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Head and Block Finishes
By Larry Carley
Clean, smooth and flat. That’s what both mating surfaces have to be to not only seal the head gasket but to make it last.
For years, most aftermarket gasket manufacturers have recommended a surface finish with a roughness average (Ra) of 55 to 110 microinches for conventional head gaskets, with a preferred range of 80 to 100 Ra. But on today’s engines, those numbers are no longer valid. Finishes that worked well on older cast iron engines with composition head gaskets don’t work so well with newer gaskets on bimetal engines and/or all-aluminum engines.
Federal-Mogul’s Fel-Pro division now recommends a surface finish of 30 to 110 Ra for cast iron head and block combinations, with a preferred range of 60 to 100 Ra for best results. For aluminum heads, the numbers are even lower: a surface finish of 30 to 60 Ra, with the preferred range being from 50 to 60 Ra.
If the engine has a multi-layer steel (MLS) head gasket, which a growing number of late model engines do, an even, smooth finish is required. OEM specifications vary, but typically range from 15 to 30 Ra or less.
MLS gaskets, which are found on Ford, Chrysler and many Japanese engines, typically have three to seven layers of steel. The outer layers are usually stainless spring steel and embossed. The inner layers provide added support and thickness. The embossed multi-layer construction reduces the load on the head bolts, which in turn reduces bore distortion for less blowby and lower emissions. The outer layers of steel are coated with a thin layer (.001˝ to .0015˝ ) of nitrile rubber or Viton to improve cold sealing.
MLS head gaskets are very durable because their solid steel construction retains torque very well and doesn’t take a compression set like composition gaskets. But the rigid nature of MLS gaskets also means they have very little conformability. That’s why MLS gaskets require an extremely smooth, flat surface finish on both mating surfaces.
Reproducing the kind of high quality surface finish needed to seal MLS gaskets requires up-to-date milling equipment and precision resurfacing techniques. Even then, it may be difficult to get a good cold seal on some of these engines.
Some gasket manufacturers have introduced "re-engineered" replacement gaskets for certain late model engines that were originally equipped with MLS gaskets. Some of these gaskets have a thicker surface coating so they can handle a rougher "aftermarket" surface finish (up to 60 Ra). Other gaskets are made of graphite or other nonasbestos materials. Even so, engine builders should have the know-how and equipment that can reproduce the high quality factory finishes so they don’t have to rely on special gaskets to solve their assembly problems.
Smoother Is Better
Anthony Usher of Rottler Manufacturing says one message that needs to be hammered home is that smoother is always better.
"People are still learning about MLS head gaskets. The smoother the gasket surface, the better these gaskets work. Some people still think they need a rough surface to hold the head gasket in place. What they don’t understand is that MLS head gaskets can actually seal on a mirror finish. The rubber coating is what seals the gasket. You don’t want a rough finish because it will damage the gasket. If the head is aluminum and the block is iron, they expand at different rates. One of the reasons for these metal gaskets is that the aluminum can slide over it without damaging the gasket. If the finish is too rough, it can cause the gasket to fail."
Usher said Rottler is always striving to achieve smoother and smoother finishes. "There are still guys out there who are grinding, but we haven’t ground anything in decades. They say they can’t cut diesel heads with prechambers or cut aluminum blocks with iron liners. But that’s not true. We can cut anything quickly, cleanly and with a dry process in most cases. We have the cutting materials today such as CBNs and PCDs that allow us to cut all kinds of materials.
Rottler recently sold a resurfacer to Hendrick Motorsports (Jeff Gordon’s team) after proving to them how much smoother they could make their heads and blocks with updated equipment. With their old equipment, Hendrick Motorsports was getting finishes in the 25 to 30 Ra range, a range that was okay but not as good as it could be. By increasing the cutting speed to 2,000 rpm and using larger 1/2-inch diameter PCD inserts, Rottler’s new machine could give them finishes as low as 3 to 5 Ra, says Usher. "Their first reaction was, ‘It’s too smooth.’ Then they realized how much better a seal they could get with the smoother finish."
Usher points out that the finish on the block is just as important as the finish on the head. "Everybody believes heads need to be resurfaced, but some don’t think the block needs to be resurfaced because the block is more stable than the head. But the block is just as important because it seals the other side of the gasket. One reason why some people don’t resurface blocks is because they don’t have the right equipment to do it properly.
