On older cast iron engines with conventional soft-faced head gaskets, as long as the surface finish on the cylinder head and block deck is roughly 60 to 100 microinches Ra (roughness average), the head gasket should cold seal and remain leak-free for the life of the engine. But on late model bimetal (aluminum head/cast iron block) and all-aluminum engines, smoother surface finishes are required for a couple of reasons.
Aluminum expands and contracts at a much higher rate than cast iron as it heats up and cools down. As a result, the head gasket typically requires some type of low friction coating such as Teflon, graphite or moly so it won’t stick to the head and be pulled apart. A smoother surface finish also helps. Most gasket manufacturers now recommend a surface finish of 30 to 60 microinches Ra for bimetal and all-aluminum engines with conventional soft-faced head gaskets. And if the engine has a multi-layer steel (MLS) head gasket, a surface finish of 25 Ra or less is typically required because OEM MLS gaskets have very little conformability and require almost a mirror-finish to seal properly.
With performance engines, smoother finishes have also become the norm. Most NASCAR engines today are running MLS gaskets, and the people who are putting these engines together typically want finishes in the low teens or even single digits! That level of smoothness can only be obtained with surfacing equipment that has the rigidity, accuracy and control flexibility to produce an ultra-smooth finish.
If there’s one thing most equipment suppliers agree upon, it’s that yesterday’s surfacing equipment can’t cut it in today’s world. Older milling machines and grinders usually can’t hold the tight tolerances that are needed to achieve really low RA surface finishes required for MLS gaskets. Lack of rigidity in the machine itself or fixturing, and play in the travel mechanism, cutter head or spindle can all have an adverse effect on the finish.
In the good ol’ days, it was possible to get a satisfactory surface finish with almost anything that could remove metal whether it was a grinder, carbide flycutter, broach or belt sander. But to achieve the high quality finishes required for today’s engines, you need a high speed resurfacer to mill cylinder heads and block decks.
In some cases, older grinding equipment can be upgraded and converted to milling by changing the head. But some equipment suppliers say you can’t get the same results using a cutter head in a modified grinder. It’s better to go with a dedicated milling machine. Yet there are combination grinding/milling machines that can do both equally well and provide the flexibility to use whichever method works best in a given situation.
Though the trend in recent years has been away from wet grinding to dry milling, grinding is still used to resurface diesel cylinder heads with precombustion chambers as well as diesel engine blocks with hard cylinder liners. Milling a diesel cylinder head with the precombustion chambers in place is difficult because the precombustion chambers are much harder than the head. Grinding eliminates the need to remove the precombustion chambers. But if a smoother finish is required than can be obtained by grinding, you may have to remove the precombustion chambers to mill the head.
Shopping For New Surfacing Equipment
Dry milling heads and blocks requires surfacing equipment that can deliver a smooth, flat, high-quality finish. The OEMs and many production engine builders use surfacers that have high speed multi-cutter heads. The more inserts there are in the cutter, the faster it can transverse the surface. Faster cutting saves time and boosts productivity in a high volume shop. But multiple inserts also increase the cost of the equipment and maintenance, and requires more skill and effort to set up than a simple single insert cutter head.
Many shops like the simplicity of a single insert cutter because it is so much easier to set up and adjust. If a cutter head has two or more inserts, all the inserts must be set to the same height or else it will leave an uneven cut on the surface. So that’s why many shops prefer the single insert setup.
To achieve the best possible finish, you should use a higher rpm and lower feed rate with a very shallow cut on the final pass (less than .001″). With a single insert cutter spinning at 1,000 to 1,500 rpm, the feed rate should probably be less than two inches per minute to achieve a surface finish in the low teens. If you bump up the cutter rpm, you can also increase the feed rate while still maintaining the same surface finish. Using a surfacer that has an adjustable rpm and feed rate, therefore, gives you the flexibility to play around with the settings so you can find the optimum combination that gives the best surface finish.
Buying a surfacer with higher rpm capability also allows the use of special cutters like shell mills, which are becoming increasing popular for resurfacing the intake side of cylinder heads.
You can also get a programmable surfacer that can do a two-step milling job. You tell the machine how much metal you want to remove for a rough cut to restore flatness across the surface of the head or block deck, then how much additional metal you want to remove for a final finish cut (and at what speed and feed). After the machine takes the first cut, it automatically recycles and adjusts the speed and feed rate before taking the final cut. Fully programmable equipment is expensive, but also frees up the operator to do other tasks, which may save labor cost and help boost your overall productivity.
