He might be right.....to a point. One can get too smooth of a surface.
Per Larry Carley:
Surface finish
The issue of smoothness is one that is worrisome to rebuilders today. For many years, most aftermarket gasket manufacturers have recommended a head gasket surface finish of 55 to 110 micro-inches RA (roughness average), or 60 to 125 RMS (root mean square). The preferred range has traditionally been 80 to 100 RA. A surface finish on both the block and head that falls within the recommended range should provide good cold sealing and long term durability (assuming everything is assembled correctly and the head bolts are torqued in the proper sequence and to the specified torque).
Recently, though, some gasket manufacturers have changed their recommendations. Engines and castings have become lighter and less rigid. Many cylinder heads are now aluminum rather than cast iron. There have also been changes in head gasket materials and designs. So the recommendations for some engines now require a much smoother surface finish.
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.
For aluminum heads, the numbers are even lower. The typical recommendation today for an aluminum head on an OHC bi-metal engine is a surface finish of 30 to 60 RA, with the preferred range usually being from 50 to 60 RA.
Smoothness has become a major issue with bi-metal engines because the difference in thermal expansion rates between an aluminum head and cast iron block create 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 and causing it to leak and fail.
Even lower numbers are recommended for some engines. General Motors 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 depends on the design of the gasket.
Very smooth surface finishes in the 20 to 30 RA range (or even less) are now required for some Ford engines such as the 4.6L V8 and its V6 modular offshoots. These engines, like a growing number of late model Japanese engines, have multilayer steel (MLS) head gaskets.
These laminated steel gaskets are extremely durable because the multiple layers of metal (each of which is coated with a thin layer of rubber) prevent the gasket from losing torque over time. The design also reduces the amount of torque that’s required on the head bolts to seal the gasket, which in turn reduces cylinder bore distortion for better combustion sealing and reduced blow-by.
When replacing the OEM head gasket on these engines, the surface must be restored to the same smoothness as it had from the factory. Some aftermarket gasket manufacturers are concerned that engine rebuilders and machine shops can achieve the high quality surfaces required for MLS gaskets, so are developing or have developed graphite or composite replacement gaskets for some applications such as the Ford 4.6L V8. These aftermarket replacement gaskets, say the manufacturers, can handle a more "traditional" (rougher) surface finish — but they may not provide the same longevity as the original MLS gasket.
"You don’t really need a 100,000 mile replacement gasket for the aftermarket," said one gasket engineer. "But you do need a gasket that will seal with a typical aftermarket surface finish."
Can the surface finish ever be too smooth? Although very smooth surfaces are required for MLS head gaskets, and smoother is generally better (up to a point) for most gaskets because it improves cold sealability, there is a limit. Most gasket manufacturers say the surface should not be smoother than about 30 RA for most non-asbestos or graphite head gaskets because of these gaskets’ 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. You don’t want too much bite when the head is aluminum and the block is cast iron because of the 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 exert even greater force against the head gasket, a surface finish that’s below the minimum smoothness spec might lead to premature gasket failure.
Surface condition
Another condition that can affect sealing 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. If scratches, pitting or erosion are present on the head and/or block, resurfacing is required to remove the flaws.
Measuring surface finish
The only way to know if the finish you’re producing on a head or block is within specifications is to measure it. The least expensive way to do this is with a simple comparator gauge. Available from some aftermarket gasket suppliers 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’re in the correct range. It’s not exact science, but it can help you determine if you’re reasonably close to the desired finish.
The best way to determine surface finish is to measure it with an electronic surface profilometer. A profilometer measures surface finish by dragging a diamond-tipped stylus across the metal. This reveals the roughness of the surface as well as other important parameters that can tell you even more about what the surface is really like.
RA, or the roughness average, is an arithmetical average of the absolute values within the area sampled by the stylus. It gives you an approximation of surface roughness, plus other information that gives a more accurate picture of the surface’s true condition (things like the maximum peak-to-valley height (called "RY"), and the arithmetic mean of peak-to-valley heights ("RZ").
The ideal sealing surface is one that has neither large peaks or deep valleys, and provides a lot of flat bearing surface to support and seal the gasket.
Resurfacing techniques
The gasket manufacturers don’t care what type of resurfacing techniques or equipment rebuilders 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 belt sanding.
However, for engines with MLS head gaskets, and even some of the aluminum heads that require a smoother than usual finish (like the 2.3L Quad Four), milling or grinding may be the only sure way to achieve the lower RA numbers required. Be warned, though, that the spindle and bearings in older milling and grinding equipment may not be designed to meet the resurfacing requirements of today’s engines. Consequently, some machines that are more than about five years old may not be capable of producing these really smooth finishes. Check with your equipment supplier if you have any doubts.
Grinding
In the past, many heads were resurfaced by wet grinding. It is still used by many shops because grinding can produce a very smooth finish. Silicon carbide grinding wheels and segments are generally recommended for both cast iron and aluminum. Grinding aluminum, however, can be tricky because the stone tends to load up with metal, causing it to overheat and score the surface. So the aluminum needs to be pre-coated with lubricant and resurfaced with plenty of coolant to prevent clogging.
A faster transverse speed may also be needed with aluminum, and the depth of cut limited to no more than .0005" to .001". The machinist should also 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
Many shops today have switched to dry milling because it eliminates the mess and maintenance that go with wet grinding. Milling allows very precise control over stock removal, and it is faster than grinding because more metal can be removed in a single pass, eliminating the need for multiple cuts.
The key to achieving the smoother finishes required by many of today’s aluminum heads is using the right combination of table feed and rpm when milling the head. This requires a variable speed table and/or multi-speed or variable speed milling head. Increasing the rpm of the cutting head and/or slowing down the feed rate produces a smoother finish. One equipment manufacturer, for example, recommends a feed rate of two inches per minute at 1,000 rpm on a milling machine with a two-bladed cutter to achieve a surface finish of 12 RA.
Carbide or PCD (polycrystaline diamond) tooling is usually recommended for milling aluminum, while carbide or CBN (cubic boron nitride) is recommended for milling cast iron.
Belt sanding (Not recommended for modern gasket sealing)
Belt sanding is a quick way to resurface a head because the head doesn’t have to be mounted in a fixture. But it isn’t as precise as milling or grinding. Results often depend on the expertise, experience and skill of the individual operator. The amount of downward pressure exerted by the operator on the head, the weight of the head itself, how the head is positioned on the sander and the condition of the belt can all affect the results. Consequently, some say belt sanding is better suited for clean-up work or resurfacing hard-to-fixture parts like manifolds and timing covers rather than resurfacing heads on late model engines.
For resurfacing aluminum, silicon carbide belts are generally recommended. Silicon carbide can also be used for cast iron, as can aluminum oxide or other ceramic based abrasives. Either #40 or #80 grit belts can be used with aluminum, but #80 grit is the preferred choice and should be used with no downward pressure on the head.
It’s also important to replace the belt regularly because a dull belt can overheat the head causing warpage and an uneven surface finish.
In sum, today’s lighter weight, bi-metallic engine designs usually demand stricter control of surface finish requirements in order to seal properly. New replacement gasket designs, along with the right equipment, is necessary to ensure your customer is happy with the performance of their rebuilt engine.
http://www.enginebuildermag.com/2013/04/performance-gaskets-surface-finishes/
