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Smooth Moves: Today's Surfaces Often Demand Different Finishes

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What Gasket Suppliers Say
Bill McKnight of Mahle/Victor-Reinz Gaskets said the surface finish requirements for modern MLS gaskets are not are critical as they once were. “You had some manufacturers saying we had to produce a 8 to 10 Ra finish to seal the head gasket. But the coatings on today’s aftermarket MLS gaskets can handle anything in the 40 to 70 Ra range with no problems.”

McKnight said the type of process or equipment used to surface a head or block doesn’t matter as long as it leaves a good finish. “You can dry mill or wet grind or sand and get good results when the resurfacing is done right.”

Jim Daigle of Fel-Pro/Federal Mogul also said today’s aftermarket MLS head gaskets don’t require the super smooth finishes originally specified by the vehicle manufacturers. “Our blue coated MLS gaskets can typically handle surface finishes from 70 to 80 Ra, while our black coating is typically designed for surface finishes of 30 Ra or less. We put our surface finish recommendations right on our packaging because it can vary from one application to another.

“Smoother is better for many applications, but is not absolutely necessary. If you’re building a NASCAR motor that’s running 280 to 300 degrees F, a surface finish of 30 Ra or less is better and recommended. Even though some of our Performance MLS gasket coatings can accommodate above 30 Ra we really encourage a 30 Ra target (or less) for any Performance MLS application. But for a typical street or strip application, we’ll use a gasket coating that is more conformable to handle a more traditional surface finish.”

One point Daigle emphasized with respect to surface finish is the need to de-emphasize Ra numbers and focus more on Rz. Roughness Average (Ra) can actually have a wide variance across a given surface profile. Rz, which is the average difference between the peak height and valley depth, is actually a much more accurate representation of true surface topography.

He said that most Asian vehicle manufacturers don’t even talk about Ra anymore, referring instead to Rz values for surface finish.
Profilometers can usually provide a range of surface parameters, including Rz (which is roughly 6X the Ra value in most conversion tables). Of course, many shops don’t own a basic hand held profilometer, let alone know how to use one. And those that do may only use their profilometer to check surface finishes if there’s a problem. The rest of the time, it stays in the box.

A profilometer is a precision instrument that typically costs $800 to $1800 for a basic hand held unit, or as much as $100,000 for a sophisticated lab model. The hand held units drag a diamond tipped stylus across the metal to measure the microscopic peaks and valleys on the surface. It then displays the various values and calculates an Ra number for the surface finish.

One dimension simple hand held profilometers cannot measure is waviness. It takes megabuck lab equipment to do that. But waviness is another critical dimension that can affect head gasket sealing, too. Waviness problems can be caused by vibrations and a lack of rigidity in milling equipment.

According to Fel-Pro, the recommended surface finishes for various engine and gasket applications are as follows:

• Cast iron and aluminum engines assembled with Permatorque MLS head gaskets should have a surface finish of no more than 80 Ra (480 Rz) or less (as compared to 30 Ra or 180 Rz for a typical original equipment MLS head gasket).

• Cast iron or aluminum cylinder heads and blocks assembled with conventional steel/fiber composite head gaskets or expanded graphite head gaskets should have a surface finish no smoother than 40 Ra (240 Rz) and no rougher than 100 Ra (600 Rz). Rougher surfaces limit gasket conformance, while smoother surfaces increase the tendency for gaskets to flow, reducing the gaskets blow out resistance. The optimum recommended surface finish for these applications is 60 to 80 Ra (360 to 480 Rz).

• The maximum amount of out-of-flat should not exceed .001˝ within three inches in any direction. For four-cylinder and V8 engines, the maximum allowable out-of-flat specification for the cylinder head and block deck surfaces is .004˝ lengthwise and .002˝ sideways. For V6 and three-cylinder engines, .003˝ lengthwise and .002˝ sideways. For in-line five- and six-cylinder engines, .006˝ lengthwise and .002˝ sideways.

• Waviness (which requires special “skidless” lab equipment to measure, should not exceed a waviness height (Wt) of .0008˝ with conventional composite or graphite head gaskets, and no more than .0004˝ with MLS head gaskets.

Surfacing Tips
The quality and smoothness of the surface finish requires using the correct feed rate and speed for the type of tool bit. This, in turn, will vary depending on the diameter of the cutter head.

To achieve the best possible finish, use a higher spindle speed and lower table feed rate with a very shallow cut on the final pass (less than .001˝).

If you are using a carbide insert to refinish a cast iron head, the spindle rpm required will typically be about 140 rpm for an 11-inch cutter, 120 rpm for a 13-inch cutter or 110 rpm for a 14-inch cutter.
With CBN or PCD inserts, the recommended spindle speeds are much higher: 1040 rpm for a 11-inch cutter, 880 rpm for 13-inch cutter, or 720 rpm for a 14-inch cutter. If the head or block being resurfaced is harder, high silicon content alloy, the speeds need to be slowed down a bit: 690 rpm for a 11-inch cutter, 580 rpm for a 13-inch cutter or 540 rpm for a 14-inch cutter.