You can spend anywhere from around $12,000 for a basic entry-level surfacer up to $50,000 or more for a top-of-the-line model. The price ultimately depends on the tooling, fixturing and other features you want. The higher the price, the more “extras” you get like programmable controls, multi-bit cutter heads, special gauging, etc.
Some surfacers are dedicated milling machines while others are dual-purpose machining centers that can also be used for boring/milling or even valve and seat work. A dual-purpose machine gives you more flexibility and takes up less floor space but limits you to performing one type of job at a time. A dual purpose machine may be a good choice if you operate a smaller shop and are limited on space, or you want one machine that can do more than one thing. On the other hand, if you have a larger, high volume shop, you would probably want a dedicated milling machine – maybe even several such machines depending on the type of work you do. You might need a standard sized machine for resurfacing car and light truck heads and blocks, and a second larger machine for resurfacing heavy-duty diesel heads and blocks.
To achieve today’s flatness and smoothness requirements, a surfacer must be a very rigid machine. The work table, cutting head, shaft and motor must all be very strong and constructed to extremely tight tolerances. Any flexing or movement in these parts will affect the quality of the surface finish.
Motors should have zero tolerance bearings. Play in the bearings may allow the motor shaft to move up and down when the motor changes speed or when the cutter encounters resistance. There should be no vertical movement in the motor or cutter head while the milling process is taking place.
The type of feed mechanism the surfacer uses is also important. Hydraulic surfacers have been around for many years and offer infinitely variable feed speeds. A hydraulic feed uses pressure to push the table along on slides. At low feed rates, though, the table may stick momentarily as it moves, creating a “stick and slip” motion that may leave an uneven finish on the head or block. That’s why many surfacers now have electric drive ball screw feed mechanisms. A motor driven ball screw mechanism provides a higher degree of control and consistency, which helps the machine achieve ultra-low Ra surface finish numbers. Some machines with ball screw mechanisms can hold tolerances to within .0001″.
Ease of setup is also an important feature to look for if you are shopping for new equipment. The fixturing should be capable of accommodating a wide variety of different cylinder head configurations as well as different lengths. If all you do are passenger car and light truck engines, all you need is a standard sized surfacer with fixturing for these applications. On the other hand, if you do heavy-duty diesel heads and/or blocks, you’ll need a larger machine with oversized fixturing to handle the larger parts.
Ease of setup includes being able to mount a head or block on the surfacer quickly, level it and align the cutter head. The “Winfield” fixture is very popular and can accommodate both heads and blocks. It usually takes only 5 to 8 minutes to set up, which can really save a lot of time in a busy shop.
Less expensive universal “economy” fixtures are also available, and may save you some money up front. But you also have to consider what your needs really are and which type of fixturing will best meet those needs year-round in your shop. You also have to compare set-up times and how it affects productivity.
Setup accuracy is absolutely essential because there’s so little margin for error on most engines today. In most cases, you will be removing very little metal and just skimming across the surface to clean it up. Some surfacers have a built-in dial indicator that makes it easier to set up the machine. Time is money and the less time it takes to set up a cylinder head or a block to be resurfaced, the more time can be spent resurfacing metal. Faster turn around means more production and hopefully more profit at the end of the day.
Milling machines can use carbide inserts, specially coated carbide, CBN (cubic boron nitride) or PCD (polycrystaline diamond) inserts.
Carbide can be used on aluminum or cast iron, is the least expensive type of insert and gives a high quality finish. Coated carbide inserts provide improved cutting action and deliver even better results than plain carbide. But carbide does not last as long as the more expensive superabrasives such as PCD or CBN.
PCD is generally recommended for milling aluminum. But PCD doesn’t like cast iron and may shatter. CBN works great on cast iron and can be successfully used to mill aluminum provided some type of lubricant is used. A light coating of soap, wax or WD-40 can prevent the aluminum chips from sticking to the CBN tooling and scratching the surface.
Some shops change their tooling back and forth from CBN to PCD depending on what type of head they’re resurfacing (cast iron or aluminum), while others use CBN on everything.
If you’re shopping for new equipment, look for a surfacer that is designed for high speed CBN/PCD tooling. Superabrasives cutting at high speeds do not like any vibration or harmonics. They must be stable to cut evenly and cleanly. So look for a rigid table, power head and a large diameter shaft for the cutter head.