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Motorhead Memo: Getting cranky

By Kip Woodring

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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!)

Whether cast (stock) or forged (Screamin’ Eagle), modern H-D crankshafts are three-piece affairs pressed together at something like 20 tons of force at an interference fit of five- to seven-thousandths of an inch. This should be more than enough strength for any realistic use you can think of, and rumor has it they have even been recently improved. That said, somehow a few are known to have shifted the individual flywheel halves where they fit on the crankpin—a phenomenon known as “scissoring.” No one seems to know the true cause, but opinions abound. Of these, the only one that’s invalid is that the cranks are “junk.” In fact, they are the best ever… so something else is responsible for the problem.

Whether cast (stock) or forged (Screamin’ Eagle), modern H-D crankshafts are three-piece affairs pressed together at something like 20 tons of force at an interference fit of five- to seven-thousandths of an inch. This should be more than enough strength for any realistic use you can think of, and rumor has it they have even been recently improved. That said, somehow a few are known to have shifted the individual flywheel halves where they fit on the crankpin—a phenomenon known as “scissoring.” No one seems to know the true cause, but opinions abound. Of these, the only one that’s invalid is that the cranks are “junk.” In fact, they are the best ever… so something else is responsible for the problem.

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.

 

Balancing act

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.

Not knowing for sure leads to “band-aid” operations like welding, as seen here, and some outfits have even gone so far as to change the shape of the pins where they fit to the flywheels from round to a hexagon. Not that these treatments don’t help! It’s just that even up-rated, “modded” cranks and better bearings offer no guarantee of a permanent fix, because there just might be more to it.

Not knowing for sure leads to “band-aid” operations like welding, as seen here, and some outfits have even gone so far as to change the shape of the pins where they fit to the flywheels from round to a hexagon. Not that these treatments don’t help! It’s just that even up-rated, “modded” cranks and better bearings offer no guarantee of a permanent fix, because there just might be more to it.

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.

 

For high-performance operations, Harley has two offerings: the heavy-duty “Lefty” bearing (#24605-07) or reversion to a Timken setup (#9028 seen on the left) and 34822-08 plus appropriate sprocket spacers. The Lefty is a stouter version of the standard roller, but the Timken is old school in that it offers both superior support and firmly locates the crank—removing a notable degree of slop in the crankshaft’s tendency to flop and flex. But is it a cure?

For high-performance operations, Harley has two offerings: the heavy-duty “Lefty” bearing (#24605-07) or reversion to a Timken setup (#9028 seen on the left) and 34822-08 plus appropriate sprocket spacers. The Lefty is a stouter version of the standard roller, but the Timken is old school in that it offers both superior support and firmly locates the crank—removing a notable degree of slop in the crankshaft’s tendency to flop and flex. But is it a cure?

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

“You can’t wear out an Indian Scout or its brother the Indian Chief. They’re built like rocks to take hard knocks, it’s the Harleys that cause the grief.” This ditty came to pass as far back as 1920 and the introduction of Charles B. Franklin’s exemplary design for the new Indian Scout, which employed gears for both primary power transmission and camshaft/oil pump/generator drive. Harleys of the period still had separate (and movable) cases for engine and transmission—so they had to use chains for primary drive. The Scout was of so-called semi-unit construction, which featured (among other things) fixed, aligned and immobile shafts from front to back, lending itself nicely to gear drive because there was far less slop to deal with. Perhaps noisier, but clearly more trouble-free, there’s no reason Harley couldn’t do the same with modern Big Twins and Sportsters… ‘cept chain drive is traditional, quieter and cheaper.

