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Tech Article

Audi C5 A6 Quattro 4.2 V8 Camshafts and Sprockets -- Overview

Last Updated: 09/24/2017
Parts Group: Camshaft with pulley
I have asked an engine-savvy tech to dismantle part of the 40-valve 4.2 V8 engine from one of my 2000 C5 Audi A6 Quattro cars, while commenting as she worked on parts relevant to the camshafts vs. crankshaft synchronization. She’s accustomed to building racing engines but she’s relatively new to Audi so she was also learning while working. It was interesting for me to hear her thinking out loud and to observe what she was figuring out as she was going along.

The camshaft(s) must be synchronized with the crankshaft on any four-stroke engine but especially interference-fit engines such as this one, otherwise piston-to-valve impact and damage can occur. Here is an example from a 40-valve 4.2 V8 engine from a 2001 C5 Audi A6 Quattro that I bought at a junkyard:

The bright marks on the center piston shows where the valves impacted the piston.

In preparation to changing the timing belt, the crankshaft gets locked in place in a known position, using a screw-in plug designed to work only when the crankshaft is in that position. As I recall this position is called “Service position” and it’s probably also top dead center for cylinder #1, which is passenger side front as I recall.

The engine has four camshafts, two per bank. These two are attached to each other with a chain that keeps them synchronized. Each bank has one accessible camshaft to which a sprocket is attached. Each accessible camshaft (one each bank) has a plate installed perpendicular to the camshaft. The front of each accessible camshaft is shaped so that the plate can be installed in only one position, and in only one direction:

On each bank, the key is shaped like a paralellogram, and has a small hole and a large hole:

Also in preparation to changing the timing belt, a locking bar with two studs (of different sizes) per bank is placed in position so that each stud fits into a hole in the plates, one plate per bank. This locks the accessible camshafts, and thus also the inaccessible camshafts, in a known position relative to the crankshaft.

One each bank, one camshaft sprocket is attached to the front of the accessible camshaft.

Front view:

Rear view:

The sprocket is NOT keyed; it can rotate freely relative to the camshaft, except that high tension from a bolt pushes the sprocket against the camshaft, thus making them rotate as one unit, so that when the belt turns the sprocket, this also turns the accessible camshaft, which via the chain also turns the inaccessible camshaft on each bank.
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