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Motorhead Memo: A gas, gas, gasket

By Kip Woodring

Truth be told, it’s been quite a while since Harleys really marked their spot. That singular improvement in our collective circumstance is really down to a couple of signs of progress, which we ignore at our own peril. Namely, improved design, better surface treatments between mating components and mainly gaskets—gaskets we pretty much takefor granted, though our grandparents would’ve marveled at them.

Ever since the internal combustion engine was invented, gaskets have undergone much experimentation for the most efficient and effective sealing method in an attempt to keep fluids in and dust, dirt and contaminants out. Through years of research, many different materials and designs have been created in attempts to develop suitable “packing” devices for all sorts of applications all over engines, transmissions, and drivetrains. Suitable materials, then as now, must be appropriate for the job, prove durable, provide effective sealing, and must be compatible with the fluid used in the system.

The packing devices used in fluid-containing power systems and components are divided into two general classes: static and dynamic. The static type is usually referred to as a gasket. One main quality of a gasket is to incorporate a material that can flow into surface irregularities of mating areas that require sealing. To do this effectively, the gasket material must be under pressure, meaning that the jointed surfaces must be tightly bolted or otherwise held together.

The dynamic type, commonly referred to as a packing or seal, is used between two parts that move in relation to each other. Certain types of seals (for example, the O-ring) may be used either as a gasket or a seal, but don’t let that confuse you, we’ll get to it.

Head-ache!
The engine gasket with the most difficult job is the one between the heads and the cylinders, and things are worse today than ever before because of higher temperatures, increased pressures, and persistent industry-wide use of only four “through” head bolts on ever-larger bores.

When head gaskets first appeared to facilitate the frequent servicing needed by early engines, they were made with traditional copper and brass. Next came various other metals and asbestos in the 1950s. Wasn’t long after that synthetics came into play, with things like Teflon incorporated into gaskets from the ’60s though the ’80s. Composite metals and impregnated fiber or graphite composites likewise found their uses in automotive head gaskets and proved their reliability well enough, in terms of durability and flexibility—until the ’90s, that is. Things like ultrathin-wall CAM precision castings, composite engine components, and the renaissance of horsepower meant those various head gasket materials were toppled with the development of the so-called multi-layer steel (MLS) type of head gasket. Since this MLS technology proved more versatile than anything that came before, it has found its own importance in advancing oil-tight, pressure-tight, incredibly durable service in serious automotive engines for the 21st century. Multi-layered steel head gaskets are used by an estimated 80 percent of modern automotive engines, as well as new Harleys— and that ain’t the half of it!

There are the “bi-metal” issues on older Harleys. Pans and Shovels use aluminum heads on iron barrels. The expansion rate of these two metals is nearly a 2:1 ratio, so as the temperature increases the head expands at twice the rate of the block, wherein these differing expansion rates scrub away at the gasket and cause head warping. That might explain the massive, continuing popularity of “blue” Teflon head gaskets for Pans and Shovels… since the gasket can “slip” against the two dissimilar materials and still hold a seal. True, when these older Big Twins were designed, they got a machined lip/groove setup to help prevent outright catastrophic failures, but the life of a head gasket in that environment ain’t great, period!

Commencing with Evos in ’84 there’s only the gasket (and a couple of O-rings) ’twixt head and barrel, or barrel and cases as far as that goes. The barrel may be alloy on the outside, but there’s a steel liner inside, yet no lip/groove arrangement, only flat surfaces relying on clamp loads and gaskets. So, let’s not presume this new mono-metal world is flat—enough—just yet, OK?

If and when your heads are going to a shop, whether for hop-up or rebuild, cleaning and surface checking should be done there. Otherwise, these critical details are your responsibility. Make sure you remove all of the old gasket, as well as any shellac, carbon, and corrosion. Especially on aluminum, be careful not to make any deep gouges or take off any metal in the process. (I’ve heard from gasket experts that savage/silly cleaning procedures such as using abrasive disks on a drill can cause unsealable unevenness almost instantly.) Check for warping using a straightedge and feeler gauges lengthwise and crosswise. On Evos and Twinkies the traditional rule of thumb says if you can slip a 0.006- inch gauge between the head and the straightedge anywhere over its entire length, the head is too warped to guarantee a decent seal. It’s pretty much the same deal for both the top and bottom (base) of the cylinders as well. Never underestimate the ability of the barrel to be so misshapen as to ruin any chance of a durable seal at the head! There’s also the infamous issue of cylinder base leaks on Evo Big Twins, mostly cured by modern gasket technology, yet still more amenable to long-term sealing integrity when the damn parts involved are flat!

