Cylinder Head Gasket Service In Modern Era Engines
A Failed Head Gasket Is Only a Symptom, Not the Root Cause
“My engine blew a head gasket and now it overheats.” That’s a fairly commonly heard knee-jerk diagnosis. However, it’s wrong. When an engine overheats or if the gasket allows coolant or combustion gasses to escape, do not automatically blame the gasket. Cylinder head gaskets can be viewed similar to a fuse. If the gasket “fails,” this is a symptom that something other than the gasket itself caused the failure.
Potential causes include uneven deck surfaces, improperly machined decks, pitted decks, cracked decks, gasket contaminants, or improper/insufficient clamping pressure. A severe overheat condition caused by a failed water pump , stuck thermostat or a cooling system restriction can cause a head to distort (warp) to the point where a consistent mating of the gasket between the block and head is compromised. Aluminum cylinder heads are more prone to warping as a result of engine overheating.
It’s important to understand that in most cases, it’s not the head gasket that caused the overheat. A “failed” head gasket is simply a by-product of the root cause. Providing the correct gasket was installed, and installed correctly, don’t blame the gasket. Rather, you need to look for the reason that the gasket was compromised.
MLS, or “multi-layer steel,” head gaskets are made with multiple layers of stainless steel. The outer layers feature embossed raised areas that act as springs, compressing under resistance and maintaining a seal. Inner layers (depending on design, one to seven layers may be involved) serve to both create the required thickness for a given application and as a support foundation.
An example of a 3-layer MLS gasket. Depending on application, gaskets may feature as many as seven layers. The center support layer serves not only as a support between the outer layers but also serves to determine the compressed thickness of the gasket.
The tempered embossed outer layers are compressed when the cylinder head is clamped in place, providing outstanding sealing performance. A special coating is also applied to the outer layers to aid in cold sealing. Unlike composite gaskets, MLS gaskets won’t degrade or burn out and do not require re-torquing. While a used MLS head gasket may be viable for re-installation, it’s always recommended to replace with new gaskets. Due to the tempered and embossed outer layers, an MLS head gasket does not stay in a compressed state when the head is clamped in place. As engine temperature rises and falls, the “spring-like” characteristic of the embossed layers continue to seal even when the cylinder head wants to pull away from the block.
Regardless of engine application, always make sure that the female threaded holes in the engine block are clean and dry. Use a special thread chaser tap to re-form burred threads and to remove any rust or debris, in conjunction with solvent and compressed air. Never use a traditional cutting tap, as this will remove critical material that can weaken the threads. Chaser taps are available for all popular thread sizes.
And yes, the cylinder head does try to move and rise under operating conditions, exerting additional force at the head bolts, causing the bolts to slightly expand in length. High temperature and cylinder pressure constantly try to push the head away from the block. In terms of temperature factors, this is particularly evident in iron block and aluminum head assemblies due to the expansion and contraction of dissimilar metals.
In short, MLS head gasket design and technology far surpasses composite gaskets in terms of durability and sealing performance. That doesn’t mean that composite gaskets are a poor choice. Older engines that were designed to be sealed with composite gaskets may still be serviced with that type of gasket, but the use of MLS gaskets will improve reliability for those applications.
Run the chaser tap by hand, remove and flush the hole. After applying compressed air, run the tap again and blow again. Make sure that each hole’s threads are clean, free of burrs and dry.
MLS cylinder head gaskets require a very smoothly finished deck surface to provide a lower friction coefficient, with a finer/smoother finish than older engine designs that called for composite head gaskets. Deck surface smoothness is measured in Ra (roughness average). The smaller the Ra number, the finer the finish.
If an engine that originally called for a composite type head gasket is going to be assembled with an MLS gasket, the deck surfaces (block and head) will likely need to be machined to achieve a smoother finish, usually in the range of 20-50 Ra (roughness average). However, for engines that originally used composite head gaskets, gasket makers such as Fel-Pro also offer MLS gaskets that feature a slightly thicker outer coating that will accommodate as rough as an 80 Ra finish, so if you plan to install MLS gaskets on an older engine, it may not be necessary to have the decks milled.
