Cylinder head resurfacing is just one of the jobs that is usually required when rebuilding an engine or reconditioning a cylinder head. It is an extremely important job because the surface finish that is put on the head (and engine block) affects not only the head gasket's ability to cold seal fluids and combustion gases, but also its long term durability. CYLINDER HEAD SURFACE FINISH SPECIFICATIONS For years, most aftermarket gasket manufacturers have said surface finishes with a roughness average (RA) of anywhere from 55 to 110 microinches (60 to 125 RMS) are acceptable. The preferred range they have recommended is from 80 to 100 RA. Even so, as long as the surface finish on the head and block end up somewhere between the minimum smoothness and maximum roughness numbers, there should not be any cold sealing or durability problems with the head gasket (assuming everything is assembled correctly and the head bolts are torqued in the proper sequence and to the specified torque, too). But like everything else, these numbers have been changing. These recommendations were primarily for older cast iron heads on cast iron blocks. As castings have become lighter and less rigid, the need for smoother, flatter surfaces has become more important. Consequently, some aftermarket gasket manufacturers now recommend 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. RESURFACING ALUMINUM CYLINDER HEADS For aluminum heads, the numbers are even lower. The typical recommendation today for an aluminum head on an OHC bimetal engine is a surface finish of 30 to 60 RA, with the preferred range being from 50 to 60 RA. Smoothness has become a major issue with bimetal engines because the difference in thermal expansion rates between an aluminum head and cast iron block creates a tremendous amount of sideways shearing force and scrubbing action on the head gasket. If the surface finish is too rough (more than about 60 RA), the metal will bite into the gasket and pull it sideways as the head expands and contracts. The cumulative effect over time can cause a delaminating effect in the gasket, literally tearing it apart causing it to leak and fail. Even lower numbers may be required for certain engine applications. General Motors, for example, specifies a surface finish of 27 to 47 RA for its 2.3L Quad Four engine when the OEM replacement gasket is used. Some aftermarket gaskets can handle a rougher finish on these engines, but it all depends on the design of the gasket. Even so, smoother is definitely better on these engines. Ford specifies an unusually smooth surface finish for its 4.6L V8 engine. This engine, like a growing number of late model Japanese engines, uses a multilayer steel (MLS) head gasket. This type of laminated steel gasket is extremely durable because the multiple layers of metal (each of which is coated with a thin layer of rubber) prevents the gasket from losing torque over time. The design also reduces the amount of torque that is required on the head bolts to seal the gasket, which in turn reduces cylinder bore distortion for better combustion sealing and reduced blowby. The recommended surface finish for the OEM gasket on the 4.6L V8 is 8 to 15 RA! Many late model engines require a mirror-like super smooth surface finish to seal the head gasket properly. MIRROR-LIKE CYLINDER HEAD SURFACE FINISH To appreciate just how smooth this is, compare the surface finish requirements for the head and block on the Ford 4.6L V8 to those for a typical cylinder bore. Honing with #220 grit stones typically leaves a finish in the 38 to 45 RA range. Honing with #280 grit stones will generally produce a finish of 18 to 25 RA. Honing with #400 grit stones, a finish of 10 to 15 RA can be achieved, and honing with #600 grit stones can push the numbers down to 5 to 10 RA. A pane of window glass, by comparison, measures about 3 to 4 RA.