"You need resurfacing equipment that is quick and easy to set up and allows you to do a block as fast as you can do a head. Our most popular resurfacer is the SF00E. It sells for about $28,000 complete. Our leveling table allows you to clamp the part and then adjust it very accurately so you can do a skim cut and remove only .002˝ or .003˝ of metal to clean it up. That’s very important on late model engines because you don’t want to change the compression ratio."
Usher says engine builders who are not resurfacing blocks are missing a significant profit opportunity and exposing themselves to unnecessary risks of future head gasket failures. "You’re just turning away money if you’re not resurfacing blocks. If you’re boring and honing blocks, you should also be resurfacing them, too."
Being able to resurface heads and blocks quickly is just as important as being able to achieve a smoother finish. Tim Whitley of T & S Machines says most engine builders work on a wide variety of different engines, so they have to be able to reset their resurfacing equipment quickly to keep the work flowing through the shop.
"With our resurfacer, you can reset it and do a head in five minutes floor-to-floor. If you’re doing two or more heads that are the same and don’t have to reset the machine, you can cut the head in two minutes flat."
Whitley says his TS3000 Block-Mate can do head and blocks, and only takes minutes to convert from one to the other. "It’s a machine that’s built for the automotive performance market, but today that includes most late model heads. The machine can handle heads up to 38 inches in length and uses a cutter head with a single CBN bit to cut in a single pass. We can cut up to .030˝ at 45 inches per minute, so it’s very fast.
"We also designed the machine to have a small footprint," explains Whitley. "In some areas, like California, shop space is very expensive and every foot counts. Our machine will fit in a 3 x 5 foot work space. The resurfacer sells for about $18,000 with fixturing."
Carbide, CBN or PCD?
Though many high speed resurfacing machines today use CBN (iron and aluminum) or PCD (aluminum only) cutting bits, carbide is still used by one out of four shops according to Tim Mera of Sunnen. Mera says most shops today want equipment that is flexible and can handle aluminum, cast iron and hard diesel heads. "Some shops use CBN for everything, but CBN is not the best choice for aluminum because aluminum erodes the face of the CBN. For aluminum, you should use PCD."
Mera said Sunnen’s most popular resurfacer for automotive applications is the HBS1355. The cutter head can handle 12 carbide bits, or CBN or PCD single bits. "Most shops don’t want to take the time to set up a two-bit head and prefer the single-bit setup."
Randy Neal of CWT Industries says carbide is still an excellent choice for resurfacing heads and blocks. "Tool geometry is the key to successfully using coated carbides. Our $18 carbide is a lot less expensive than PCD or CBN and lowers the cost per surface. Other manufacturers are using CBN and PCD because they have a fixed spindle speed. With our equipment, you can adjust spindle speed and transverse speed to achieve as smooth a finish as you need. We can get as low as 7 Ra with carbide." Neal says his company’s resurfacer is an "industrial quality" machine that sells for $18,000 ($25,000 with fixturing).
Terry Wagner at Storm Vulcan agrees that carbide is still a good choice. "Our Style 85 resurfacer has an eight-carbide cutter head and allows variable speed and feed for achieving a high quality finish. We’re working on a new CBN resurfacer but it won’t be ready for a few months. Our existing machine which can do both blocks and heads, is capable of producing finishes of 15 Ra or less, and sells for $17,995 complete.
Wagner believes some heavy-duty engine builders are still grinding heads with hard prechambers, but that 90 percent of older grinding machines have been converted to milling machines.
Ed Kiebler at Winona Van Norman says, from his perspective, CBN and PCD are the most popular cutting bits today. "Carbide is a thing of the past and grinding is even further back." Kiebler says. Winona Van Norman’s most popular resurfacer, the SM100M, comes with a 2-bit cutter head, which allows fast surfacing and eliminates harmonics that can sometimes occur with a single-bit head. The machine sells for less than $16,000 tooled up with CBN and can handle blocks up to 24 inches high.
Bill Wessel at Peterson Machine Tool agrees that CBN continues to be the most popular choice for milling heads and blocks. "Our best moving machine right now is the Model 900MVA, which sells for about $15,000. We have larger resurfacing machines and are still selling platten grinders mostly for doing manifolds. But most of the interest is in the smaller machines."
Besides the type of tooling that is used, fixturing is also important. "I can’t imagine anyone surfacing heads today who wouldn’t benefit from a Winfield fixture," says Winona’s Kiebler. "The Winfield fixture is the hottest thing going today. It can handle any head and takes only 5 to 8 minutes to set up instead of 25 minutes. The time savings can really add up when you’re looking at a stack of heads."
