Sealed With Science: Tips for Perfect Head-Gasket Sealing

Forget smartphones, ultra-high-definition televisions, and even that gizmo from Amazon that allows you to order a pizza just by talking to it. The best part about the latest advancements in technology is how it is now easier than ever to make big power in street and race engines.

These days, horsepower freaks have a multitude of options when it comes to making power, but whether we are talking a supercharger, turbocharging, EFI, or even an old-school stroker kit – it all comes down to squeezing more air and fuel into the combustion chambers to create a bigger boom.

Of course, the more power you make, the more stress you create on the engine. In the old days, blown head gaskets were often issues when making big power. These days, it doesn’t have to be, but you still have to take the proper precautions. It all comes down to selecting the right components and preparing them properly. In this article we’ve got a few tips on things to do that will definitely help cut down significantly on the chances of a blown head gasket. Oh, we’ll also dispel a few myths that can actually do more harm than good, too.

Be Smooth

A blown head gasket is almost always blamed on – you guessed it – the head gasket. However,if you haven’t laid the proper foundation by properly machining the decks of both the block and cylinder heads, every head gasket will fail. Both the deck of the block and the cylinder heads should be machined so that they are flat and parallel to the centerline of the crankshaft to within a couple thousandths of an inch.

You can’t achieve that with hand tools – no matter how careful you are. It doesn’t matter how good you are as an engine builder, unless you are trying to drive some jalopy out of the junkyard or get the demolition derby car through one more night of carnage, it’s always a good idea to have a machine shop clean up the decks for both new engines and rebuilds. In the long run, it will always be worth it.

On the left is a standard composite gasket, which is basically a set of stainless steel fire rings around each cylinder with a different material holding it all together. Composite gaskets usually work for your standard rebuild, but once you ramp up the power, failure rates can begin to increase dramatically. The JE Pro Seal MLS gasket on the right is the next step in sealing technology. Constructed entirely from high-grade stainless steel, each layer is stress relieved after the embossing process to ensure that it lays flat to provide even pressure across the head.

Composite vs MLS

MLS gaskets have been available now for several years and are no longer just for expensive, high-end race engines. MLS (which stands for Multi-Layer Steel) gaskets are a step up from standard-issue composite gaskets thanks to their construction. Composite gaskets are made with steel fire rings for each cylinder surrounded by other materials. In the event the cylinder head lifts even a bit, combustion gases can escape past the steel fire ring and burn the gasket in the brief moment before the head is pulled back into place.

MLS gaskets are constructed from three or more layers of steel. The outer layers are embossed around the combustion chambers as well as the water jacket and oil gallery holes leading between the block and cylinder heads. With this configuration, if the cylinder head lifts the embossments act as springs to help maintain proper sealing to keep the combustion gasses where they belong.

MLS gaskets also do a superior job sealing between dissimilar materials. These days, aftermarket aluminum cylinder heads are becoming a lot more popular as the prices for a quality head are coming down. When you first fire up an engine with a cast-iron block and aluminum heads, the heads absorb heat faster than the block, causing it to expand more quickly. An MLS gasket is better at handling that difference in expansion rates than a typical composite gasket. In the event you get into pre-ignition or detonation, the more robust MLS gasket is also better at withstanding the tremendous spike in pressure those events create.

Notice that the embossing surrounds not only each cylinder bore, but also the water and oil passages. The Pro Seal gaskets use a proprietary process to add a sealant coating to the outer layers. This helps seal off small imperfections in the machined surface of either the block or cylinder heads to eliminate possible coolant or oil leaks.

Just Because They Used to Do it That Way…

Fathers taking the time to teach their sons and daughters how to work on cars is a great thing. What’s not a great thing is when they accidentally pass along bad, or outdated, info.

A perfect example is copper head gasket spray. In the old days, racers and engine builders would try to get overmatched head gaskets to handle extra power by coating them with a copper-based spray. This would fill in surface irregularities in the deck of either the block or cylinder heads and also help dissipate heat from hot spots (often between the side-by-side exhaust ports on small-block Chevys).

But that old trick is now bad info, which should be left in the past. Spraying an aftermarket coating on a quality MLS gasket can actually hurt its performance. For example, JE’s Pro Seal MLS gaskets are all constructed from stainless steel and given a multi-layer surface coating that includes a “primer” to help it stick to the stainless gasket.

Then there’s a high-strength Viton layer to mold to minor surface imperfections on the head or block, and finally a top-layer silicon coating which is especially good at sealing the coolant and oil passages moving from the block to the heads. Modern surface coatings on quality MLS gaskets do a better job than a hand-sprayed copper “paint” ever could, so save your money and leave that stuff on the parts store shelf. All that needs to be done is to properly clean all metal surfaces before the gasket is laid down.

Dorton gave us this very interesting tip: When using head studs, always make sure the studs bottom out in the hole (if possible) instead of on the shoulder of the stud where the threads end. This helps limit warping the cylinder bores once the heads are torqued into place

Clamping Loads

No matter how good the head gasket is, it is still extremely dependent on proper clamping load from the cylinder head fasteners to help it work properly. Achieving a good, uniform clamping load across the gasket is dependent on three factors: fastener hardware, consistent torque on the fasteners, and the pattern used to tighten fasteners.

