Harley-Davidson Guts & Stuff
By the 1990s, the Evolution proved to be a phenomenal engine and was a critical ingredient to the revival of Harley's success. Although the Evo motor revitalized the charisma, feel and rumble that made a Harley appealing, it also was saddled with design and production flaws, like crankcase breakage, left-side case leaks, weeping cylinder base gaskets and pulled cylinder studs. The early to mid-1990s Evo cases were the most prone to catastrophic disaster. Building an 80ci engine beyond roughly 80 to 85hp with the early '90s-style cases became risky at best. That meant that aftermarket components were required to fix the Evo's design and production flaws. The factory realized the Evo bottom end needed a complete redesign to make it robust enough to handle mild modifications let alone serious performance work. Moreover, EPA regulations were dictating the elimination of the clicking and whining noises from the cam gears.
The factory's solution to these issues was to develop an engine that would not only fix known problems but also one that would take Harley-Davidson down the road and into the future. In other words, an engine that didn't leak, had room for performance potential and satisfied EPA regulations while maintaining the traditional 45-degree, V-twin, air-cooled and pushrod design. To satisfy the EPA's reduced noise edicts, Harley decided to abandon the gear-driven cam design and replace it with a chain-driven system. But to maintain the traditional V-twin silhouette, Harley had to utilize a two-cam design. As history would prove, the new motor became known as the Twin Cam, because the cam arrangement embodied the major difference between the new motor and its predecessor, the Evo. The Twin Cam was introduced in mid-1998 and shipped in 1999 rubber-mounted big twins. Although the Twin Cam design became the new motor's namesake, it also became a source of ongoing frustration and trouble for many owners.
Cam Bolts & Drive Gears
Some very early 1999 TC models were plagued by breakage of the cam bolt holding down the large drive sprocket attached to the rear cam. Other 1999 models encountered problems because the keyway on the rear camshaft drive sprocket sheared. The keyway was used to secure the drive gear to the camshaft. The factory fixed those problems by replacing the faulty hold-down bolt and the drive gear's cast-in keyway with multiple splines on the drive gear and end of the camshaft.
Outboard Rear Cam Bearings
A serious problem also developed with the outboard rear cam bearing located in the cam support plate. Initially, Twin Cam engines were manufactured using two roller bearings to support the twin camshafts on the right side. Due to the manner in which the rear ball bearing was subjected to forces, it often failed. The factory supposedly fixed the problem in late 2000 by upgrading the cam support plate and replacing the ball bearing for the rear camshaft with a roller bearing. The front cam retained a ball bearing for outboard support. A service bulletin was issued by the factory in July 2000 describing the fix. To date, no less than four cam support plate designs have been used, so you must make sure you have the correct support plate when performing any upgrades.
Inboard INA Cam Bearings
Another cam bearing that many engine builders feel uncomfortable using is the INA bearing used to support the Twin Cam's two camshafts on the left side or inboard side. The INA bearings are located in the right-side crankcase just above the crank's large pinion bearing. Harley experimented by using INA bearings in place of Torringtons on a few Shovelhead models and then converted totally to the INA with the Evolution. INA bearings have intermittently caused problems when used on the left side of the V-twin's camshaft(s). As such, wise engine builders replace the INA bearing with a B-148 Torrington.
Other serious issues involve the silent cam chains that drive the camshafts and the spring-loaded tensioners used to keep the chains in adjustment. The 1999 to 2006 Twin Cam engines, except those shipped with the 2006 Dyna models, use two silent chains and two spring-loaded adjusters. There are several issues with this setup, both for stock and performance-modified engines.
To reduce cam gear whine, the Twin Cam's timing setup uses silent chains and drive sprockets instead of the traditional spur-gear drive mechanism. The Twin Cam's silent chains are constructed of six plates aligned side by side. This results in an inverted tooth or "silent" type of timing chain that provides a smooth and relatively quiet mechanism. Silent chains are nothing new and have been used by Detroit automakers for a long time. Although silent chains are silent, they also stretch, contributing to inaccurate timing. For this reason, automotive hot rodders have traditionally upgraded silent timing chains with roller chains, which are lighter, more durable and can handle higher rpm.
Additionally, many high-performance cams require higher valve spring pressures to control valve float and bounce. Higher valve spring pressure places greater loads on the camshaft, resulting in greater demands on the stock spring-loaded chain tensioners. Consequently, the spring tensioners become overloaded, leading to a loose timing chain and inaccurate cam timing. Tests have proven that the design of the stock silent chain and spring-loaded tensioner mechanism allows cam timing to vary as much as 4 degrees and much more when heavy valve springs are used.
