VALVE TIMING
DESCRIPTION - TIMING DRIVE SYSTEM
The timing drive system has been designed to provide quiet performance and reliability to support a non-free
wheeling engine. Specifically the intake valves are non-free wheeling and can be easily damaged with forceful
engine rotation if camshaft-to-crankshaft timing is incorrect. The timing drive system consists of a primary chain and
two secondary timing chain drives.
OPERATION - TIMING DRIVE SYSTEM
The primary timing chain is a single inverted tooth type. The primary chain drives the large fifty tooth idler sprocket
directly from a 25 tooth crankshaft sprocket. Primary chain motion is controlled by a pivoting leaf spring tensioner
arm and a fixed guide. The arm and the guide both use nylon plastic wear faces for low friction and long wear. The
primary chain receives oil splash lubrication from the secondary chain drive and oil pump leakage. The idler
sprocket assembly connects the primary and secondary chain drives. The idler sprocket assembly consists of two
integral thirty tooth sprockets and a fifty tooth sprocket that is splined to the assembly. The spline joint is a non –
serviceable press fit anti rattle type. The idler sprocket assembly spins on a stationary idler shaft. The idler shaft is
press-fit into the cylinder block. A large washer on the idler shaft bolt and the rear flange of the idler shaft are used
to control sprocket thrust movement. Pressurized oil is routed through the center of the idler shaft to provide lubri-
cation for the two bushings used in the idler sprocket assembly.
There are two secondary drive chains, both are inverted tooth type, one to drive the camshaft in each SOHC cyl-
inder head. There are no shaft speed changes in the secondary chain drive system. Each secondary chain drives
a thirty tooth cam sprocket directly from the thirty tooth sprocket on the idler sprocket assembly. A fixed chain guide
and a hydraulic oil damped tensioner are used to maintain tension in each secondary chain system. The hydraulic
tensioners for the secondary chain systems are fed pressurized oil from oil reservoir pockets in the block. Each
tensioner also has a mechanical ratchet system that limits chain slack if the tensioner piston bleeds down after
engine shut down. The tensioner arms and guides also utilize nylon wear faces for low friction and long wear. The
secondary timing chains receive lubrication from a small orifice in the tensioners. This orifice is protected from clog-
ging by a fine mesh screen which is located on the back of the hydraulic tensioners.
HB
ENGINE - 4.7L SERVICE INFORMATION
9 - 1303