Twin Cam Engine - Chain Driven Cams And A Twisting Crank

(1.)Introduced for the 1999 model year, the Twin Cam engine offered room for performance enhancements and satisfied EPA noise regulations while maintaining the traditional 45-degree, V-twin, air-cooled and pushrod design.

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.

(2.)The rear camshaft is driven by a silent chain kept in adjustment by a spring-loaded tensioner. The chain is powered by a sprocket mounted on the crank's pinion shaft. Silent chains stretch and spring-loaded tensioners do not operate well under heavy valve spring pressures, resulting in inaccurate valve timing. The bolt fastening the large chain sprocket and the sprocket's keyway are prone to failure on very early 1999 Twin Cam engines.

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.

(3.)Removing the outer silent chain and sprockets reveals the entire cam support plate. The shaft at the bottom center of the support plate is the right-most end of the crank's pinion shaft. The two shafts above the pinion shaft are the rear cylinder camshaft (left) and the front cylinder camshaft (right).

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.