surfaces. Many of these gaskets have anti-friction Teflon® or moly-based surface coatings that help the gasket resist shearing forces in bimetal engines. Others that are engineered for cast-iron engines may have raised silicone sealing beads printed on one side to provide improved cold sealing.

If a chemical sealer is used on a coated head gasket, it may have a chemical reaction with the coating material, preventing the gasket from sealing properly or causing it to fail.

If RTV silicone is applied to a head gasket, it may flow when the head is clamped down and enter the combustion chamber and/or cooling jackets. What’s more, RTV silicone is not resistant to gasoline and would quickly dissolve, leaving gaps that could become leaks.

The added thickness of a heavily applied sealer on a head gasket could also cause uneven loading and loss of torque retention which could also create leaks or cause the head to warp.

That’s Tight
Sealing hinges on the fasteners as much as it does the gaskets. A leak-free seal requires proper loading and torque. This is especially critical with head gaskets because of the high pressures they must withstand in a racing environment.

The amount of torque that’s applied to each head bolt or stud nut, as well as the order in which the bolts or nuts are tightened, determine how the clamping force is distributed across the surface of the gasket. If one area of the gasket is under high clamping force while another area is not, it may allow the gasket to leak at the weakly clamped point. All the head bolts must be tightened in a specified sequence and torqued to a specified value to assure the best possible seal. The operation requires an accurate torque wrench that is properly calibrated.

With pan and cover gaskets, overtightening can crush the gasket, causing it to slip or split. Many gaskets have grommets that control the amount of crush, and others have metal or plastic carriers that provide added support and reinforcement. Using an inch-pound torque wrench is recommended on the fasteners for these types of gaskets to make sure they are clamped with just the right amount of loading.