Once the cylinder head has been removed, check deck flatness using a precision machinist’s straightedge and a feeler gauge to look for warpage. Check in four locations. Here a head is checked from front to rear. If warpage is found beyond specification, the head must either be surface milled or replaced.
Here a head is checked diagonally left front to right rear. Always check with the automaker’s tolerance specifications, but a generally accepted maximum warp is about 0.003-inch on a V6 deck, 0.004-inch on a V8 deck and about 0.006-inch on a straight-six deck.
The head deck is then checked in the opposite diagonal direction. For an aluminum head, a generally accepted maximum is about 0.002-inch in any direction, and less than 0.001-inch across any 3-inch surface.
For cast iron block and cast iron head combinations that use a composite gasket (non-asbestos/graphite), acceptable surface finish is in the 60 to 100 Ra range. For iron blocks with aluminum heads, the acceptable range is about 20 to 50 or 50 to 60 Ra (check with the service manual). Simply understand that MLS gaskets require a smoother finish.
Since the average repair shop will not be equipped to perform deck resurfacing, either for cleanup or to produce a finer finish, if this is needed, it’s best to let a local and experienced) engine machine shop handle this.
The head is also checked for warp across deck width (inboard to outboard).
Whenever replacing a cylinder head gasket, always check surface flatness on both the block deck and head deck, using a precision straightedge and a feeler gauge. Lay the straightedge on the deck from front to rear at the center. Next, position the straightedge from corner to opposing corner (diagonally), checking both diagonals (front right to rear left and front left to rear right).
On an iron overhead-valve engine (pushrod engine), a generally accepted maximum warp is about 0.003-inch on a V6 deck, 0.004-inch on a V8 deck and about 0.006-inch on a straight-six deck. For an aluminum head, a generally accepted maximum is about 0.002-inch in any direction, and less than 0.001-inchacross any 3-inch surface. Always check with the automaker’s tolerance specifications.
Note: If you’re dealing with an OHC engine, also check for warp along the OHC cam bore(s) using a precision bar. If the deck is warped but the cam bores are straight, it may be possible to save the head by resurfacing the deck (of course, depending on how much material needs to be removed and stay within the safe clearance limits of pistons to valves.
However, if the cam bore is warped, the cylinder head will either need to be replaced or align-bored/honed. In this case, the customer may be better served by replacing the head with either a new head or a quality remanufactured head.
Note that by milling the OHC head deck, you’re affecting camshaft timing, so again, refer to the manufacturer’s tolerance specifications. If excess material needs to be milled in order to achieve flatness, it may be possible to order an MLS gasket that features added thickness to compensate for material removal.
When installing MLS cylinder head gaskets, install them dry, with no added sealant. It is absolutely critical to make sure that the block deck and head deck are clean and dry.
Carefully wipe the decks with a fast-drying solvent such as alcohol, using a lint-free towel before installing the gasket. Most MLS gaskets feature a special coating (Viton or Nitrile, usually about 0.001-inch thick) that both allows the head to move during expansion and contraction (especially important when an aluminum head is secured to a cast iron block, due to different rates of expansion/contraction) and aids in cold sealing.
Never wipe the gasket with any type of solvent, as this can easily ruin/remove the special surface coating. Also, because you need to prevent damage to this coating, keep the gasket in its package prior to installation to prevent accidental coating damage that might occur by handing, laying on a workbench, etc. Avoid getting dirty or oily fingerprints on the gasket surfaces.
If the block deck is found to be warped, the block must be resurfaced or replaced. This is usually not an issue on iron blocks but can be present on some aluminum blocks that have been exposed to a severe overheat condition.
TORQUE AND TORQUE-PLUS-ANGLE
When tightening a threaded fastener, it is critical to refer to the automaker’s service manual to determine if the re-use of original head bolts is recommended or not. If the bolts are of the T-T-Y (torque-to-yield) design, they should not be re-used, as they may have stretched beyond their point of elasticity.
While some may think that a bolt is a rigid component, this is a fallacy. Bolts, depending on design and clamping load application, slightly stretch in length when fully tightened to a specified torque (or torque-plus-angle), similar in some respects to a rubber band. Head bolts are designed to slightly stretch within their designed elastic range in order to obtain proper clamping force.