Typically, the OEM’s use torque-to-yield head bolts to clamp the cylinder heads to the block. Torque-to-yield bolts are one-time use only. So once you pull them off, throw them away. Usually, these bolts can be identified because they will have a flat gray surface coating, but if you aren’t sure, always play it safe and buy new head bolts.

You can buy replacement torque-to-yield head bolts relatively cheap, but we would advise against it. Instead, invest in a set of quality fasteners made by aftermarket manufacturers such as ARP. Not only are these fasteners less prone to failure, they also allow higher clamping loads and are reusable through multiple engine rebuilds.

Although we told you no coatings are necessary, it is helpful to apply a thin film of silicone on both sides of the “ear” of each gasket, because this area can sometimes weep fluid.

When buying new fasteners, you can choose between head bolts and head studs. Most high-performance engine builders use head studs whenever possible because they can provide a more consistent clamping load. This is because the head stud is threaded into the block without any resistance. Then the head is installed and clamped in place with nuts on the head studs. This allows the studs to be placed into almost pure tension rather than torsion.

Head bolts, on the other hand, must be spun into the block as torque is applied. This means the bolts can be twisted as the last bit of torque is applied, making the overall clamping pressure sometimes less consistent than it could be. Head bolts are, however, sometimes necessary because they make it much easier to pull a set of heads while still in the car. Once the bolts are removed the cylinder heads can be lifted straight off. This often isn’t possible with head studs because the heads have to be slid out and up until they clear the studs, and the car’s fenders or other parts can get in the way. So before purchasing new fasteners, plan ahead and determine whether you might be pulling the heads with the engine still in the car.

Dorton uses high pressure lube to lubricate the threads as well as the washers on each head stud. Other lubricants like Ultra-Torque also work well, just make sure you are consistent and use the right torque correction factor for the type of lubricant you are using.

When installing the head fasteners, it is critical that they never be installed dry. This allows friction between the threads of the fastener and the block to create too much resistance, which can cause clamping loads to vary wildly. Instead, some sort of lubricant should always be used. ARP has created a thread-lubricant they call Ultra-Torque we’ve found works very well. Other engine builders swear by old-school “High Pressure Lube,” which comes from a variety of manufacturers. What shouldn’t be used as a lubricant is motor oil, which is too thin to provide good consistency.

Different lubricants provide different levels of “lubricity,” which is a fancy way of saying how slick they are. It is important to know exactly what lubricant you are using because it will affect the force applied with your torque wrench. A torque wrench measures resistance to a twisting force. Meanwhile, proper loading by the fasteners requires them to be in a specific amount of tension in a straight line down the length of the bolt or stud. Essentially, they are tightened to the point that the bolt or stud stretches a few thousandths of an inch, putting it into tension. The fastener reacts to being in tension by trying to pull back to its original length, pulling the cylinder head tighter to the block.

Manufacturers like ARP provide proper torque specs for bolts using different lubricants. It is important that you follow the exact specs for the specific lubricant you are using because they account for the lubricity, or slipperiness, of the lubricant. For example, if you want to properly load the head gasket while lubricating the head bolt threads with Ultra-Torque you may need to set your torque wrench to 70 ft/lbs, but if you are using motor oil which does not do as good a job at lubricating the threads, your torque wrench should be set higher. Make sure you always use the correct torque setting for both the fastener and lubricant you are using.

Torque the head bolts in three stages with at least 15 lb-ft between each pull. If your final torque is 75 lb-ft, begin by pulling each bolt to 45 lb-ft, followed by a pull to 60 lb-ft and then making your final pull to 75 lb-ft.

Finally, the order in which you torque the head fasteners is also important. There are lots of diagrams and charts on the internet showing the correct order to tighten the head bolts for practically any cylinder head, but once you understand the concept you won’t need them. Always start with the center most bolt or head stud, and then work your way out in a spiral pattern. That helps spread the loading over the head gasket evenly to give it the most strength possible.

Proper Prep

Last, but certainly not least, always take a few moments to prepare both the block and heads before beginning assembly. Even with a new engine build, use a thread chaser to make sure all the threads in the bolt holes are clean, straight, and free of burrs. If you don’t have a set of thread chasers, you can use a thread tap if you are very careful. This will help make sure you get proper loading of the head fasteners without anything in the threads to screw up the torque readings.

And while you are at it, take a moment to chamfer any sharp edges created by the machining processes used to create the component. Sharp edges not only create weak points but can also keep the cylinder head from sitting perfectly flat. You can use sandpaper or a deburring tool to cut down any burrs and sharp edges. It only takes a few minutes to do, but it can be the difference between a great engine build and one that causes endless frustration.

No matter what type of engine you are building, there is a simple method to ensure you torque each bolt in the proper pattern. Begin with the bolt in the center of the head (or at least the closest) and then work your way outward in a spiral pattern. It doesn’t matter if your spiral is clockwise or counter-clockwise, just make sure you do it the same on each pass, and you will be fine.

Article Sources

About the author

Jeff Huneycutt

Jeff Huneycutt has been in the automotive industry long enough to collect more project cars than he can afford to keep running. When not chasing electrical gremlins in his '78 Camaro, he can usually be found planning unrealistic engine builds.
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