Summer is here and chances are you've repaired an engine that has fallen victim to the sweltering heat of the long, hot summer days, where poorly maintained cooling systems filled with sub-par coolant have helped head gaskets rupture and straight molecules morph into warped, twisted slabs of metal. Do you know the surface finish of that last cylinder head that you bolted back onto the block? Does your machine shop measure - or even have the equipment to measure - the Ra value of machined metal? Did you correctly repair the root cause of that last head gasket rupture?

Much has been written regarding the correct method of repairing failed head gaskets, so when I went looking for state-of-the-art information on this subject, I picked the brains of the guys that do this stuff for a living; the engineers at Fel-Pro. In the Aftermarket Engineering Department, Jim Daigle, product manager, and Jerry Rosenquist, chief product engineer, gave me a glimpse inside an extremely experienced crew of engineers - their team of eight engineers has a combined 160 years of gasket engineering experience.

Here's an example of what Jim and Jerry bring to the troubleshooting table.

"On Dodge 2.2L engines we noticed terrible aeration in the cooling systems of these vehicles. We found that the lower hose-to-water pump junction was allowing air to be pulled in, but it didn't leak coolant under pressure. This was the root cause of overheating leading to head gasket failure. Double-clamping the hose solved the problem."

For this article, I'll walk through a typical head gasket repair that will include Jim and Jerry's engineering insight. We will break this procedure down into three main areas:

• Why did this head gasket fail?
• How do I measure my cylinder head and correctly resurface my head and/or block?
• How do I handle fasteners, threads, torque and preparation for installation?

Head Gasket Failures Caused by Engine Overheating

To quote Jerry: "I can't believe how many people don't take care of the underlying root cause before replacing a head gasket. The gasket is a safety valve - if it is clamped up tight and it stays cool, it will last forever."

If the engine overheats and exceeds its normal operating range, the elevated temperatures can cause extreme stress in the cylinder head, which may result in a head gasket failure. This is especially true with aluminum cylinder heads because aluminum expands about two times as much as cast iron when it gets hot. The difference in thermal expansion rates between an aluminum head and cast iron block leads to the term "scrubbing," where the head gasket can be torn each time the engine is thermally cycled - especially if the Ra value is incorrect (more on this later). This added stress caused by overheating can also cause the head to warp. Clamping force in critical areas can be lost and allow the head gasket to leak.

This four-valve engine cut-away shows the combustion area and coolant jackets.

Overheating can be caused by anything that decreases the cooling system's ability to absorb, transport and dissipate heat. Examples include a low coolant level, loss of coolant (through internal or external leaks), poor heat conductivity inside the engine because of accumulated deposits in the water jackets, a defective thermostat that doesn't open, poor airflow through the radiator, a slipping fan clutch, an inoperative electric cooling fan, a collapsed lower radiator hose, an eroded or loose water pump impeller, or even a defective radiator cap.

Jim and Jerry agree cooling systems are badly neglected. In addition to the normal causes of overheating - low coolant level, thermostat stuck closed, cooling fan inoperative - their engineers ask them to dig deeper. Both say in a lot of cases, OE radiators are barely capable of performing to par, and aftermarket replacement radiators are a complete unknown. Add to this a radiator with a partial restriction, and that is when temperatures start to climb. How do they know this? If they have a problem with a head gasket, they run it in an engine on one of their dynos. Then, they slightly restrict coolant flow and see the gasket blow.

"We have also seen water pumps with impeller-to-housing clearances that don't allow for good pump action," said Jim. "We have compared OE and aftermarket pumps with flow rates that pale in comparison to brands such as Edelbrock or Weiand. Weiand was one of the first companies to have a water pump dyno. That's how critical this clearance is to proper cooling system function."

Check the Thermostat, Senders, Switches, Fans and Flow

Water pmp impellers.

When I suspect that an engine is overheating due to a defective thermostat, I check it first by feeling the upper hose. When the thermostat opens and hot coolant flows, I like to monitor the coolant temp with the dash-board gauge and compare that value with the reading from the CTS PID on my scan tool. With the scan tool hooked up I also want to see the coolant fan switch open and close at the proper temperatures. Pulled out your gun lately? Your non-contact temperature gun, that is. While all of this is happening, I shoot the radiator with my gun, searching for localized areas where the temperature is too low, which indicates low or no coolant flow through a portion of the core.

Missing fan shroud? A missing fan shroud can reduce the fan's cooling effectiveness by as much as 50 percent (depending on the fan's distance from the radiator) which may be enough to cause the engine to overheat in hot weather or when working hard. Leaking fan clutch? Defective fan clutches are a common and often overlooked cause of overheating. The shear characteristics of the clutch fluid gradually deteriorates over time, with an average loss in drive efficiency of about 200 rpm per year. Eventually, slippage reaches the point where effective cooling is no longer possible and overheating results. (On average, the life of a fan clutch is about the same as a water pump. If one needs to be replaced, the other usually does too.)

Radiators must be checked for plugging, both inside and out. Bugs, debris and bent fins can limit the radiator's ability to rid the cooling system of heat. Internal corrosion and an accumulation of deposits can likewise inhibit coolant circulation and reduce cooling. Back-flushing, especially when done with a cooling system flush machine, is a good way to help keep too much debris from plugging up the cooling system from within.

