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Motorhead Memo: Trouble-free motorcycle transmission tips

By Kip Woodring


It was once an article of faith, a basic tenet of the brand, a routine ritual, a duty and a necessity. Then the factory started using various “threadlockers” on almost all critical fasteners and we forgot. That can be a bad thing.

I’m talking about the fundamental task of checking and rechecking for tight nuts and bolts on your Harley. Look, I don’t really give a damn that you take it in for service and “they” are supposed to do that for you. It doesn’t matter that most hardware on modern Harleys stays put pretty well without a lot of attention. This simple imperative remains: it falls to you, the owner, to stay on top of this just like checking oil levels and tire pressure before every ride. You see, to this very day, the fundamental nature of Big Twin vibration and loads can cause fasteners to come loose, break or fall out, rubber-mounted or counter-balanced engines notwithstanding! You’ll find that truth mostly evident in lost seat screws and loose exhaust bolts these days. Therein lies a tale.

When the six-speed transmission was introduced on Dynas back in 2006, the factory initially instituted special service policies to monitor and minimize any potential problems in the field. Perhaps six months later, things seemed just fine and the “special” reverted to ordinary. Now all Big Twins are equipped with this transmission but displacement and torque have risen considerably since the days of 88″ Dyna Glides. You may be aware that one of the design features of these gearboxes is a “reverse” helical gear pattern, which, when engaged, quite simply pushes pretty damned hard on the trap door on the right end of the box. You might also be conscious of the fact that the exhaust system bolts up to said trap door. The loading and unloading of the trap door, during routine riding, coupled with exhaust “shakes” (especially on rubber-mount models, like Dynas and Tourers) has the net effect of trying desperately to loosen the fasteners that hold that door. This phenomenon is assisted further by typical aftermarket exhaust installations—which often do not utilize fasteners with threadlocker, and/or reuse the “one-time use” Harley muffler clamps! If you do not routinely check the torque and tightness of these critical fasteners—most especially the ones that attach the exhaust to the trap door—it can be a recipe for disaster. First the screws work loose, then the leaks start, then the trap door cracks, then the bearings and shafts inside the gearbox try to get anything but parallel and there ya go—busted box! It’s a classic version of the old saw, “For want of a nail, the shoe was lost,” only in this case (literally) it’s a couple of screws that were allowed to come loose. The moral of the story? Keep your trap shut tight and it’s all good—for the bearings and gears within.

Maybe not the cleverest segue into more conversation about that other aspect of the helical gear business (its big push) and the beneficial effects of superior gear oil, but hey, we’re here so why not?

But first—consider a few more basics regarding gears, bearings and boxes. New surface-hardening techniques and metallurgy have enabled the manufacture of smaller gears for a given horsepower rating. However, these changes also result in much more aggressive surface loading, and the thickness of these super-hard materials is often razor-thin. These harder surfaces are more resistant to particle-induced sliding wear, but they are highly susceptible to particle-induced rolling wear, which occurs at the pitch line of the gear tooth where load is transferred. When a particle is squeezed in the load zone, it can fatigue the hardened material, resulting in a spalling (spiked) metal deformation or pitting (dents), and/or a combination producing a proud area around a dent’s crater. The plastic deformation that occurs where a surface is dented can dramatically alter the material’s physical properties, making it more susceptible to wear and fatigue. Making matters worse, new gearbox designs tend to run hot, increasing the risk to the lubricant—meaning there are two essential proactive aspects to lubrication management: The oil must be right for the job, and must be free of contaminants. Put another way, “wear and tear” is an expression frequently used to describe the aging of any mechanical system, but inadvertent use and abuse might be more correct since whatever you wanna call it, it commonly occurs as a result of the contamination and degradation of the lubricant that can be reduced dramatically as a result of some basic measures to protect the gearbox from the wear, tear or abuse it generally receives. Here are some important ones:

The viscosity level of gear oil is the most influential property to be considered in protection of internal metal parts. Most emphatically, not that gibberish about weight, but rather the centistokes values at the two commonly measured temperatures—namely 40 degrees Centigrade/100 degrees Fahrenheit ambient (if you will) and 100 degrees Centigrade/212 degrees Fahrenheit (so-called) operating temperature. There are so many choices that I can’t begin to cover them all in this limited space. What you can do is search for this info in the MSDS sheets available for most any fluid you might be considering—or call the maker. Harley, in their normal secretive fashion, doesn’t offer enough specific data on their own recommended gear oils. Frustrating, but at least there’s this: SYN3 and Formula+ are both rated at 159 CST (at 40 degrees C). Sadly, one must extrapolate the rating at 100 degrees C, using that bit about the percentage of viscosity change at operating temperatures… meaning factory gear lube is going to range between 18.8 CST and 26 CST… hot!

Our transmissions need gear oils with precise and capable viscosity levels to avoid premature part failure and provide optimum protection. If gear oils are too thin, they will not protect the faces of gear teeth from metal-on-metal wear. If gear oils are too thick, they will create viscous drag, energy inefficiencies and poor performance during component operation—not to mention adverse effects on bearings. Synthetic gear oils have precise viscosity levels because they are created in a laboratory, by engineers, to supply specific chemical properties and molecular uniformity. They hold their viscosity levels over time far better than traditional gear oils as well. Viscosity is what allows gear oils to protect gear teeth faces from wear during operation. Viscous fluids stick to gears during their many revolutions and create a thin layer of protection between gear teeth. Conventional petroleum-based fluids break down quickly under extreme temperature spikes, losing their viscosity and strength between drain intervals. This molecular breakdown and viscosity loss can be very detrimental to gear teeth when under heavy loads. Regardless of your opinion of synthetics as motor oils, there is virtually no argument as to their superiority in gear lube applications. They run cooler, offer better protection and can actually increase efficiency as much as four percent. The only trick is to find lubes of the proper viscosity that are not just labeled “full synthetic,” but in reality are 100 percent synthetic—such as Redline Shock Proof Heavy, Schaeffer, Torco, Mobil, B&M and a scant few others.

OK—that’s all well and good for Big Twins with separate gearboxes, but what about Sportsters/Buells, which share gear oil with the clutch? Well, personally, I’ve found that Big Twins are perfectly happy with gear oil viscosities closer to 30–35 CST @ 100C in the first place… meaning that the recommended “standard” rating is a little on the light side. Since Harley figures their own lubricants should be interchangeable between Big Twins and Sportys, this is understandable. This light rating is, however, pretty much dead-on for XLs and Buells, precisely because there’s a clutch involved. So, in a sense one can benefit from a mindset of “high-performance automatic transmission fluid” perspective when selecting gear lubes for these bikes. ATF in cars always involves friction materials as well as gears and bearings—same as Sportsters—so it stands to reason there are formulas out there that can improve not only gear protection but clutch performance as well. A couple that come to mind are B&M Trick Shift synthetic and Redline Shock Proof Light, but in the end it’s up to the individual rider to determine what’s best for his own machine and the uses and abuses—not to mention wear and tear—it will be subjected to.



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