Since 2007, Harley-Davidson three-piece crankshafts (sprocket flywheel, pinion flywheel and crank pin) have been assembled… robotically. This is a first. A first that, coincidentally, seems to come at a time when so-called “scissoring” has become an issue. (At least with hot-rodders on the Internet!)
Scissoring amounts to a situation wherein the press fit on the crank pin slips, allowing the flywheels to go out of alignment and develop a “wobble,” causing excessive oscillation (runout) on the pinion and sprocket shafts, damaging the cam plate, oil pump, cam chain tensioner and main bearings. Understandably, this phenomenon has a tendency to make us think that the Harley TC crankshaft is a piece of junk. Well, quite simply, this is B.S.! Fact is, these cranks are, in many ways, the best ever fitted to a production Harley engine. As delivered unto a set of cases on the production line they are—in fact—straighter than a string! What’s weird is H-D’s own specification for “acceptable” runout on the ends of these cranks once they are in the crankcases. To anyone who’s familiar with traditional tolerances of 0.001″–0.0015″, a factory spec for late-model (six-speed) Twinkies of a whopping 0.010″ (measured in the cases on the right-side shaft) seems unacceptably sloppy. It’s not! But hang on, ‘cause we’re about to find out just what might be.
Straight and “true” is one thing… “balanced” is quite another. Historically, properly balancing a Harley flywheel assembly involved everything from drilling holes on the rim to inserting plugs to grinding certain areas—and by all means weighing rods, piston and ring assemblies—then factoring the percentage of balance (usually in the range of 55–60 percent of “flailing mass”) to achieve smooth running within a certain rpm range. For about a hundred reasons I haven’t time or space to get into here, this was frequently fine art by skilled practitioners and always and unavoidably a compromise. You simply cannot find the perfect balance for a 45-degree V-Twin at all revs for all purposes. So the best cranks were the ones best balanced for a specific task. In old-time, solid-mount chassis, getting this right was of paramount importance. With the advent of rubber mounts and counter-balancers… not so much. Balancing has become one of those obsolete issues and more slop isn’t just tolerable, but accepted fact. On the other hand, because of that simple fact, here’s how it’s done today, ironically with invariable “precision” as the watchword.
The engineers at Harley-Davidson have figured out (and designed in) where the heavy spot in the flywheel should be located and have built a casting mold that creates a cavity within the casting of each flywheel, which should bring the crank assembly in balance (within tolerance) automatically… in theory. In practice, this method doesn’t allow for much core shift in the forging/casting of the flywheels. Yet, core shift occurs. When this happens, it changes the location and depth of the cavity, allowing individual flywheel halves to be out of balance. If the cavity happens to be closer to the mainshaft, it leaves too little material to be properly counterbalanced. Or, if further away, perhaps not enough material can be removed from the flywheel. Variations in depth and/or location of this cavity can also interfere with a proper degree of rotation from the centerline of the crank pin. Slop like this on each flywheel half can cause the crankshaft assembly to be either overbalanced or underbalanced—but not by much.
Twist and shout
The Harley-Davidson crankshaft also has an enormous amount of “torsional vibration,” which happens each time the air/fuel mixture inside the combustion chamber is ignited. The rapid rise in cylinder pressure applied to the top of the piston becomes the force that is applied to the crankshaft through the connecting rod to make the crankshaft rotate. The pulse from each cylinder firing is like a huge hammer blow that hits with such intensity that it actually deflects and twists the crankshaft. This twisting action and the resulting rebound (as the crankshaft snaps back in the opposite direction) are collectively known as torsional vibration. If not adequately controlled, it will cause main bearing failure, main shaft bending, main shaft twisting, crankshaft shifting and possible crankshaft breakage. Harley cranks, with their one-rod journal, have main bearings located a little more than two inches away from the center of this behavior. Twinkies also have rod angles that create serious leverage where the crank pin connects to each flywheel half. So, try to visualize what’s really going on as the engine runs. Basically the crank flexes, but in a way that (when applied to old British parallel twins) was referred to as a “jump rope” fashion. Meaning, as the force comes thundering down in the middle, the ends try to move up in an equal and opposite reaction. Add the natural “spring” effect inherent in metal, and picture the dynamics when the pistons move back up and all those enormous loads are reversed. You can imagine how hard this behavior is on both the crank and the main bearings that support it. Lugging an engine makes this much worse because, rather than a relatively smooth series of speedy fluctuating rotations, this phenomenon becomes a series of near stops and starts, which hammer crank and bearings all the more!