But flatness isn’t the only sealing surface consideration. The level of smoothness or “tooth” is also important. This “surface finish” on the face of the heads and barrels is measured in micro-inches and typically given an RA or RMS rating. As a rule of thumb, with cast-iron components and a conventional head gasket, both surfaces need a finish having a 30 to 110 RA rating, with 60 to 100 RA preferred. With aluminum heads mounted on cast-iron barrels, 30 to 60 RA is recommended, with 50 to 60 RA preferred. If the surface is too rough (say more than 113 RA on an old Iron Sporty), it may be too rough to seal properly and the head gasket will leak. If the surface is too smooth (less than 54 RA on Pans and Shovels), it may not provide enough “grip” to prevent the gasket from flowing or scrubbing. But if you’re working on a late-model Evo or Twinkie with MLS head gaskets (multi-layer steel type, also known as “RCE” for “Rubber Coated Embossed”), follow the gasket maker’s specs. This variety, depending on who makes ’em, may only tolerate a maximum of 30 RA. (Obviously, the smoother the better, and 30 RA is close to polished.)

On older bikes, if the gasket of choice has faces of graphite or a soft synthetic material such as Teflon, or has elastomeric (Print-O-Seal) beads around potential trouble spots, don’t apply any sticky stuff, gasket goo, or “cheap insurance” solutions. It’ll soften and likely ruin that carefully manufactured surface. Thing is, head gaskets are only one relatively minor (though obviously high-profile) arena of innovation in engine sealing.

From paper to poly-somethingorother
Seals, like gaskets, are made of materials that have been carefully chosen or developed for specific applications. These materials include tetrafluoroethylene (TFL, commonly called Teflon), synthetic rubber (elastomers), flouroelastomers (Viton, to most of us), cork, leather, metal, and asbestos (until lately, anyway).

Any Harley rider with a memory that dates back even to Shovelheads remembers cork. Cork has several properties which make it ideally suited as a sealing material in certain applications. The compressibility of cork seals makes them well suited for confined applications in which little or no spread of the material is allowed, like its use in the pushrod tubes. Cork can also be cut to any desired thickness and shape to fit any surface and still provide an excellent seal. The bad news with cork is its tendency to crumble, especially where temperatures might exceed 2,750 degrees. Use it where there is high pressure and/or high flow velocity, and small particles can break off. Modern oiling (and filtration) won’t put up with much of that crap, so these days cork’s use is best limited to inspection covers, especially since modern rubber/composite versions of virtually all critical pre-Twin Cam Harley applications exist today.

Speaking of rubber (the synthetic kind, since natural rubber is pretty useless) a simple rubber O-ring will almost always be the most satisfactory choice of seals in static applications if the fluids, temperatures, pressure, and geometry permit. On the other hand, standard O-rings are not specifically designed as rotary seals. When infrequent rotary motion or low peripheral velocity is involved, you might get away with it (like Harley did on sprocket shaft spacers with the last 4-speed Big Twins) provided there are consistent surface finishes and any eccentricities are accurately controlled. As rotary seals, O-rings perform satisfactorily in only two application areas: (1) low-speed applications where the surface speed of the shaft does not exceed 200 ft/min. And (2) high-speed moderate-pressure applications, between 50 and 800 psi. O-rings can’t compensate for out-of-round, heavily loaded, or eccentrically rotating shafts—high speed ones in particular. For those situations there are those nifty lipped (some doubly) and spring-equipped steel-backed, rubber-coated doodads we think of when the term “oil seal” is invoked! More about “real” oil seals another time. Until then, there are four things you need to know.

Four factors of engine sealing
When sealing any seam, these are critically important:

  1. The condition of the surfaces in terms of straightness, flatness, smoothness, and cleanliness. (Gouges, scratches, and goop, not to mention warping, simply will not do!)
  2. The quality of the gasket design and material. (Do your homework here, especially when it comes to updated solutions to traditional issues on older Harleys. Times have indeed changed!)
  3. The care taken during installation. (Clean, slow and by the book works best. You do have the book, right? Or at least the info the gasket maker supplies?)
  4. The clamping pressure. (Torque wrenches, accurate torque specs, a safe cracker’s “touch” and a load of common sense. Guess which one of these is in shortest supply?)

Keep these in mind when you approach any assembly job. Remember, even if there’s never enough time to do it right, somehow there’s always enough time to do it over!

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