The installation of gaskets is one of the most important aspects of rebuilding an engine. Gaskets provide a seal between mating surfaces so oil, coolant, vacuum and pressure donít leak. As long as all the gaskets are installed properly, everything should be fine, and the engine shouldnít leak. But, as every rebuilder knows, even a small leak can create an expensive warranty problem for you and your customer.
Nobody wants oil leaks, coolant leaks, vacuum leaks or combustion leaks, so in this article weíre going to review some basic installation tips and tricks that can help you minimize the risk of gasket failure.
Keep it clean!
For a head gasket to seal properly, the head and engine deck surfaces must be clean, smooth and flat. Any foreign material such as dirt, carbon, old gasket material or abrasive residue on either surface may interfere with sealing by preventing firm contact between the mating surfaces and gasket. Debris can act like a bridge, and form pockets that create a path for leakage to occur.
Debris can also become embedded in the surface of the gasket and damage it. Debris may even damage the surface of the head and/or engine deck, which can lead to repeat head gasket failures unless the damage is diagnosed and repaired.
To avoid these kinds of problems, make sure the surface of both the head and block are perfectly clean before installing the head gasket.
Old gasket material, carbon, rust, scale, dirt and other debris may be removed from the head and block by applying a gasket removing compound, by using a general purpose degreaser, and/or by scraping the surface of the head with a scraper or wire brush.
Using an abrasive pad in a drill may also seem like a quick and easy way to remove gasket residue, but itís risky because the abrasive can also remove metal, leaving an uneven surface that may not seal properly. For this reason, some gasket manufacturers do not recommend using drill-powered abrasives to remove old gasket material.
When cleaning aluminum surfaces, use a non-metallic (brass) scraper so you donít gouge or scratch the relatively soft metal.
Itís also a good idea to make sure the combustion chambers in the head are perfectly clean, too. Hard carbon deposits may flake loose when the head is being installed and end up between the gasket and head.
Heads and blocks that have been cleaned in a bake or open flame oven and then bead blasted to remove the ash usually come out clean with no residue left on the gasket surfaces. But heads and blocks that are cleaned in a spray washer or soaked in a hot tank or cold tank may require additional scraping or bead blasting to remove stubborn residue from gasket surfaces. For this reason, itís important to always inspect the gasket surface to make sure thereís no residual debris left that could cause a sealing problem. In other words, never assume parts are 100% clean just because theyíve been washed.
Resurfacing a head (milling, grinding or belt sanding) usually removes all traces of debris. But milled, ground and sanded surfaces can sometimes pick up debris from the resurfacing operation itself. So again, donít assume the head is clean just because itís been resurfaced. Check it and clean it if necessary to remove any metal chips or residual traces of abrasive or oil. Ordinary soap and water works best for a final cleaning.
Once a head has been cleaned, handle it with care to keep it clean.
ē Place it on a clean work surface, preferably one thatís covered with clean rags or towels. Or, place the head inside a clean plastic bag and keep it sealed until itís time to install the head.
ē Never set a clean head on the ground or shop floor. The weight of the head can cause dirt to stick to the head and end up on the gasket.
ē Clean hands are a must when installing the head gasket and head. Greasy fingers can pick up a lot of dirt that may end up on the gasket or head surface. Grease and oil can also interfere with a good seal, so keep the gasket as well as the head and block surfaces as clean and dry as possible during gasket installation.
Smooth and flat
For head gaskets to seal properly, the surface finish and flatness of the head and block must be within acceptable specifications. For cast iron head and blocks with a conventional head gasket, both surfaces should have a finish between 30 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. For a late model engine with MLS (multi-layer steel) head gaskets, follow the vehicle manufacturerís recommendations, because some of these engines require a very smooth finish (most are less than 30 Ra).
Flatness is important, too. On most pushrod engines with cast iron heads, up to .003˝ (0.076 mm) out-of-flat lengthwise in V6 heads, .004˝ (0.102 mm) in four cylinder or V8 heads, and .006˝ (0.152 mm) in straight six cylinder heads is acceptable. The maximum allowable limit for out-of-flat sideways in any head is .002˝ (.05 mm) ó with no sudden irregularities that exceed .001˝ in any direction.
Aluminum heads, on the other hand, should have no more than .002˝ (.05 mm) out-of-flat in any direction. If the cl earance 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 straight edge, and down the OHC cam bores with a straight edge 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 thatís 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 ó 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 carefully 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.
Use sealers with 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 of some type is used on a coated gasket, it may react chemically with the coating material, preventing the gasket from sealing properly or causing it to fail. A sealer may also undermine the anti-friction characteristics of a Teflon or moly-based surface coating on a head gasket, causing it to stick when it shouldnít. This would increase the shearing forces on the gasket, which could also lead to early failure.
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. Whatís 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.
On late model engines with MLS head gaskets, some do specify the use of a sealer. So always follow the gasket supplierís recommendations when it comes to the use of sealers.
