In a perfect world, opening an engine to service internals ought to happen rarely, if ever, and many modern engines can go tremendous mileages before major work if well maintained. Given enough time, however, the head will have to come off, and when it does, the most critical sealing surfaces in the engine become an important reliability issue.
What’s the best gasket? In theory, no gasket at all. Perfectly polished, perfectly flat head and block deck surfaces wouldn’t need a gasket at all, as the surfaces would “cold weld”, but in the real world, a deformable sealing layer makes reasonable machining tolerances possible. Working against that seal is sealing surface condition, heat, compression, corrosion, and differential expansion from different metals in the head and block. Good cooling and ignition system performance helps, but a poorly installed head gasket has a limited chance to survive in today’s lightweight, high-RPM engines.
With few exceptions, (many from GM’s V-8 stable), multi-layer steel is the technology of choice in head gaskets and with good reason. Aluminum alloy heads and blocks, and especially aluminum heads on iron blocks expand and contract at different rates, causing movement at the mating surface. MLS technology tolerates the movement while containing compression and combustion pressures. In some cases, MLS gaskets replace conventional types where failures are common. “The really recent example was the Neon application”, relates Jim Daigle, product manager for Fel-Pro. “That was an application where multi layer steel is used with bi-metal applications where the motion of the head to the block will tend to tear the gasket. In those applications multi-layer steel gaskets perform better. In the Neon application, the multi-layer gasket replaces composite. In almost every case, our testing shows that you need to replace a multi-layer steel gasket with another multi-layer.”
Some technicians claim that composite designs are more conformable and therefore more tolerant of variation in the block deck or cylinder face surfaces. “There may be some truth to that in that it seals initially with less machining”, says Daigle, “but if the motion of the gasket tends to tear it, it ultimately won’t do its job. The fact is, many MLS aftermarket gaskets out there are OEM designs.”
While modern multi-layer head gaskets tolerate the movement between head and block, they’re not as forgiving of a warped or contaminated mating surface. Alloy heads are very susceptible to warpage with overheating, often bowing enough to lose clamp force across the centre two cylinders. Careful examination of the failed gasket provides useful clues, and if an overhead cam won’t spin in it’s bores, it’s an especially severe case. Many good shops resurface automatically in any case of suspected overheating in a head gasket failure. Checking for flatness where it’s not so obvious can be done successfully using the tried-and-true feeler gauge method. Fel-Pro has the following recommendations for flatness:
Maximum Out of Flat, head and block combined (unless otherwise specified by manufacturer):
L3 and V6 Engines
L4 and V8 Engines
Fel-Pro also recommends a surface finish between 80 and 100 RA microinches. Too rough a surface will scuff and damage the gasket, while too smooth a finish (in the mid ’50’s RA) may cause the opposite problem by not holding the gasket in place during the engine’s heat cycle. Newer technologies using multi-layer steel gaskets, commonly used on late model Japanese engines, use super-smooth finishes (in the 20’s RA) which are impossible to achieve in a typical repair environment, so correcting surface flaws on these engines requires a competent machine shop. You just can’t wrap a piece of emery cloth around a two-by-four anymore. Checking surface finish can be done by comparing the look and feel of the surface with a standard “comparator”. Precision machine shops often use electronic profilometers, which allow more consistent results, but checking the surface after the head or block returns from the machine shop is a good practice.
Avoid adding sealants
Before multi-layer steel gaskets dominated OE designs, installers often used sealants on head gaskets for best performance. An increasing number of aftermarket MLS head gaskets ship with polymer sealants already applied, but a considerable number of technicians apply coatings before assembly. While many experienced techs have long histories of success with this practice, gasket manufacturers don’t recommend it with modern designs. According to Marty Novil, director of static sealing for Freudenberg-NOK (Corteco):
“Multi-layer steel is meant to seal a very smooth surface finish. Any scratch, as little as one thousandth of an inch, can affect sealing, especially if it goes across a coolant or high-pressure oil passage. The black rubber that you see on the gasket is as little as one thousandth (of an inch) of rubber. It’s intended to seal surface irregularities about one third of the thickness of the rubber, so that suggests the kind of surface finish you need. On the plus side, if it doesn’t seal, you’ll usually see it immediately. The reason is that on a MLS gasket, the rubber seals the surface irregularities on the flange, while the embossments do the real sealing of the pressure, either combustion, high-pressure oil or coolant. They provide a ‘living spring’ seal.”
And what do you do if OE conditions can’t be met? Novil feels that part of the problem in attaining OE-recommended levels of surface finish is knowing what’s possible with given equipment: “Many machine shops get to 50-60 RA surface finish, and they need something around 20-30. Some say they can seal MLS with 50, but in fact they may have a better surface finish that they think. There’s a lot to measuring surface.”
Is there a situation where sealants are necessary? According to Novil, “If I had an extremely rough surface and I couldn’t do anything about it, I’d use a specialty aerosol spray gasket product with a very light coating, not heavy or dripping.” For non-emergency, bay-level head work, however, the right finish with a modern aftermarket gasket with integral sealing built in is the best practice, unless otherwise specified by the gasket maker. Says Novil, “if I had an OE surface on the block that was clean and straight, I wouldn’t coat it.”
Assuming the sealing surfaces are clean, smooth and straight, the right gasket is carefully installed and everything lines up, there is still an important consideration: bolts. The way a technician clamps a clean, straight head to a similarly prepared block can be more important than the gasket itself. While it’s obvious that bolt-tightening sequences and cleanliness are vital, a significant number of technicians are still re-using torque-to-yield head bolts. The requirement to discard TTY bolts is more than just marketing hype; the “yield” in torque-to-yield means that the steel assumes a permanent stretch, necking and elongating slightly. The bolt may look normal, but if reused, TTY bolts will not give consistent clamping regardless of the torque applied. Over-torquing TTY bolts to induce further stretch risks distortion and, at best, a blown gasket.
When torquing up those new bolts, use the correct technique, and the correct torque-measuring tool. Many modern applications use degrees or angle, rather than applied torque as the measure of clamp force, and for especially with TTY bolts, angle produces very consistent results. The cleanliness of bolt threads and lubrication are important factors in getting the correct clamping force, as is the condition and cleanliness of the area under the bolt head and washers. There is no such thing as “too clean”, either on the bolts themselves or in the tapped holes they thread into. Chasing threads should be a part of the
block deck preparation. If bolts are re-used, a die can be run over the thread area.
Modern head gasket service is very much a “good news/bad news” scenario. The good news is that aftermarket problem-solving products can salvage a poor OE design and give improved service life and customer satisfaction. The down side is that modern engines are much less tolerant of poor surface finish, contamination or sloppy service procedures that the cast iron engines of the previous generation. Nothing about cylinder head gasket service is rocket science, except the technician’s need to keep it clean, and think about the installation.