The power is actually created in the top end of the engine by the combustion process pushing down on the pistons, and this energy has to be transferred into a rotary motion by the crankshaft. The crankshaft rotates in the crankcase main bearings, but these bearings have to do a lot more than just merely have a simple shaft spin in them.
Whilst the right hand (pinion shaft) bearing has to cope with the constantly fluctuating loads produced by the valve train, and both bearings have to restrain the crankshaft from flexing excessively, the left hand (sprocket shaft) bearing has to transmit virtually all of the power that the engine produces to the primary drive. Crankshafts also thrust sideways, and this also has to accomodated.
Earlier Harley-Davidson engines used roller bearings on both sides, and a thrust washer between each flywheel and the face of each main bearing outer race, to both take the side thrusts, and control the endfloat. This was fine for the low power outputs of the sidevalve and early overhead valve era, but as power outputs and rpm levels increased, a stronger solution was needed.
When the "K" model high performance unit construction sidevalve engine was released in 1952 it featured a pair of "back to back" Timken taper roller bearings on the sprocket shaft to better cope with the higher rpm levels and power output anticipated. Not only did this immensely strong configuration do this, it also reduced the crankshaft endfloat requirement, properly handled the side thrusts, and helped prevent the flywheels shifting in use.
This bearing carried through to the Ironhead Sportster of 1957, and the slightly redisgned version of 1977 carried through until 2003. The Panhead of 1955 gained the larger Big Twin Timken bearing, which was modified in 1969, and used right through the Shovelhead and Evolution era and into the early Twin Cams untill 2002.
So think about this for a moment. In the K model and XL model, these Timkens were used for 51 years with one small modification. The Big Twins used this bearing style for 47 years, again with just one minor modification. None of this longevity was down to Harley not bothering to design a better bearing, it's classic American over-engineering, and it works. Timken bearing failures are unheard of in this application.
The replacement was first used as the pinion bearing when the Twin Cam B (Softail) was introduced in 2000. In 2003 the Softail and Twin Cam A (Dyna and FL) got this bearing on both sides of the crankshaft. The Sportster got this bearing on the sprocket shaft when the engine was redesigned for the rubbermount in 2004. This new bearing has not been without issues, and had 4 upgrades in 6 years, before being replaced with the current incarnation in 2007. So, 6 versions in 7 years! Compare that with the Timkens which had 2 versions in half a century.
So, why the change to the INA roller bearing? Simple, cost. Whilst the Timken bearing assembly is only about 20% more expensive than the INA roller, it is a lot more time consuming to install. Effectively two bearings instead of one, and the end float has to be tested and correctly set. (We'll deal with the Timken bearing crankshaft installation in another tech article).
Although this change is a major step backwards in engineering terms, the INA bearing failures aren't that common in stock engines. However, the bigger picture involves the crankshaft. Harley-Davidson have rightly come in for criticism regarding both the original alignment (or lack of it), and the flywheels shifting in use. The fact is that the Timken bearings support the crankshaft better and reduce the instances of the wheels shifting. We've seen the stronger, and better trued, Screaming Eagle (Jims Machine) cranks shift .015" in the INA bearing, and not move at all in the Timkens. We've also been back inside some of our HQ113 builds to experiment with different cams, and the potentially troublesome late flywheels haven't shifted with our Timken conversions, even with 130ft/lb of torque pulling a heavy Glide along.
All of our bigger builds that involve splitting the crankcases get the Timken conversion, irrespective of whether or not we use a welded crank. .003" is the runout limit for using gear drive cams, and .007" will trash the oil pump, so we strongly advise this useful insurance anytime the 2003 and later Twin Cam engines come apart.