Get your bearings
The other thing that has changed in the construction of Twin Cam engines is the type of main bearings used. From 2003 on, the tried (and very true) double-tapered Timken-type left bearing has been dropped in favor of an INA-type roller bearing. This was done to save time on the assembly line. Seems they simply slide a crank into a roller bearing as the engine is assembled, rather than take the time to install and “set up” a Timken. I mean, assuming that you can slip a shaft into a bearing, you must acknowledge that there’s plenty of slop in the arrangement… right? Then there’s the fact that both crank bearings are roller-type, with the same amount of slop in them. Seems to me that means, as the crank tries to roll complete with high-frequency flex (as we’ve just discussed), it has to coexist with a whole lotta rock, as well! (I think it’s worth noting that H-D sells both a heavy-duty roller—the so-called “Lefty” bearing—and a means to convert back to a Timken for high-performance applications. Why do you think that would be, if the standard setup isn’t a little bit sloppy?)
As we try to paint a mental picture of all this motion and commotion, it pays to encompass the differences in function of the ends of the crank, as well. We know well enough what happens in the middle with those rods and pistons flying up and down, banging on the pin the whole time, but the other functions are equally important… and stressful. Everybody worries about what might happen to the oil pump and cam plate if there’s a problem on that end of the crankshaft. But what about the other end… the one that carries the load back to the clutch and transmission? The pinion shaft only has a light load by comparison, and that load varies little. It’s actually the driveshaft that has its work cut out for it—ironically cutting it no slack… or slop.
I know the hot-rod heroes among us figure that you need to weld the crank (whether cast or forged), upgrade the bearings and more to ensure the best odds against the dreaded scissoring. All of which is insurance for sure, yet it might still occur… ask ’em and they’ll tell ya. A weld with 20-thousandths penetration on a crank cheek that’s almost an inch thick and shakin’ back the whole time (to me) is like a glued-together house in an earthquake—not sure I’d trust it in extremes.
On the other hand, it might just be that Harley-Davidson has already offered the solution, or at least a valuable aid to correcting the overall source of the issue, in the form of a new Screamin’ Eagle part (#36500020)—namely, a manual primary chain tensioner. Yup. Seems to me, once again, the automatic tensioner supplied as standard equipment on all six-speed Big Twins since 2007 is likely the true culprit or at least, for sure, a major contributor! (Never mind that I’m already on record as being highly suspicious of the stock tensioner’s role in transmission bearing failures.)
Slop the hogs?
As suggested, the trend for Twin Cam engines has been to increase tolerances (add slop) everywhere but in the primary drive—interestingly the most highly, variably and intermittently-loaded area of the drivetrain. Looser main bearings, less tension on the bearing-less cam/oil pump drive—yet running in conjunction with a tensioner that can only get tighter and has a rep for getting too tight in many an instance? WTF?
Stands to reason that holding tightly onto one end of a flailing, rotating assemblage would tend to make the other end move in ever-larger ellipses. Ever mess with one of those toy gyroscopes? Notice that while it spins and one end is “located” on a surface, the other end is moving in dizzy circles? And what happens when the gyro slows? The other end slips around, too. Hold a spinning gyro pinched between your fingers and feel the buzz as it struggles to escape your grasp… and there are no pistons and rods moving up and down while that “flywheel” spins! The tighter you hold, the more pronounced the effect. I believe it’s much the same with a crankshaft that’s allowed plenty of movement everywhere along its length… except on the end that has to transfer power to the gearbox and clutch. In that scenario, something’s got to give—and it just might be the crankpin slipping… Huh?
Chain drive for primary transmission of power is archaic anyway. It was useful when Harley had separate casings for both engine and transmission, but those days are gone. Yet chains remain! In a “unit” or even semi-unit construction—where shaft centers are fixed—there are better ways. We are all familiar with primary belts and almost all other motorcycles actually use gears for primary drives. The benefit is a much more consistent and invariable loading of all components involved… not the least of which is the crank!
However, overloading the drive side was never an issue with manual chain tensioners. Surprise! That’s because they left enough (needed) slop in the chain to allow the crank bearings and flywheels to merrily roll along… and keep rolling! The bottom(end) line here is this; installing the new SE tensioner and manually checking for proper tension in the primary chain cannot help but make the crankshaft more reliable and less likely to scissor.