There was a time when it was expected that an engine, even a relatively new one, would leak. These days, engines are expected to remain leak-free well past the five-year mark. This drastic improvement in sealing technology has to be one of the most significant developments in engine construction over the past decade, but it has come with a price and it is not absolute.
There are a couple of main areas of sealing that have seen significant changes. Perhaps the most often recognized is that of cylinder head sealing.
While graphite and metal core gaskets are still to be found, the movement to multi-layer-steel (MLS) gasket construction is ongoing.
MLS gaskets are complex devices made up of several layers of materials. They are designed to retain sealing even when the head and block are moving in relation to each other. The movement is typically small–only fractions of a millimetre–and is due to differing expansion rates in the head and block materials, as well as the tendency for the head to lift off the deck as combustion pressures rise.
Simply put, clamping force sufficient to hold the head down during cold operation is often not possible considering the degree of expansion that aluminum heads undergo as they are heated. Sufficient clamping force to cold-seal a conventional gasket might lead to crushing the gasket when temperatures rise.
A case in point is the Chrysler Neon problem. These engines were equipped with a graphite-type gasket from the factory (against the advice of the supplier), which proved to be inadequate to the job. This was subsequently replaced with an MLS-style gasket.
And than there’s the often quoted worst-case scenario of the Ford 3.8L engine that had too low a clamping force to seal, leading to a much publicized recall of hundreds of thousands of vehicles, not to mention a revised torque-down specification.
Still, when an aluminum head gets hot, it tends to swell and bow up in the middle, causing a loss of clamping force across the centre portion of the head gasket. As mentioned, this can be problematic. Without proper clamping force, coolant and combustion gases can begin to escape, eating away at the gasket as they do so.
In severe cases, the distortion in the cylinder head can cause the camshaft to bind in the bores. None of these scenarios are particularly appealing.
Even so, it’s not all about torque and MLS. Neither of these can fend off the cumulative effects of a poor surface finish. Increasingly, aftermarket machine shops and cylinder head suppliers have come to terms with the stringent new demands.
To seal properly, the finish on the face of the cylinder head and block deck must be relatively flat, smooth and clean. This has always been the case. The challenge has come in the increased emphasis on surface finish, as well as just how smooth that surface has to be.
Surface finishes are measured in microinches (millionths of an inch). Ra stands for Roughness Average, and is a simple average of the heights of the peaks and depths of the valleys on the surface. RMS stands for Root Mean Square, and is a mathematical technique for describing the amount of variation across the surface. The difference between RA and RMS is about 10%.
The surface finish on the face of the head and block is critical for proper sealing. Even the best-engineered gasket cannot seal properly if the surface finish is wrong.
The surface finish for most cast-iron engines should be in the range of 60 to 120 microinches (nominally about Ra 80). For aluminum engines, this tightens to 20-50 microinches (nominal Ra 30). For engines equipped with MLS gaskets, the surface finish is an extremely fine Ra of 7-15 microinches.
The difficulty with surface finishes as fine as this is that they are difficult to achieve, require properly maintained equipment, and are virtually impossible to determine with the unaided eye.
As a rule, the smoother the surface finish the better. There are also a variety of tools, some simple and inexpensive, others highly sophisticated and highly accurate, but requiring an investment, to help determine surface finish results.
In some cases, shops have been able to borrow these sophisticated devices and by closely recording procedures to get specific results, have been able to achieve proper surface finishes.
For the most part, the appropriate surface finish on the cylinder head is achievable by properly equipped machine shops serving the aftermarket, but this leaves the variable of the engine block that may never have left the service bay. Technicians should be advised to ensure that they are careful when cleaning any gasket residue off the block. Aggressive cleaning using abrasive discs or scrapers can cause surface imperfections that can lead to premature gasket failure. Preparing the surface for the new gasket is at least as critical to its longevity as choosing the right gasket.
Information in this article was provided by Federal-Mogul Corporation, Dana Corp., and Sunnen.
TOP 10 TORQUE TIPS
1. Head bolts must be in perfect condition. Only clean, undamaged threads will allow for accurate torque readings.
2. Threaded holes must be round and clean. Dirty or deformed hole threads in the engine block can reduce clamping force as much as dirty or damaged threads on the bolts. Run a bottoming tap down each bolt hole in the block.
3. Lubricate bolts threading into blind holes. Add some lubrication to the underside of the bolt head, too. When bolt holes extend into the coolant jacket, use a thread sealer to prevent coolant leakage.
4. Torque-to-yield (TTY) bolts stretch when they have been properly torqued, and so it is often recommended that they not be reused. Once used, they may not provide proper clamping force, leading to possible leakage. Replacement isn’t always specified, but it may be a good idea.
5. Check bolt condition and length. Even non-TTY bolts can become stretched or damaged. Also, bolt lengths can vary on an engine. Using a bolt too long for a blind hole–whether because it is intended for a different position or is stretched–may provide a torque reading, but not clamping force.
6. Resurfaced heads may have a reduced height sufficient to require different length bolts or deeper washers. As mentioned, if a bolt bottoms out, it will apply little or no clamping force on the head and may allow the gasket to leak.
7. Don’t forget to use steel washers in aluminum applications that call for it. They prevent the galling of the surface and help distribute the clamping load.
8. Use torque sequences and values as specified, and be sure to use the latest information. Torque procedures can be amended to address specific sealing problems.
9. Make sure you have an accurate torque wrench and torque-to-angle indicator for TTY bolts.
10. Ensure that you know which applications recommend retorquing and which do not. Many modern gaskets are engineered to eliminate this procedure, but for some it is unavoidable. If a gasket requires retorquing, the engine should be brought up to normal operating temperature and shut off. Retorquing should be done while the engine is still warm on cast-iron applications. On aluminum head or block applications, the engine should be allowed to cool.
INTAKE MANIFOLD LEAKS CAN CAUSE SERIOUS PROBLEMS
Due to complex crossflow designs, sealing intake manifolds is more difficult than it used to be. And, due to the importance of carefully managing the air-fuel ratio, it is also more important.
Some intake manifold leaks cause changes in vacuum pressure. Others still can allow coolant to contaminate the engine oil.
Vacuum leaks can cause the on-board computer to make adjustments in fuel flow and engine timing, which in turn can result in driveability problems or abnormal combustion (preignition or detonation). Abnormal combustion is the most common cause of damage to the head gasket. It can even result in damage to hard parts such as pistons and spark plugs.
If an exhaust crossover port passes through the intake manifold gasket, some leaks may result in a noisy engine, p ower loss or even a blown gasket.
And, of course, leaks such as these can occur in combination, which can further frustrate the technician’s efforts.
For example, some General Motors 3.4L V-6 engines may experience a high idle with idle air control (IAC) valve counts at zero. This is usually caused by intake manifold gasket failure near the vehicle’s number five cylinder.
If the vehicle experiences high idle during cold starts, this is an indication that the intake manifold gasket is starting to fail, whereas continuous high idle indicates complete gasket failure.
Also, standard composition intake manifold gaskets on late-model General Motors 7.4L (454 CID) engines tend to fail prematurely due to an insufficient bolt load. These gaskets generally leak coolant at the rear of the engine, where it is difficult to detect.