The gasket manufacturers don’t care what type of resurfacing equipment or tool bits engine builders use to resurface heads and blocks as long as the Ra numbers end up in the recommended range. It is possible to achieve an acceptable surface finish for a soft-faced head gasket on most cast iron and even aluminum heads by milling, grinding or even belt sanding. But for late model engines with MLS head gaskets, the smoother surface finishes usually require milling using equipment designed for this purpose.
A converted grinder may be able to mill heads and blocks, but the spindles and table drives in many of these older machines may not be able to hold close enough tolerances to achieve a really smooth, flat finish. One equipment manufacturer said grinding and milling machines that are more than five years old are probably incapable of producing consistent results and should be replaced.
Randy Taylor of DCM Tech said holding accurate tolerances is extremely important on today’s engines as well as performance engines. "We’ve put our new 3820 resurfacer into some high profile NASCAR shops because it can hold tolerances to within a tenth of a thousandth. When these guys tear down an engine, all they want to do is remove the gasket impression. They’re running close to 12:1 compression ratios, so they can’t remove much metal. Accuracy in this kind of situation is absolutely essential. So, too, is a high quality finish. Our equipment can achieve the kind of numbers required by the new metal head gaskets. We can get into the mid-teens."
Grinding Versus Milling
Grinding cast iron and aluminum with a silicone carbide grinding wheel can be tricky because the stone tends to load up with metal, causing it to overheat and score the surface. Precoating aluminum with lubricant and using plenty of coolant can reduce clogging. Increasing the transverse speed and limiting the depth of cut to no more than .0005˝ to .001˝. also helps. It’s also important to dress the grinding wheel often to keep the grain open – but not on the final pass so the stone will leave a smoother finish.
Milling, by comparison, is much easier. Dry milling eliminates the mess and maintenance that goes with wet grinding. Milling also allows more precise control over the amount of metal removed, and is faster than grinding because more metal can be removed in a single pass eliminating the need for multiple passes.
The key to achieving a smoother finish is using the right combination of table feed and rpm when milling the head. A variable speed table and/or multi-speed or variable speed milling head allows you to find the best cutting combination. Increasing the rpm of the cutting head and/or slowing down the feed rate produces a smoother finish.
A milling machine with a multi-bladed cutter head will give the fastest cutting speed because there are more tool bits chipping away at the metal with each revolution of the head. Even so, most shops use a single cutter head or one with two bits.
To achieve the lower Ra surface finishes 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. With a single bit, the feed rate might have to be slowed down even more – as low as one inch per minute at 1,000 rpm to get a really smooth finish.
Though most shops that mill heads do not use any coolant, milling with a coolant has a number of advantages. Coolant helps wash away swarf for a cleaner more consistent cut. It also keeps the tooling and work piece cooler which reduces heat buildup, distortion and tool wear. Consequently, milling with a coolant generally improves the overall process and improves the surface finish.
When resurfacing an aluminum head that has a lot of lime built up in the water jackets, the hard calcium deposits around the water jacket openings can sometimes be picked up by the tooling and dragged across the surface leaving a groove. Removing the deposits before the head is resurfaced can eliminate this particular problem. EB
Flatness is another aspect of surface finish that needs to be mentioned, too, because a surface that’s out of flat won’t seal no matter how smooth it is. Near perfection is required in many of today’s engines for a good cold seal. Out-of-flat lengthwise should not be more than .003˝ (.076 mm) in a V6 head, .004˝ (.102 mm) in a four cylinder or V8 head, or .006˝ (.152 mm) in a straight six head. The maximum allowable limit for out-of-flat sideways in any head is .002˝ (.05 mm) – with no sudden irregularities that exceed .001˝ in any direction.
Any head that fails to meet these specs needs to be resurfaced and/or straightened in the case of a warped aluminum head.
Another dimension that needs to be mentioned but often receives little attention is waviness. Waviness ("WT") refers to the subtle undulations in the surface left by milling or grinding operations. One reason why nobody in the aftermarket pays much attention to OEM waviness specs is because it takes a very sophisticated and expensive profilometer to accurately map it. So people tend to ignore it. Yet waviness can cause problems if it is beyond the acceptable range.
According to Fel-Pro, the maximum acceptable waviness height is .0008˝ (.020 mm) for peak spacing greater than .100˝ (2.5 mm), and .0005˝ (.013 mm) for peak spacing between .030˝ and .100˝ (0.8 to 2.5 mm). Waviness peak spacing should be no less than .030˝ (0.8 mm).