Honing cylinder bores is obviously an entirely different process than resurfacing cylinder heads or blocks. For one thing, the walls of a cylinder bore must have a certain amount of crosshatch to retain oil so the bores can�t be too smooth. But neither can they be too rough because a rough finish can wear the rings excessively as they seat in. The ideal finish for a cylinder bore is a "plateau" surface where the sharp peaks have been knocked off but the valleys remain. This provides plenty of bearing area to support the rings but also leaves the crosshatch for proper ring lubrication. More importantly, it virtually eliminates ring wear during the initial break-in process because the surface already has the profile of a broken-in cylinder. This kind of surface is produced by going in with a final honing step that uses a fine grit stone (#600) or a flexible abrasive. Although an unusually smooth finish may be required for the Ford 4.6L V8 and certain late model Japanese engines that have MLS head gaskets, smoother is generally better for all engines because it improves cold sealability. One thing you do not want on the surface of the head or block is scratches. Every scratch is a potential leak path along which fluids and pressure can migrate. If a scratch is deep enough, coolant may find its way into the crankcase or cylinders before the engine is fired up. Or, combustion gases may force their way past the gasket into the cooling jacket or an adjacent cylinder eventually causing the gasket to burn out and fail. Either way, it�s bad news. So the best way to avoid cold sealing and durability problems is to take the proper steps when refinishing the head and block to ensure the surface finish is within the recommended limits of the gasket manufacturer and/or original equipment manufacturer. CAN A SURFACE FINISH BE TOO SMOOTH? Can a head or block surface be too smooth? After all, the smoother the surface the better the initial cold seal of the gasket and the less likely you are to have problems with coolant and combustion leaks. Though most gasket manufacturers do not specify a minimum smoothness spec for aluminum heads that have MLS head gaskets, they do recommend a minimum of 30 RA for engines with aluminum or cast iron heads and a nonasbestos or graphite head gasket. The reason for doing so is because soft-faced head gaskets require a certain amount of lateral support from the head and block. When the head is bolted to the block, the metal on both sides bites into the gasket to help hold it in place. Too much bite when the head is aluminum and the block is cast iron creates sideways shearing forces that result from the expansion and contraction of the aluminum head. Yet a certain amount of support is necessary to keep the combustion gases in the cylinders from distorting the gasket and blowing past it. This is especially critical in the areas with narrow lands and between the head bolts where there is nothing to keep the gasket in place but the gasket itself. In high output or heavy-duty applications where combustion pressures exerts even greater force against the head gasket, a surface finish that is below the minimum smoothness spec might lead to premature gasket failure. HOW TO MEASURE SURFACE FINISH Most engine rebuilders have no idea how smooth a surface finish they are actually putting on the heads and blocks they resurface. As long as the head gasket seals initially and does not fail during the warranty period, they assume they are doing everything right, and maybe they are. Then again, maybe they are not. They may not be seeing the long term consequences of their actions because the head gasket does not fail until the engine is out of warranty. In many instances, a premature head gasket failure because of leakage or burn through can be traced back to the surface finish that was put on the head and block.

Of course, there may be other factors involved, too, like engine overheating, detonation, etc., or mistakes that were made during engine assembly such as not torquing the head bolts properly. But if a head gasket fails, there is always a reason why. Paying closer attention to the details of resurfacing, therefore, can help eliminate this as a potential source of trouble. Most people can't look at a finish and tell if it is 20 RA or 60 RA. So the only way to know if a resurfaced head or block is within the proper range is to measure it. Judging surface finishes by appearance alone, or even feel, is not a very accurate means of controlling quality. Most people can't tell the difference between an acceptable finish and one that is not unless it is really bad. Even then, their judgment may be shaded by outdated notions of what is "good enough" and what is not. If they are resurfacing aluminum heads the same way they have always done cast iron heads, chances are the surface is too rough. One way to judge surface finishes is with an inexpensive comparator gauge. Available from at least one aftermarket gasket manufacturer as well as various tool suppliers, a comparator gauge has sample patches etched on a metal plate that indicate the different surface finish ranges. By placing the comparator gauge next to a resurfaced head and visually comparing and feeling the sample patches on the gauge to the head you can get an approximation of whether or not you are in the correct range. But it is not very exact, and it is often hard to tell just how close you actually are to a given range of numbers. A far better method of judging the quality of your work is to actually measure the surface finish with an electronic profilometer. The least expensive profilometers start at around $1500, and go up from there depending on the features they offer. A profilometer eliminates any uncertainty or guesswork about surface finish by accurately measuring the texture, peaks and valleys on the surface. PROFILOMETER A profilometer measures surface finish by dragging a diamond-tipped stylus across the metal. This reveals the roughness of the surface …

Fonte: AA1Car.com