Building a stout bottom end is crucial when modifying an engine for increased performance. The Twin Cam engine provides many enhancements over its predecessor, the Evolution, including beefier and stronger crankcases, reduced oil leakage, improved cooling, and more capability for increased displacement and power. Despite these benefits, the TC is not without its warts. Over the years, four critical bottom-end issues have surfaced with the TC engine. Two of those-the lack of a left side Timken bearing on 2003 and later engines, and B-Motor balancer issues-were discussed in the No. 40, Vol. 7 issue of Hot Bike. The other two major issues are crankshaft shifting and the cam drivetrain. In this month's segment, we'll address crankshaft shifting, which can cause catastrophic engine damage, while briefly touching upon the cam drivetrain. Short Block Charlie in Tempe, Arizona, invited Hot Bike to follow along as he modified a Twin Cam crankshaft to help prevent shifting.
A seldom talked about Twin Cam engine problem is crankshaft shifting. Unlike the Evolution and Shovelhead cranks, the TC crank is a press-together unit where the crankpin is a "hard" press fit into the two flywheels halves. Under most circumstances, this design works well. However, in some cases, under hard deceleration and acceleration or ground-pounding tire burnouts, the three-piece pressed-together crankshaft can twist out of true, sometimes as much as .030-inch, or even more in the worst situations.
Ideally, the crankshaft should be trued during assembly to within .001-inch. Under high power and torque conditions, a TC's flywheels can shift on the crankpin, causing the crank assembly to become severely out of true. This results in a severe wobbling of the right-side pinion shaft, which is located in the camshaft's gearcase. A wobbling pinion shaft often causes catastrophic damage to the engine's oil pump and cam support plate along with a loss of oil pressure and oil scavenging. Early warning signs include loud noises from the engine's gearcase area, ticking lifters, and oil dumping out of the cylinder head breathers. Sometimes, the unfortunate bike rider can limp home with the damaged engine. In worst-case situations, the bike needs to be trailered home. Nevertheless, the problem is serious, costly, and a big inconvenience.
The fix for a shifted crankshaft requires complete disassembly of the engine. Generally, the crankshaft, oil pump, camshafts, cam support plate, and cam drive mechanism require replacement. Additionally, debris from the damaged parts ends up circulating throughout the engine, destroying cylinder walls, pistons, rings, lifters, and anything else it contacts. Much of the debris ends up collecting in the oil tank, so the tank requires cleaning. In addition, if you have an oil cooler installed, it too becomes contaminated and should be replaced because coolers are difficult to clean thoroughly. If the engine is under warranty, you may be lucky enough to get it replaced. However, the problem is waiting to zap you again, because a new engine usually includes the same weaknesses as the damaged engine.
Oddly enough, not all Twin Cam cranks shift out of true and result in catastrophic engine damage. The problem is typically hit and miss. Stock cranks (both stroker and non-stroker) are usually the most susceptible to shifting problems, although some aftermarket cranks have been known to twist, too.
 1. The Twin Cam engine provides...  1.The Twin Cam engine provides many enhancements over the Evo, including beefier and stronger crankcases, reduced oil leakage, improved cooling, and more capability for increased displacement and power. But to realize its true potential, the Twin Cam requires a few key modifications to eliminate the weak links, starting with the crankshaft. Once the modifications are made, you can build a ground-pounding TC engine without worrying about walking home after whacking the throttle wide open. |  2. Stress from hard deceleration,...  2. Stress from hard deceleration, acceleration, and burnouts can shift a TC's pressed-together crankshaft and cause .030 inch or more of mainshaft runout. Ideally, shaft runout should be .001 inch or less. A wobbling pinion shaft will damage the oil pump and cam drive mechanism while contaminating the engine's oil supply, including the oil tank and any oil cooler. |  3. Here is Short Block Charlie...  3. Here is Short Block Charlie TIG-welding the crankpin on a new stroker crank. Tack welding the pin at both ends helps eliminate potential crankshaft shifting problems. This crank was checked for true before welding, and then retrued after welding to .001 inch or less of runout. Installing a Timken bearing in the left case (2003-later) also helps minimize crank problems when horsepower reaches roughly 100 and higher. |