ocles_inc

This write-up tries to explain a design that used to perplex me. I hope you also find it useful.

Also, as to used parts for this type of engine, I do sell them. I also sell complete running engines in cars that you can test drive, take to your mechanic etc. before you buy. By the time I offer a running engine for sale it'll typically have had its timing belt and related parts replaced with new parts from Blauparts, Please ask for specifics and pricing if you're interested.

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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.

CRuby

Double check your info. I've read it's #5 cylinder on the 4.2, & it's #3 cylinder on my 2.7 .

LilRedHead

What kind of girl works on racing engines and touches an Audi? I think you must be confused haha

For those who don't know, you MUST torque the bolt that sandwiches the cam sprocket(My info is somewhere near 150 ft-lbs) to the camshaft or the result is it will works its way lose and you get a expensive lawn ornament.

Because the sprockets can move it's pointless to mark the cam sprockets because they can shift and make the lines useless. However if you don't change your cam seals, you shouldn't have to worry about it because they shouldn't be shifting. This is however bad practice since any difference in the belt or other part of the pulley system will make the alignment off.
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4Driver4

...which means if they are off, they will still be off, and/or if there is any different in water pump or idler or belt, they will be off.
Any time you are in there, you should be setting cam timing.

LilRedHead

Nearly every engine I've looked at had someone in before me leaving timing marks. Very bad practice yes, were they off, about 1-2 degrees from the other cam (usually the right cam set) so I'd obvious that a lock bar is a must. Credit due most engines ran pretty dang close to fine but yeah you are right indeed.

ocles_inc

Thank you for the correction. :-)

~Tanya