In recent years, there has been a proliferation of new engines from the worlds automobile manufacturers. Many of these engines have simply been revisions of earlier designs, improved to bring them up to par with todays standards of performance. Redesigned cylinder heads, low friction valvetrain components, lower tension piston rings, tighter build tolerances, new materials and sequential fuel injection systems have all produced significant horsepower gains, better fuel economy, lower emissions, reduced noise, vibration and harshness (NVH), and improved durability.
Other recent engines have been more revolutionary than evolutionary in their design. One such engines that began as a clean sheet of paper was General Motors 2.3L Quad 4, introduced in 1988. The Quad 4 was the first mass produced modern multivalve engine built by a domestic vehicle manufacturer. The Quad 4 broke new ground with its dual overhead cams, four valves per cylinder, and aluminum head on a cast iron block. The engines relatively high power output rivaled that of many larger displacement V6 and even some V8 engines. The added heat and pressure placed a considerable strain on the head gasket, which turned out to be the Achilles heel on this particular engine.HEAD GASKETS
As the Quad 4 engines accumulated miles, the difference in thermal expansion rates between the aluminum head and cast iron block produced a sideways shearing or scrubbing motion that was intensified by high operating temperatures. The OEM gasket that was originally used in this engine had a type of coating that would provide a good cold seal when the engine was assembled. But in service, the coating stuck like glue to the head and block. With each successive thermal cycle, the gasket was literally pulling itself apart. After 50,000 or 60,000 miles, this often led to cracking of the combustion armor and failure of the head gasket. Coolant would then start to seep into the cylinders causing the engine to overheat.
General Motors ended up paying for a lot of head gaskets to be replaced, and promptly changed to a new gasket design and OEM gasket supplier. The Quad 4 is now on its third OEM head gasket design from yet another OEM gasket supplier. The latest design, which is an armored graphite head gasket, appears to have solved the Quad 4 sealing problems as far as GM is concerned. But what happens when the Quad 4 and other high tech engines with similar sealing requirements need to be rebuilt? Does the aftermarket have the know-how and right kind of resurfacing equipment to seal these engines properly?
Ray English of AADCO in Solon, Ohio, says no. English has a contract with General Motors to redo their Quad 4 heads so he knows what he is talking about. He says most shops do not have what it takes to reproduce an OEM finish, which is essential when you are using an OEM head gasket. Though there are a number of aftermarket gaskets available for the Quad 4, English says the OEM gasket that GM recommends requires a very smooth, flat surface finish: 0.7 to 1.2 micrometers RA (27 to 47 microinches) with no more than 0.2 to 0.6 micrometers waviness. This dictates that the heads be milled. English says it was difficult to meet both of GMs parameters using carbide tooling, but that it has been no problem at all since he started using polycrystaline diamond (PCD) tooling.
English says the sealing problems encountered in the Quad 4 are really no different than those experienced by many other bimetal engines such as the Chrysler 2.2L and 2.6L engines and some of the Japanese engines. "Aluminum expands at 1.7 times the rate of cast iron. Every time the engine heats up, expands, cools and retracts, it crushes and tugs at the gasket. After so many such cycles, either the head cracks or the gasket fails."
"One thing that GM is now doing differently with the Quad 4 is a new bolt tightening procedure with the Victor gasket. Bolts 1 through 8, the long bolts, are first torqued to 30 lbs. Then the bolts 9 and 10, the two short ones, are torqued to 26 lbs. Then all 10 head bolts are given an extra 90 degree twist. The procedure works great and has virtually eliminated any failures from loss of torque," said English.
HEAD GASEKT SEALING REQUIREMENTS
The sealing requirements for the Quad 4 as well as most of todays overhead cam, multivalve, bimetal, high output engines are much more demanding than those for an all-iron pushrod V8, V6, straight six or four cylinder engine. But the refinishing techniques that the aftermarket has traditionally used to resurface heads and engine decks may not produce a good enough surface to seal some of todays engines.
The two basic issues here are surface finish and gasket design. Most aftermarket head gaskets are designed to seal rougher surfaces than an OEM gasket sees in a brand new engine. Though this approach has worked well in older, high mileage engines, it may not work so well on some of the newer engines that require a very smooth, flat sealing surface, or applications that require an OEM type of head gasket instead of an aftermarket gasket.
A case in point is Fords 4.6L "modular" V8. This engine has a "multi-layered steel" (MLS) head gasket (which may also be referred to as a "rubber coated embossed" (RCE) gasket). This type of head gasket is extremely durable and requires less torque on the head bolts. Lowering the load on the bolts reduces cylinder bore distortion, which in turn improves combustion sealing for less blowby, emissions and oil consumption. But to seal properly, the OEM gasket requires an almost perfect surface finish. The specified factory surface finish is 8 to 12 microinches RA.