If a bolt is over-tightened beyond its designed elasticity, the bolt fatigues and is no longer able to maintain the initial clamping load. This allows the cylinder head to move beyond it’s sealing point, resulting in (among other issues) coolant, oil and/or combustion pressure to escape past the head gasket.
Use great care when handling/installing the head gasket. All surfaces must be kept dry. Avoid dirty or oily fingerprints and scratches.
Pay attention to gasket orientation. Some gaskets are marked, indicating the front. Always inspect the gasket placement on the block deck to make sure that all critical holes are located as needed.
This Honda B18 head gasket is an example. Note the passage crossover.
Torque-to-yield head bolts are designed as one-time-use bolts and should not be re-used (again, refer to the service manual). If an upgrade is desired, aftermarket head bolts are readily available from sources such as ARP, which provide higher tensile strength and call for a torque-only installation (no additional angle rotation needed). In those cases, the bolt maker will recommend a torque value, which may differ from that listed by the automaker.
It’s important to understand that any type of head bolt may be tightened following a torque-plus-angle method, but if a torque/angle spec is provided, do not blindly assume that the bolts are torque-to-yield. While most modern engine service specifications may call for a torque plus angle tightening, that does not necessarily mean that the bolts are torque-to-yield. If not T-T-Y, the original bolts may be reusable. Again, check the service manual.
In some cases, the engine may feature cylinder head studs instead of bolts. If studs are being installed, do not apply excessive torque to the stud as you install it to the block deck. Studs should be installed finger-tight (some may call for a slight preload of perhaps 5 or 10 ft.-lbs.). The stud only requires full thread engagement to the block deck. Clamping load will be applied as the nuts are tightened to the specified value.
Lubricate head fastener threads using the lubricant specified by the engine service manual. If using aftermarket fasteners, the fastener maker may recommend a different lube. The type of lubricant will directly affect the applied torque value.
Also apply lube to the underside of the head bolt head (or to the washer/nut if studs are used).
Over-tightening studs to the block can easily result in a splayed condition, making head installation difficult, in addition to placing undue lateral stress at the studs and where the studs contact the sides of the bolt holes in the head.
Regardless of the use of bolts or studs, it’s also very critical to follow the automaker or fastener maker recommendations for thread lubrication. Depending on the application, the recommendation may call for the use of engine oil or a specific thread lube that reduces thread friction. The recommended torque may be based on the use of oil or a “more-slippery” lubricant.
Using the wrong torque and lube combination can result in insufficient or excessive thread engagement pressure. For instance, if the application calls for 70 ft.-lbs. with the threads lubed with engine oil, and you tighten to only 50 ft.-lbs., the head fasteners may not achieve proper stretch/clamping load.
By the same token, if a lube that greatly reduces friction is applied and the same bolts are torqued to the specified 70 ft.-lbs., you may actually be over-stretching the bolts beyond their elastic point.
With the head gasket properly registered to the dowels in the block deck, carefully position the head in order to engage the head onto the dowels. Avoid sliding the head around, which can nick the head deck surface or damage the gasket.
The point: Make sure that you use the proper lubricant and tightening value combination for the type of fasteners being installed. Especially when dealing with aluminum heads, you want to avoid the underside of the head bolt (or nut) from digging into the softer aluminum surface. The use of hardened washers is normally recommended. This also provides a bearing surface for the bolt head underside or nut, aiding in achieving a more accurate torque/clamping load.
Be sure to properly lubricate all contact areas (threads, washers, and underside of bolt heads). Any excessive friction, resulting from dry threads or bolt head underside can result in inaccurate (and inadequate) clamping load. Improper clamping load will compromise the head-to-block stability, and thereby undermining the head gasket’s ability to seal.
Pay attention to head bolt lengths, as length can vary depending on the engine application.
Torque-to-yield head bolts can usually be easily identified by an undercut between the bolt head and threaded area.