With a single CBN or PCD insert cutter spinning at 1,000 to 1,500 rpm, the feed rate should probably be less than two inches per minute on the final cut to achieve a surface finish in the low teens.

Cylinder Bore Finishes
The surface finish in the cylinder bores is just as important for proper ring sealing as the surface finish is on the block and heads for proper head gasket sealing.

Regardless of what type of rings or cylinder liners are used in a block, rings usually seat best and last the longest when the cylinder bores are given a plateau finish. A plateau finish essentially duplicates a “broken-in” bore finish, so there is less scrubbing and wear on the rings when the engine is assembled. What’s more, if the surface is finished correctly it will provide plenty of flat, smooth bearing surface to support the rings while also retaining oil in the crosshatch valleys to lubricate the rings.

The only exception to this is in motors where there is a lot of bore distortion. If the bores go out of round when the head bolts are torqued down, the rings may not seat as well allowing increased blowby and oil consumption. Thinner rings that can conform to the bore will work better in these kind of applications, but it’s also a good idea to use torque plates when honing when honing the bores to simulate the distortion that occurs when the cylinder heads are installed. The other option is to go with a slightly rougher “peaked” finish to seat the rings.

Most ring manufacturers recommend using a two- or three-step honing procedure to achieve a plateau finish. First, rough hone to within .003˝ of final bore size to leave enough undisturbed metal for finish honing. For plain cast iron or chrome rings in a stock, street performance or dirt track motor, hone with #220 grit silicon carbide stones (or #280 to #400 diamond stones) to within .0005˝ of final size. Then finish the bores with a few strokes using an abrasive nylon bristle plateau honing tool, cork stones or a flexible abrasive brush.

For moly faced rings in a street performance, drag or circle track motor, hone with a conventional #280 grit silicon carbide vitrified abrasive, then finish by briefly honing to final size with a #400 grit vitrified stone or #600 grit diamond stone (or higher), plateau honing tool, cork stones or a brush.

For stock and street performance engines with moly rings, an average surface finish of 15 to 20 Ra is typically recommended. for higher classes of racing, you can go a little smoother provided you don’t glaze the cylinders.

For moly or nitrided rings in a performance motor, hone with #320 or #400 vitrified stones, and finish with #600 stones, cork stones, a plateau honing tool or brush.

If the cylinders are rough honed with diamond, they can be finish honed with a finer grit diamond, a fine grit vitrified abrasive or a plateau honing tool or brush. Because diamond is a harder material and wears more slowly than conventional abrasives, it cuts differently and may require more honing pressure. But many newer diamond stones now use a more friable bond that stays sharp and doesn’t load up, allowing the stones to cut smoother and leave a rounder, smoother bore finish.

When using diamond honing stones instead of vitrified abrasives, you generally have to use a higher number grit to achieve the same Ra (roughness average) surface finish. For example, if you have been using #220 grit conventional stones to finish cylinders for plain cast iron or chrome rings, the equivalent diamond stones might be a #280 to #325 grit. If you have been using #280 grit conventional stones to hone for moly rings, the diamond equivalent might be #400 to #550 grit stones. The actual numbers will vary somewhat depending on the brand and grade of the stones.
Bristle style soft hones (plateau honing tools) have mono-filament strands that are extrude molded with a fine abrasive material embedded in the strands. The filaments are mounted in different types of holders for use with portable or automatic honing equipment. Another type of brush uses molded abrasive balls that are mounted on flexible metal shafts so the balls can easily conform to the surface. Brushing helps sweep away torn and folded metal on the surface while removing many of the sharp peaks to make the surface smoother.

As with any type of machine shop equipment, proper technique is required to do the job right with these tools, so be sure to get  the necessary instruction from your supplier.

With the right plateau honing techniques, you should be able to get the surface down to an average roughness of 8 to 12 Ra or less, with RPK (relative peak height) numbers in the 5 to 15 range, and RVK (relative valley depth) numbers in the 15 to 30 range. These numbers are meaningless unless you have a surface profilometer that can measure them (which a growing number of shops now have).

When finishing a performance block with nickel silicon-carbide liners, the microscopic pores in the coasting do an excellent job of retaining oil for the rings. Consequently, the bore can be finished to a super smooth finish of 4 to 6 Ra or less to reduce friction even more. Such low numbers would be too smooth for grey cast iron and would likely starve the rings for proper lubrication.

Dennis Westhoff of Sunnen cautions that honing coated cylinders is not the same as honing conventional cast iron cylinders. “Engine manufacturers and racers are developing new thermal spray coatings for cylinder walls that contain a mix of ceramics and other materials,” said Westhoff. “We are working with these people to develop a database of honing procedures that can achieve the best surface finish. The porosity in many of these coatings retains oil quite well, so it is usually possible to go with a much smoother plateau finish in the bores.” Westhoff’s advice for engine builders who may be working with coated cylinders is to find out what type of coating is being used, then call the machine supplier to find out what combination of honing stones and honing procedures will produce the best finish.

For contact information for suppliers of surfacing machines and abrasives, as well as gasket manufacturers, click on our online Buyers Guide tab.