Similar gaskets are found on a number of late model Japanese engines, including 1990 & up Honda Accord 1.8L, 1990 & up Honda 1.5L, 2.2L & 2.3L, 1988 to 1991 Mazda 3.0L V6, 1990 & up Mazda SOHC & DOHC 1.8L, and 1992 & up Mazda 1.8L. The MLS gaskets in these engines have two to five layers of heat treated steel, each covered with a thin (.001 in.) coating of nitrile rubber or Viton. These gaskets also require an exceptionally smooth surface finish, though not as polished as that specified by Ford. Surface finish requirements range from 8 to 40 microinches RA, which is considerably smoother than the 54 to 113 RA (60 to 125 RMS) surface finish that is usually recommended for traditional aftermarket gaskets.
As for flatness, near perfection is also required for a good seal. The maximum allowable limit is .002 in. (.05 mm) of total distortion (that is, block plus the head combined) sideways across the face of the head or block. Again, this is considerably less than the standard aftermarket limits which normally allow up to .003 in. (0.076 mm) out-of-flat lengthwise in V6 heads, .004 in. (0.102 mm) in four cylinder or V8 heads, and .006 in. (0.152 mm) in straight six cylinder heads.
It is not difficult for the OEMs to achieve such smooth surface finishes when they manufacture a brand new engine in a controlled environment. Automated milling machines with multi-bladed cutter heads (up to 16 tool bits per head) and polycrystaline diamond tooling provide the precision needed to produce the extremely smooth surface finishes necessary to seal todays engines with MLS steel head gaskets. But few engine rebuilders have such sophisticated equipment.
Because of this, some say it is better not to resurface the head or block on an engine that requires an MLS steel head gasket unless it is absolutely necessary. Unfortunately, resurfacing is usually necessary by the time these engines need to be rebuilt. And that creates a problem for those who lack the know-how or equipment to duplicate the OEM finish. Close enough is not good enough on these engines because the OEM gasket will not seal properly unless both mating surfaces are within the specified surface finish limits. Gasket shellac may give a temporary seal, but it won not last.
What is needed is one of two things: either better resurfacing equipment that is capable of duplicating an extremely smooth OEM surface finish, or an aftermarket replacement gasket that can seal with the same loading as the OEM gasket yet handle the rougher surface finishes commonly produced by current aftermarket refinishing techniques.
Several aftermarket gasket suppliers have developed conventional gaskets that can replace MLS gaskets on some engines. Nonasbestos and graphite service gaskets are currently available for some engines (such as the Quad 4 and Ford 4.6L V8), but not most of the Japanese applications. So for the applications where an aftermarket gasket is not available, you have to use an OEM-style MLS gasket and try to reproduce the highly-polished OEM surface finish that is required.
RESURFACING CYLINDER HEADS
To produce an OEM surface finish with a low RA value, a milling machine is recommended. Milling machines run the gambit from less expensive models with a two-bladed cutting head, fixed rpm and feed rate to top-of-the-line machines that have "precision pocket" multi-bladed heads and infinitely variable rpm and feed rate. Though many shops would love to have the best machine that money can buy, many feel they can get satisfactory results with a more modest model.
A multi-bladed cutter will give the fastest cutting speed because there are more tool bits chipping away at the metal with each revolution of the head. In a high volume production application, a multi-headed cutter would save considerable time. But multi-bladed cutters require more tool bits which are expensive and take longer to adjust. Each bit must be set to the same height so they cut evenly. So for these reasons, many shops prefer a milling machine with a simple, two-bladed cutter.
To achieve an OEM surface finish with a low RA using a two-bladed cutter, the feed rate must be slowed down to maintain the proper cutting rate in surface feet per minute. One equipment manufacturer recommends a feed rate of two inches per minute at 1,000 rpm to achieve a surface finish of 12 RA with a two-bladed cutter. A four-bladed cutter, on the other hand, could do the same job in half the time.
Some shops, though, will resurface a head using only one tool bit in a two-bladed cutter. Doing this eliminates the need to set both tool bits to equal heights, but it also requires an even slower feed rate (as low as one inch per minute at 1,000 rpm), which increases the time it takes to resurface the head even more. So the temptation is to use the same feed rate as before. But doing so cuts too quickly and leaves a rougher finish that may not meet the OEM spec. Such a short cut may work on older engines that do not require extremely smooth surface finishes, but it will not work on the newer engines that have MLS had gaskets.