Don’t believe me? Here’s info from Fel-Pro regarding T-T-Y:
Bolts are elastic by nature, meaning that within their elastic range, they’ll stretch as the load on the bolt increases. As long as the bolt is not stressed beyond its proof load (the maximum load a bolt can withstand and still behave in an elastic manner), it will return to its original (static) length once the torque is relieved. However, conventional bolts can be an issue when they are used on aluminum heads, given that the expansion rate of this type of head can stretch typical bolts past their yield point. This can be overcome by designing and offering better fasteners to address these conditions.
Traditionally, a torque specification is a calculated number. This torque specification applies the proper clamp load to the joint while accounting for friction provided by the threads and the underside of the bolt head. Today’s engine requires higher clamping forces (due to increased combustion pressure) which cannot be achieved with conventional small-diameter bolts. Unfortunately, using a larger diameter bolt is not the answer, as the larger the bolt is, the less it will stretch. Remember, bolt stretch is how we get maximum clamping load.
Used by many manufacturers, especially on engines with aluminum heads and in conjunction with MLS head gaskets, T-T-Y head bolts are engineered to stretch within a controlled yield zone. Once they reach this zone, they maintain a more precise and consistent level of clamping force across the entire head-to-block mating surface. The bolts are stretched into their elastic range, the stretching approaches the bolt’s elastic limit, permanently stretching it.
Getting to the precise yield zone is accomplished by tightening bolts to a certain torque spec, then turning the bolts a predetermined number of degrees. Rotating the bolts by a set amount of degrees and putting the fasteners in the yield zone will account for “clamp load scatter” or variations due to conditions, such as assembly lubricant type, bolt and bolt hole thread condition and surface finish of the fasteners.
Why is torque-plus-angle tightening specified? As a result of engineering research, once a baseline of applied torque has been achieved, engineering has shown that a specific amount of bolt stretch and clamping load will be achieved by applying a specified number of additional rotations of the bolt head, which is a more accurate method of applying clamping load than relying on torque application alone as the tightening process addresses friction.
A torque-plus-angle approach was not designed by mad scientists who are trying to complicate your life. This approach is designed to achieve a more accurate clamping load. In order to service fasteners that require torque-plus-angle, you need not only a quality torque wrench but a means to achieve the specified degrees of additional bolt head rotation as well.
While there are inexpensive angle gauges that attach to a ratchet wrench or breaker bar, the easiest (and most accurate) method is to use one of the widely available digital torque wrenches that also feature a degree mode (these are available through suppliers such as Snap-on, Mac Tools, GearWrench, ACDelco, Matco and others). Simply tap a button to set a torque value, and rotate the bolt until this setting is achieved (most provide both an audible and visual alert). Then change to the degree mode and continue to tighten.
SERVICING HEAD FASTENERS
Before removing any head fasteners, allow the engine to cool to room temperature. Removing fasteners while the engine is hot can easily result in warpage of an aluminum head. Always try to remove head bolts in the reverse order of tightening (refer to the service manual). Loosen bolts in steps rather than all at once.
Before installing new head bolts, take the time to clean all female threads by using a thread chaser, not a thread tapping tool! A thread chaser cleans and re-forms threads without cutting or removing critical thread material. Clean with solvent and compressed air. Any debris (rust, dirt, old thread sealant, etc.) or thread burrs will create excess friction, providing a false torque reading.
Lube the new bolts per the OEM recommendations. Do not apply an excessive amount of oil or other lube into any blind threaded holes, as this will create a hydraulic lock and will prevent the bolt from achieving proper clamping load. If a threaded hole is open and passes into a water jacket, an appropriate thread sealer must be applied to the bolt threads. Always follow the OEM tightening sequence and torque/angle specs. If unsure about the tightening sequence, start at the center of the head and work out in a widening spiral pattern. The sequence is important in order to evenly spread the clamping load across the entire head.
When tightening head bolts, it’s best to do this in steps, with about 15 ft.-lb. increments. For example, if final torque is specified as 75 ft.-lbs., first tighten all bolts (in proper sequence) to 45 ft.-lbs., then 60 ft.-lbs., to the final 75 ft.-lbs. This approach provides more control for clamping load accuracy and clamping load uniformity.