Another problem associated with spring-loaded tensioners is premature wear of the tensioner pads. Silent chains have sharp edges, leading to rapid wear of the pads. Debris from the tensioner pads circulates with the engine oil and can score parts and clog small orifices. Pad debris has been known to clog a piston oiler, holding the check ball open. The result is reduced oil pressure, ticking lifters, oil puking and a variety of other problems. To slow tensioner pad wear, some engine builders have resorted to polishing the outside edges of both silent chains. But this is only a Band-Aid solution.
A serious Twin Cam problem that is seldom talked about is crankshaft shifting. Under hard deceleration, acceleration or burnouts, the Twin Cam's pressed-together crankshaft can twist out of true, sometimes as much as .030 inch or even more in worst circumstances. An ideally trued crank should be trued to within .001 inch. A crank severely out of true causes the end of the pinion shaft (right side) to wobble severely. Early warning signs are loud noises from the engine's gearcase area, ticking lifters and oil puking out of the breathers. The result is catastrophic damage to the oil pump and loss of oil pressure and scavenging. For starters, the crank, oil pump, camshafts, cam support plate and cam drive mechanism need replacement. Furthermore, debris from the damaged parts circulates throughout the engine, trashing cylinder walls, pistons, lifters and anything else in its path. The oil tank also ends up contaminated and requires cleaning. If you are on warranty, you may be lucky enough to get a replacement engine, never really knowing what exactly went wrong or why it went wrong. But the problem is waiting to bite you again, because your new engine has the same flaws as the old, damaged engine. On the other hand, if you're one of the unlucky ones, you get a big repair bill.
Some Achilles' Heel Fixes
There are several fixes for the previously described problems. First, most of the early Twin Cam engines with potential cam bolt breakage and drive gear keyway shearing problems should already have been fixed. The fixes are simple, but you have to remove the gearcase cover to inspect the problematic parts. Second, make sure your engine has a roller bearing installed in the cam support plate for the rear cam. The roller bearing is easily distinguishable from the ball bearing used for the front cam. Additionally, whenever working inside the cam gearcase, check the inboard cam bearings. If you have the stock INA bearings, replace them with proven Torrington B-148 bearings.
The best yet most costly method for eliminating all silent chain and spring-loaded tensioner problems is to install a set of gear-driven cams. Andrews Products and S&S Cycle offer gear-drive cams. Gear-drive cams will produce more accurate cam timing while eliminating the structural weakness of the cam's drive system. However, be sure to check the cam gears for proper clearance. A too-tight fit will produce a whine or destroy the gears in a thousand or so miles. A loose fit will result in gear clatter.
Harley-Davidson updated the silent chain and spring-loaded tensioner mechanism with a roller chain and hydraulic tensioner setup in 2006 on Dyna models and in 2007 for all other models. This cam drive mechanism is superior to the silent chain design. The roller chains require a different sprocket design than silent chains, and the tensioner pads have been replaced with nylon. However, I have seen one engine where the new nylon pads were worn out in about 20,000 miles. The ball and roller bearings in the cam support plate have also been eliminated, leaving the camshafts riding directly on the support plate. Only time will tell how well this design works. Andrews Products is offering a conversion kit to upgrade 1999 to 2005 silent chain Twin Cams to roller chain drive.
Finally, not all cranks come out of true and cause catastrophic engine damage. Although it seems to be a hit-and-miss proposition, stock cranks (both stroker and nonstroker) seem to be more prone to problems, although I have seen some aftermarket cranks twist. The fix for eliminating a twisting crank is to start with a good crank, TIG-weld the crankpin on both ends and then retrue the crank. The photos show Short Block Charlie TIG-welding the crankpin on a stock factory 43/8-inch stroker crank. Installing a Timken bearing on the crankcase left side on 2003-and-newer models is also a wise upgrade to help prevent crank twist.
The horsepower per cubic inch developed by the Twin Cam and Evolution engines is roughly the same, assuming both engines are built to the same standards, but the Twin Cam engine includes many refinements over the Evo, including stouter crankcases and the potential for increased displacement. However, to realize its true performance potential, the Twin Cam's cam drive mechanism and crankshaft must be set up properly to eliminate the weak links. When built correctly, the Twin Cam will give you trouble-free operation and peace of mind.