MILLING MACHINE TOOLING
Another aspect of cutting heads on a milling machine that will affect the surface finish is the type of tooling used. Though some people say they have had success using cubic boron nitride (CBN) tooling on aluminum, Dennis Marble of Sunnen Products Company in St. Louis, MO says he does not recommend CBN tooling for resurfacing aluminum heads.
"Though CBN has a lot of desirable characteristics, it is primarily for making fast, deep cuts on cast iron and hard steel. Most CBN tooling has a negative rake which is ideal for cast iron and steel. But you should use tooling with a positive rake on nonferrous metals such as aluminum."
Marble says either carbide or polycrystaline diamond (PCD) can be used to resurface aluminum heads. "Carbide is probably the most cost-effective choice for the average engine rebuilder because it is relatively inexpensive compared to PCD. And carbide can also be used on cast iron, which means you do not have to change tooling when refinishing different kinds of heads. PCD would make more sense in a high volume shop that did nothing but aluminum heads."
Marble said it is relatively easy to obtain extremely smooth surface finishes and high production rates using carbide tooling -- provided a multi-bladed cutter is used. In one test that Sunned did for an OEM engine manufacturer, a 16-bladed cutter was used to shave .005 in. off an aluminum head at a feed rate of 32 inches per minute. At such a speed, it took only 35 seconds to surface the head. Yet it still produced a finish of 12 RA. Slowing down the feed rate to 19 inches per minute with the same equipment setup increased the surfacing time to one minute but improved the finish to 5 RA!
Ray English at AADCO says he can easily produce surface finishes of 6 RA using his equipment with PCD tooling. And as stated earlier, he feels the results are more consistent with PCD tooling than carbide. English also says that the longer life of PCD tooling more than offsets its higher initial cost.
"We paid $2500 for our PCD tooling, and we have run over 1,000 cylinder heads without touching the tooling. Nor do we expect to have to do anything to the tooling for a long, long time to come. With carbide, we would run maybe 200 to 300 pieces before we would have to start working on the wiper again.
"We also have to machine thermostat housings, manifold faces and cam housings, so we have three milling machines all set up with PCD tooling to handle these jobs, too. We are running them flat out at 4,000 rpm on a 5-inch cutter with a feed rate of as fast as we can go, and we are still getting surface finishes of 12 RA.
"If I had realized how good PCD is, I would have switched to it years ago. I had always considered PCD to be too expensive compared to carbide. But when you look at the price per piece, PCD is cheaper than carbide. Of course, you have to have the volume to justify the cost."
English also said that it is important to have a profilometer so you can check the accuracy of your work. "If you do note a profilometer, you do not really know what you are getting. Most people cannot look at a finish and tell you if it is 100 RA or 20 RA. So you need some means of measuring the surface to find out what kind of finish you are actually getting.
"Some time back, General Motors sent out a number of Quad 4 heads to various shops to see what kind of finish was being applied when the heads were resurfaced. When GM got the heads back and measured them, they discovered the finishes varied all over the place, even from the same shop! Most were up around 90 to 100 RA. It was obvious that nobody was coming anywhere close to duplicating the OEM finish, so GM wanted to develop a service procedure that would more closely match the OEM finish. We started milling with PCD tooling and that proved to be the answer," said English.
The best advice you can follow with respect to resurfacing cylinder heads and blocks is to use the surface recommendations of the gasket supplier when installing aftermarket head gaskets, and to follow the vehicle manufacturer's surface recommendations when installing an OEM gasket (particularly MLS gaskets).
CYLINDER HEAD INSTALLED HEIGHT
Another sealing requirement for todays overhead cam engines is maintaining the installed height of the cylinder head. The amount of metal removed when the head is resurfaced can change the installed height and retard cam timing, reduce the volume of the combustion chambers, and consequently increase compression. Too much compression can cause detonation problems, which in turn can lead to premature head gasket failure and overheating.
When the head (or block) is resurfaced, therefore, the amount of metal that can be safely removed is very limited. Some heads, like the Quad 4, have several "indicator pads" in the casting that show how much material can be safely removed before the minimum height specification is exceeded. If the pads on the Quad 4 measure .020 in. or less, GM says the head should be replaced. Head gasket shims are available, though, that can restore proper head height to salvage such heads. The question with shims is how they might affect the long term durability of the head gasket.
Many aftermarket gaskets are made with extra thickness to compensate for metal removal when the head is resurfaced. The added thickness helps to maintain proper valve timing, combustion chamber volumes and compression. For applications where such gaskets are available, they can save heads that otherwise would have to be scrapped. But until the aftermarket comes up with something similar for the newer engines that have MLS head gaskets, no such alternative exists.