For metal, fiber or cork pan and valve cover gaskets, sealers and adhesives are OK. But, donít use these products on coated, rubber or silicone gaskets.
To hold pan and cover gaskets in place during assembly, a multi-purpose quick drying contact adhesive can prevent a gasket from slipping out of place.
Watch those fasteners
When gaskets are installed, a good seal depends on proper loading and torque. This is especially critical with head gaskets because of the high pressures they must withstand when the engine is running.
The amount of torque thatís applied to each head bolt as well as the order in which the bolts 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. So all the head bolts must be tightened in a specified sequence and torqued to a specified value to ensure the best possible seal.
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.
Paying attention to the following suggestions should help reduce gasket problems caused by improper tightening:
1. Make sure all the head bolts are in perfect condition with clean, undamaged threads. Dirty or damaged threads can give false torque readings as well as decrease a boltís clamping force by as much as 50 percent! Wire brush all bolt threads, carefully inspect each one, and replace any that are nicked, deformed or worn.
2. Dirty or deformed hole threads in the engine block can reduce clamping force the same as dirty or damaged threads on the bolts. Run a bottoming tap down each bolt hole in the block. The tops of the holes should also be chamfered so the uppermost threads wonít pull above the deck surface when the bolts are tightened. Finally, clean all holes to remove any debris.
3. For head bolts that screw into blind holes, lightly lubricate the bolt threads as well as the underside of the bolt heads with engine oil. For head bolts that extend into a coolant jacket, coat the threads with a flexible sealer. Failure to coat the threads may allow coolant to leak past the bolt.
4. Many engines today use "torque-to-yield" (TTY) head bolts that stretch slightly when installed. This provides more even head loading and allows the bolts to hold torque better for improved head gasket sealing.
When the bolts are installed, theyíre first tightened to a specific torque ó then tightened an additional amount thatís measured in degrees of rotation. This final twist stretches the bolts to their yield point and creates the elastic clamping force that provides more even loading across the head and gasket.
Because TTY head bolts stretch slightly (only a few thousandths of an inch), some auto makers say they should not be reused when the cylinder head is removed. Reusing TTY bolts will cause them to stretch further, which increases the risk of breakage. A stretched bolt also will not hold the same torque load as before, which may cause a loss of clamping force resulting in head gasket leakage. The best insurance may be to replace all TTY bolts when rebuilding and reinstalling a cylinder head with new head bolts.
5. Check bolt lengths. Make sure you have the correct length bolts for the application and for each hole location (some holes require longer or shorter bolts than others). Bolts should also be measured or compared to one another to check for stretch. Any bolt found to be stretched must be replaced because (1) it may be dangerously weak; (2) it wonít hold torque properly; and (3) it may bottom out when installed in a blind hole.
6. When installing head bolts in aluminum cylinder heads, hardened steel washers must be used under the bolt heads to prevent galling of the soft aluminum and to help distribute the load. Make sure the washers are positioned with their rounded or chamfered side up, and that there is no debris or burrs under the washers.
7. Resurfacing a cylinder head decreases its overall height, so be sure to check bolt lengths to make sure they wonít bottom out in blind holes. If a bolt bottoms out, it will apply little or no clamping force on the head, which may allow the gasket to leak.
If a head has been milled and one or more head bolts may be dangerously close to bottoming out, the problem can be corrected by either using hardened steel washers under the bolts to raise them up, by using a thicker head gasket or by installing a steel or copper shim under the head gasket to restore proper head height and compression.
8. Always look up the specified tightening sequence and recommended head bolt torque values for an engine before installing the head gasket. Donít guess.
9. Use an accurate torque wrench to tighten standard type head bolts in 3 to 5 incremental steps following the recommended sequence and torque specs for the application. Tightening the bolts down gradually creates an even clamping force on the gasket and reduces head distortion. Itís a good idea to double check the final torque readings on each head bolt to make sure none have been missed and that the bolts are retaining torque normally. If a bolt is not coming up to normal torque or is not holding a reading, it means trouble. Either the bolt is stretching or the threads are pulling out of the block.
With TTY head bolts, a "Torque-To-Angle Indicator" should be used in conjunction with a torque wrench to achieve proper bolt loading. After the head bolts have been torqued to the specified value, using the angle tool to accurately measure the additional degrees of additional rotation eliminates guesswork and assures more consistent results.
10. If a head gasket requires retorquing, run the engine until it reaches normal operating temperature (usually 10 to 15 minutes), then shut it off. Retighten each head bolt in the same sequence as before while the engine is still warm. If the engine has an aluminum cylinder head or block, however, donít retorque the head bolts until the engine has cooled back down to room temperature.
On some applications with retorque style head gaskets, it may be necessary to retorque the head a third time after a specified time or mileage interval due to the design of the engine. Always follow the vehicle manufacturerís recommendations.