Considering the unstable nature of the “breathed on” 1941 design of the pre Twin Cam crankshaft, together with the time-tested reliability of the camshaft gear drives used with these cranks; I find it difficult to rationalize the root cause of fractured S&S gear teeth as being a crankshaft issue. I’m not suggesting that crank runout doesn’t contribute; I’m saying that there are many tens of thousands of pre Twin Cam machines running around with crank runout approaching double digits, that grind away mile after mile without breaking teeth off the cam drive gears. My sense is that the root cause has more to do with the metallurgical properties or design of the S&S gear than the runout of a crankshaft.
Gear driven valve trains have been around since the beginning of motorcycling. In the last 40ish years prior to the Twin Cam gear drives, fractured cam or pinion gear teeth were unheard of, unless there was an issue with foreign object damage. The gearmesh of some generator equipped Big Twin’s and Sportsters were often set up so tight, that you could hear the gearcase whine over the exhaust from a block or two away; this was not at all uncommon; yet no broken cam or pinion teeth.
The three and five piece “bolt together” crankshafts used in the pre Twin Cam machines are extremely flexible. The tapers on both the shafts and flywheels become polished with mileage; evidence of significant movement of the shafts and crankpin in the tapers. Flywheel shift of the pre Twin Cam crankshaft is a common occurrence. It is not unheard of to see evidence of contact between a flywheel and the oil scraper in the crankcase of these early machines; yet no broken cam or pinion teeth.
It seems to me that wrist pin thrust became a problem for HD when they eliminated the tapers and integrated both left and right shafts into the flywheels. The issue with this crankshaft is that the right side Gear-shaft is much smaller in diameter than the left side Sprocket-shaft, so the right side flexes more under load. This right side flex causes the flywheel assembly to “wobble” when loaded; setting up a side-to-side “whipping” action of the connecting rods.
The attached photo illustrates the effect of a whipping connecting rod on the wrist pin retainer of an EVO piston. It is my strong sense that some evidence of wrist pin thrusting will be present in any EVO with 20k + miles and configured with floating wrist pins and wire retainers. Machines that are ridden hard generally show stronger evidence at an earlier mileage. The area most affected by this crankshaft flex is the right side wrist pin retainer groove of the front piston; yet no broken cam or pinion teeth.
There is some “give” in the PTO side taper of the earlier five piece assemblies which seems to offset the wobble affect to some small degree. Another variable affecting wrist pin thrust are the floating wrist pins used in the EVO’s. The earlier Shovelhead wrist pins were setup with a tighter interference fit than the “drop-in” fit of the EVO’s, so wrist pin thrusting in the Shovelheads was not an apparent issue.
The Twin Cam crankshaft addresses the shortcomings of its predecessor in a couple of ways. The diameter of the right shaft was increased to mirror the left PTO side. The increase in diameter reduces flexibility while the commonised diameters improve stability. The crankpin diameter was increased by almost 1 inch to provide a stable anchor for each flywheel half. It is my sense that the “press together” design was chosen to address inherent stability issues with the 60 + year old taper-fit crankpin design previously used.
All things being equal, the Twin Cam crank assembly is much more rigid and stable than its predecessor; that said, it is my sense that high output machines would benefit from welding the crankpin to the flywheel. Folks have been welding press-together cranks in high output applications for decades.
On another somewhat related topic; the Twin Cam’s chain driven cams were developed in an effort to control engine noise and reduce manufacturing costs. In the pre Twin Cam engines, HD’s effort to match the cam and pinion gears was expensive, but necessary to manage engine noise. A specific tooth profile and finish was required as well precisely matching the center-to-center distance of the gears. Some flexibility with the center distance was required to accommodate a normal production variance occurring in the manufacturing of the gearcase covers. HD produced 7 different sized pinion and 7 different cam gears for the EVO engine. The difference in pitch diameter between sizes is a few ten thousandths of an inch. It is my sense that the chain is as quiet or quieter, at least when new, and it is a fraction of the cost to produce.
HD still does the precision gear fit in the XL engines. As I remember it, at one time there were over 250 possible gear combinations produced for the XL 4 cam engine. You used to be able to order cam gears by color code for the XL, now if you need a cam, you get all four matched to a cover.
All of this is just my humble opinion, but if you print this post and bring it to Starbuck’s along with $4, they will pour you a cup of their fine coffee; be sure to tell them that djkak sent you.
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Topic: Problems with S+S gears drive (Read 9852 times)