Building A Stout Twin Cam Crank
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.
Modifying the Crankshaft
In order to properly modify a Twin Cam crank to eliminate twisting, you have to start out with a crank in good condition. It is best to begin with a new crank, but a used one in good condition can also work. Charlie starts by checking the crankshaft assembly for true and makes any necessary adjustments so there is no more than .001 inch runout on the mainshafts. Next, the crankpin is TIG-welded on both ends to the flywheel halves, and then the crank is retrued to within .001 inch or less. The photos show Short Block Charlie TIG-welding the crankpin on a new Twin Cam stroker crank.
If your engine has experienced a twisted crank, most likely the oil pump has been damaged as well as the cam support plate. If those parts need replacing, it is a wise idea to replace them with the 2007-later (2006-later for Dynas) high-volume oil pump and a compatible cam support plate. Since a damaged engine must be disassembled, this is the perfect time to make engine performance upgrades, such as a stroker crank, big-bore cylinders, head modifications, and high-performance cams.
Regarding the camshafts, you have a choice of either sticking with chain-driven cams or switching to gear-drive versions. Chain-driven cams use two chains and either two spring-loaded adjusters with flat silent chains (’06-earlier) or two hydraulic operated adjusters and roller chains (’06 Dynas and all ’07-up). The hydraulic adjusters are an improvement over the earlier spring-loaded style, but both designs have limitations. The early style spring-loaded tensioners are notorious for experiencing premature wear on the chain pads. Typically, 18,000 to 25,000 miles is the maximum you’ll get on a pair of spring-loaded pads. Also, the debris from the pads can clog internal oil passages. The verdict is still out on pad wear for the hydraulic tensioners. Andrews Products is offering a conversion kit to upgrade ’99 to ’05 silent chain Twin Cams to roller chain drive.
Another concern with chain-driven cams is accurate cam and valve timing. Some high-performance cams require higher valve spring pressures to control valve float and bounce. Since higher valve spring pressure places increased loads on the camshaft, greater demands are placed on the stock chain tensioners. As a result, the tensioners can become overloaded while chain stretch increases, leading to inaccurate cam timing.
Gear-drive cams from Andrews Products and S&S; Cycle eliminate the structural problems associated with stock chain tensioners while producing more accurate cam timing. But keep in mind that gear cams must be checked for proper gear clearance. A fit too tight will produce a whine and can destroy the gears in short order, while a loose fit results in gear clatter. Gear cams are less tolerant of a shifting crankshaft than chain-driven cams because proper gear clearance must be maintained. This is an additional reason to TIG weld the flywheel’s crankpin when installing gear cams. In fact, some aftermarket shops are hesitating to install gear cams unless the crank is properly modified to minimize the potential for shifting.
The Twin Cam is an excellent engine and includes several refinements over the Evolution, including stouter engine components and the potential for increased displacement. However, to realize its true potential, the Twin Cam requires a few key modifications to eliminate the weak links. The modifications start at the crankshaft and extend to the cam drivetrain.