“You can’t wear out an Indian Scout or its brother the Indian Chief. They’re built like rocks to take hard knocks, it’s the Harleys that cause the grief.” This ditty came to pass as far back as 1920 and the introduction of Charles B. Franklin’s exemplary design for the new Indian Scout, which employed gears for both primary power transmission and camshaft/oil pump/generator drive. Harleys of the period still had separate (and movable) cases for engine and transmission—so they had to use chains for primary drive. The Scout was of so-called semi-unit construction, which featured (among other things) fixed, aligned and immobile shafts from front to back, lending itself nicely to gear drive because there was far less slop to deal with. Perhaps noisier, but clearly more trouble-free, there’s no reason Harley couldn’t do the same with modern Big Twins and Sportsters… ‘cept chain drive is traditional, quieter and cheaper.

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?)

 

Tightrope walking

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?

As it is—the mischief wrought by sloppy and/or scissored cranks is mostly noted on the oil pump/cam plate side of the engine. With all that out of the way you can almost envision how movement curtailed on the drive side of the crank (which is never checked for runout) can whip across to this side—amplified all the way—leading to excessive runout and eventual damage. It doesn’t start out this way! In virtually every case (pun intended) this issue occurs with accumulated mileage, in my view, after the automatic primary chain tensioner has gotten too tight, ironically removing every bit of slop in the only place it’s really necessary!

As it is—the mischief wrought by sloppy and/or scissored cranks is mostly noted on the oil pump/cam plate side of the engine. With all that out of the way you can almost envision how movement curtailed on the drive side of the crank (which is never checked for runout) can whip across to this side—amplified all the way—leading to excessive runout and eventual damage. It doesn’t start out this way! In virtually every case (pun intended) this issue occurs with accumulated mileage, in my view, after the automatic primary chain tensioner has gotten too tight, ironically removing every bit of slop in the only place it’s really necessary!

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.

I hope it’s obvious by now that chains have a whippy nature. In service in a primary they also load and unload the top and bottom runs around the sprockets—mightily—every time you get on or off the throttle. The resulting shock loads are brutal enough on main bearings, crank ends, clutches and trans shafts and bearings… without taking out every bit of “give” in the damn thing. Compensator notwithstanding, the jerks and bangs involved with over-tensioned chains are not entirely (or properly) absorbed by the slop elsewhere in the assemblage. So if it were me, I’d install one of these (Screamin’ Eagle #36500020) manual chain adjusters. Sure it means more labor at service intervals, and more expense for a new gasket every time you deal with it, but that seems to be a small price to pay if it helps keep your crank from taking “hard knocks.” “Arrange for your chain to take up the strain and you might just save the day. It ain’t that much work, just an occasional look and adjustment the old-fashioned way.”

I hope it’s obvious by now that chains have a whippy nature. In service in a primary they also load and unload the top and bottom runs around the sprockets—mightily—every time you get on or off the throttle. The resulting shock loads are brutal enough on main bearings, crank ends, clutches and trans shafts and bearings… without taking out every bit of “give” in the damn thing. Compensator notwithstanding, the jerks and bangs involved with over-tensioned chains are not entirely (or properly) absorbed by the slop elsewhere in the assemblage. So if it were me, I’d install one of these (Screamin’ Eagle #36500020) manual chain adjusters. Sure it means more labor at service intervals, and more expense for a new gasket every time you deal with it, but that seems to be a small price to pay if it helps keep your crank from taking “hard knocks.”
“Arrange for your chain to take up the strain and you might just save the day. It ain’t that much work, just an occasional look and adjustment the old-fashioned way.”

 

3 comments

  1. Great article. Thanks

    [Reply]

  2. My 2003 FXDL has a manual primary chain adjuster. It started puking oil into the breather after 40,000 miles. It had cam chain tensioner’s changed at 28,000 worn but still together. Crank is scissored out of spec. oil pump damaged not scavaging oil from crank and cam cavity causing excessive oil carried thru breather. Cam plate damaged pinion bushing damaged. The automatic primary tensioner in 2007 Ultra Glide has taken transmission main shaft bearing and seal, rear primary seal, clutch and crank seal and bearing out at 37,000 miles of a ladies bike I’m repairing. These same problems exist in the new twin cams. Don’t fix what isn’t broken but this has been established as bad design.

    [Reply]

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