When dealing with torque-plus-angle specifications, torque in three steps to the initially specified torque value, then (again, in proper sequence) continue to tighten by the additional degrees of angle specified for that engine.
When using your torque wrench while applying torque to a bolt, do this in a smooth and slow movement. Avoid jerky motions or fast movement.
You may not be aware of this, but only about 15% of a torque value specification is used to actually achieve torque application. The remaining roughly 85% of applied torque is used to overcome anticipated friction.
Note that not only does proper torque application in the recommended sequence assure proper clamping load, this helps to avoid unwanted cylinder bore distortion. Believe it or not, when cylinder heads are clamped down, the cylinder bores in the engine block may be subjected to slight distortion (out of round and taper). To minimize this, it’s important to follow the correct procedures whenever installing a cylinder head, to avoid issues of poor piston ring seal.
Begin tightening head fasteners according to the specified sequence. Starting at the center, the tightening pattern will usually follow an outward-spiraling pattern. The correct sequence is critical in order to spread the clamping load evenly. If the correct pattern is not followed, this can result in distortion (warped head) and leaks past the gasket. Never fully tighten fasteners in one step. Tighten in several steps, again following the service manual specs. Avoid fast or jerking movements with the torque wrench.
GASKET INSTALLATION TIPS
1. Both block and head decks must be absolutely clean and dry.
2. Use care when handling head gaskets. Avoid contamination by oil, dirty/oily fingerprints, etc. Keep gaskets protected/covered until ready for installation. If a gasket becomes kinked by handling, discard it and replace with new.
3. Pay attention to gasket orientation. Many gaskets will feature a “FRONT” designation. On V-type engines, head gaskets will be specific to right and left locations.
Today’s digital torque wrenches can provide very accurate torque application. Some tools, such as the Snap-on unit shown here, allows switching from torque to angle tightening without the need to change tools or use adapters.
4. Install MLS gaskets dry. Only apply sealant if indicated, where extended gasket ears may require sealant (this will depend on the specific application).
5. Head dowel pins or dowel sleeves must be installed to the block deck prior to gasket and head installation. These dowels provide proper register for the gasket and head. Always use new dowels to avoid burred or distorted dowels.
Digital torque wrenches allow quick changes in requested value and depending on the model, allow settings in ft.-lbs., Nm or in.-lbs.
6. Lubricate the new head fasteners according to vehicle manufacturer or fastener maker instructions. Lube both male threads and the underside of bolt heads. Avoid dry friction points.
7. Do not apply excess oil/lube to bolt threads that will enter a blind hole to avoid hydro-locking.
Once the specified torque has been applied, a digital torque/angle wrench is easily changed to angle mode. Unlike using a simple angle gauge adapter, some digital torque/angle wrenches allow achieving desired angle while ratcheting instead of having to perform one continuous pull.
8. Apply thread sealant to bolt threads that enter water jackets.
9. Pay attention to head bolt lengths. Depending on the application, length may vary based on location.
10. If head studs are being installed, install finger-tight to the block. Only apply torque to the nut.
While not common on production engines, if you need to install head studs as opposed to bolts, install studs only finger-tight (although some makers may call for a very light preload of 5 or 10 ft.-lbs.). Never apply full torque when installing the studs into the block. The clamping force will be achieved when torque is applied to the nut. Over-tightening studs to the block will easily result in a splayed condition.
1. Tighten head fasteners in several steps per service manual instructions. Never fully tighten in one step.
12. When using a torque wrench, use slow and smooth movement. Avoid fast and jerky movements.
13. If torque-plus-angle is required, use a separate angle gauge or a digital torque wrench that also features an angle mode.
14. Always follow the recommended tightening pattern/sequence. No exceptions.
Because of the nature of MLS design, head gaskets may be ordered in custom thicknesses via the thickness or addition of the center layers. This allows you to save a cylinder head that may have required resurfacing that made the head too short. For example, if an aluminum head had to be resurfaced by removing, say, 0.020-inch of deck material, you may be able to order a gasket that is 0.020-inch thicker than the original OEM thickness. Gasket makers such as Fel-Pro and Cometic offer varying thickness gaskets for a range of popular engine applications.