When the cooling system is refilled, use a 50/50 mixture of ethylene glycol antifreeze and water. This will give freezing protection down to -34 degrees Farenheit, and boiling protection to 265 degrees Fahrenheit, in a pressurized system with a 14 psi radiator cap. When refilling the cooling system, be sure you remove any trapped air from the system. Air pockets can interfere with proper coolant circulation and cooling. Some cars (mostly front-wheel drive) may have one or more "bleeder valves" for venting trapped air from the cooling system. On some vehicles, it may be necessary to temporarily loosen a heater hose to get all the air out of the system.

Don't Forget Basics

Check belt tension and condition. A loose belt that slips may prevent the water pump from circulating coolant fast enough and/or the fan from turning fast for proper cooling.

The condition of the hoses should also be checked. Many hoses suffer the effects of electro-chemical degradation (ECD), where the insides of the hoses crack and dissolve as a result of low electrical charges breaking down the rubber. When replacing hoses, look for "ECD resistant" hoses.

Sometimes a lower radiator hose will collapse under vacuum at high speed and restrict the flow of coolant from the radiator into the engine. This can happen if the reinforcing spring inside the hose is missing or damaged.

Head Gasket Failures Caused By Detonation and Preignition

Head gasket profile.

Detonation (also called "spark knock") is an erratic form of combustion that can cause head gasket failure as well as other engine damage. Detonation occurs when excessive heat and pressure in the combustion chamber cause the air/fuel mixture to auto-ignite. This produces multiple flame fronts within the combustion chamber instead of a single flame kernel. When these multiple flames collide, they do so with explosive force that produces a sudden rise in cylinder pressure accompanied by a sharp metallic pinging or knocking noise. The hammer-like shock waves created by detonation subject the head gasket, piston, rings, spark plug and rod bearings to severe overloading.

Mild or occasional detonation can occur in almost any engine and usually causes no harm. But prolonged or heavy detonation can be very damaging. So if you hear knocking or pinging when accelerating or lugging your engine, you probably have a detonation problem.

If detonation is occurring, check for the following: fuel octane that is too low, an EGR system that may have been disabled, over-advanced ignition timing, stuck hot air intake, boost pressure that is too high, carbon buildup in the combustion chambers and knock sensor operation.

Preignition

Another condition that is sometimes confused with detonation is "preignition." This occurs when a point within the combustion chamber becomes so hot that it becomes a source of ignition and causes the fuel to ignite before the spark plug fires. If this is left unchecked, a hole can be burned through the top of a piston.

Carbon deposits form a heat barrier and can be a contributing factor to preignition. Other causes include an overheated spark plug (too hot a heat range for the application) and glowing carbon deposits on a hot exhaust valve (which may mean the valve is running too hot because of poor seating, a weak valve spring or insufficient valve lash).

Flatness, Ra (Surface Finish) and Waviness

If a head gasket has failed as a result of severe engine overheating, both the face of the cylinder head and block deck should be checked for warpage. The heads sometimes warp because aluminum has a coefficient of thermal expansion that's two times greater than cast iron. If an engine with an aluminum head overheats, the head will swell considerably more than a cast iron head. If the head and block are not flat, the new gasket will have uneven loading and will fail.

Flatness can be checked by placing a straight edge on the face of the cylinder head or block, and then using a feeler gauge to check any gaps between the straight edge and head or block. If the amount of warpage exceeds the following maximum limits, the head or block is not flat enough to hold a good seal against the head gasket and should be resurfaced:

Maximum out of flat (total of head and block combined)
3 cylinder and V6 engines
Length=.003 in. / Width=.002 in.
4 cylinder and V8 engines
Length=.004 in. / Width=.002 in.
Straight 6 cylinder engines
Length=.006 in. / Width=.002 in.

Use a straight edge and feeler gauge to check the flatness of the face of the cylinder head and block deck for warpage before replacing the head gasket.

How to Measure Surface Finish

There are two ways to check the surface finish on a head or block. One is to use a simple and relatively inexpensive surface comparator gauge.

The gauge is a flat piece of metal with small samples of various finishes reproduced on its surface by a belt sander, rotary broach and surface grinder. By comparing both the feel and appearance of the samples on the gauge to the machined surface on the head or block, you can estimate the approximate surface finish in Ra or RMS.

The other method is one that actually measures the surface finish. It requires the use of a special instrument called a surface profilometer. The handheld electronic instrument (which can cost from $1,200 to $4,000 or more) drags a diamond-tipped stylus across the surface to measure and calculate its surface texture.

A surface comparator is critical because the surface finish on the face of the head and block is extremely important. The surface finish should be a minimum of 30 Ra microinches to 110 maximum Ra microinches (150 to 1000 RMS), with a recommended range of 60 to 100 Ra (400 to 800 RMS).

If the surface is too rough (more than 110 Ra), it may be too rough to seal properly and the head gasket will leak. If the surface is too smooth (less than 30 Ra), it may not provide enough "grip" to prevent the gasket from flowing or scrubbing.

Because Ra value is so important, and it is nearly impossible to detect 20-30 Ra with the naked eye, Jim explored ways to get an inexpensive profilometer into the hands of technicians. "Unfortunately," he says, "these tools are still about $3,000, so for now, you have to trust your machine shop."

Any type of standard refinishing technique that's commonly used in a machine shop can achieve an acceptable surface finish of 30 minimum to 110 maximum Ra (150 to 1000 RMS) for a conventional head gasket if done correctly. Most equipment is also capable of achieving the super smooth finishes that are required for the new laminated steel head gaskets.

Jerry says: "Surface refinish equipment leaves a lot to be desired. When you take a head or block to a machine shop, be sure they have the knowledge and equipment to do the job right. Does the shop use a belt sander? Don't even consider using them.

"Also, find a shop that has a surface comparator and knows how to use it. If you ask them about this and they give you that deer-in-the-headlights look, just turn and walk away."

In addition to surface finish, waviness is another factor to consider. Waviness describes the undulations or waves across the surface of the metal which are usually associated with milling machines. A waviness height of up to .0005 inches is OK, provided the distance from peak to valley is between .030 inches and .100 inches. It's also important that there be no sudden irregularities in the surface that exceed .001 inch. There should also be no more than plus or minus .001 inch of out-of-flat across 3 inches in any direction.

Waviness is similar to parallelism in a brake rotor and can lead to head gasket failure. This condition can actually be induced by older resurfacing machines, or one that is out of calibration.

MLS Gaskets and Surface Finish

When Dodge Neon engines were blowing head gaskets, Fel-Pro developed the Multi-Layer Steel gasket to replace the original composite gasket. The new design took care of the problem. The new MLS gasket worked because it prefers a glass-smooth surface of around 20 to 30 Ra. "Aftermarket coatings are applied to our multi-layer steel gaskets that will make them seal to 60 Ra, to help compensate for differences in machining," said Jim.

Fasteners, Threads, Torque and Surface Prep

Blown head gasket.

When a head gasket is installed between the cylinder head and engine block, tightening the head bolts compresses the gasket slightly, allowing the soft facing material on the gasket to conform to the small irregularities on the head and block deck surfaces. This allows the gasket to "cold seal" so it won't leak coolant before the engine is started.

The head gasket's ability to achieve a positive cold seal as well as to maintain a long-lasting leak-free seal depends on two things: its own ability to retain torque over time (which depends on the design of the gasket and the materials used in its construction), and the clamping force applied by the head bolts.

To begin with, how do you prepare your gasket surface? If you use those rotary surface conditioning disks with an angle grinder, beware. If you get over-zealous with the disk and remove metal, you've just created a spot that will have low clamping force. If that same metal, or pieces of the conditioning disk, migrate into the oiling system, a bearing can be destroyed. If you're using this method instead of scraping, be careful.

Old gasket material, carbon, rust, scale, dirt and other debris may be removed from the head and block by applying a gasket-removing compound and/or by scraping the surface of the head with a scraper or wire brush.

When cleaning aluminum surfaces, use a nonmetallic (brass or plastic) scraper so you don't gouge or scratch the relatively soft metal. It's also a good idea to clean the combustion chambers in the head, too. Hard carbon deposits may flake loose when the head is being installed and end up between the gasket and head.

Clean those bolt threads! Dirty or damaged threads can give false torque readings as well as decrease a bolt's clamping force by as much as 50 percent. Dirty or deformed hole threads in the engine block can reduce clamping force the same as dirty or damaged threads on the bolts. Jerry suggests to prepare the head bolt holes using a bottoming tap, then blow the hole clean with air. If the head bolts are going into a blind hole, lightly lubricate the threads and the underside of the head with engine oil, but don't use so much that the hole will hydrostatically lock. If bolts share a coolant passage, use thread sealant.

Jerry says that lubrimatic grease also works well for head bolts, because it has a coefficient of friction that is just right for the application. Some moly greases are too slippery and can actually lead to pulled threads.

"When torquing," Jerry says, "some techs never calibrate their torque wrenches, and should have them taken away. Even with calibrated torque wrenches, techs will sometimes jerk on the wrench, rather than use a smooth, continuous pull. There is a 20 percent difference between these two methods of torque wrench use." 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.

Many engines today use "torque-to-yield" (TTY) head bolts. These are specially designed bolts that yield slightly when installed. This provides more even head loading and allows the bolts to hold torque better for improved head gasket sealing. When dealing with TTY bolts, if in doubt about their history, replace them. TTY bolt heads will snap if they become over-stretched.

When performing this task, we technicians must take care of three things to ensure a successful head gasket repair: 1) Surface finish must match the gasket and be flat, 2) Clamp load must be even and accurate across the entire head, and 3) The environment must be cool in the combustion chambers.

Brian Manley is a vocational automotive instructor for the Cherry Creek school district in Aurora, Colo. He is an ASE master certified automobile technician and a former member of the National Automotive Technicians Education Foundation (NATEF) board of trustees. He can be reached at manley_brian@hotmail.com.

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