msvphoto

I just did the Timing Belt (and all the related stuff...) on our '95 A6q Avant this weekend. My first time on an Audi 12v V6 (but I have done timing belts on 3 Audi I5s and several other types of engines as well). According to various published proceedures, and the pages from the Bentley manual CD I printed, one is supposed to use a puller to pop the cam sprokets free of the cams. On attempting this the cam rotated (thankfully the sprocket did not break free) which (obviously) scared me. I used the cam lock tool, but the proceedure I was using (combination of 12v.org and another from the FAQ here, as well as the Bentley manual on CD) indicated that the tool is to be removed when using the puller on the sprockets. It then dawned on me that I did not need to remove the sprockets to remove the old belt, and I wasn't changing cam seals (they were dry). So, why then does one need to break the sprockets loose???? I mounted up the new belt (with the cams locked) and set up tension and everything looked fine. Finished up the job and the car runs great! Can someone explain why that step is in the proceedure????

4Driver4

Unlike the 5 shooters that you are used to, the cams on the V6 do not have woodruff keys. Cam timing is fine tuned by using the specified procedure. Your car may run fine, but your cam timing might be off a few degrees. You may have left .000356 miles per gallon and .02 horsepower on the table.

SoCal Avanti

Loosening the cam bolts arent absolutely necessary, but It just makes the job of geting the belt to mesh into the teeth of the sprockets easier. It allows for more precise timing. If you dont loosen the bolts, there is a very good chance that you will move one or both cams a fraction from the exact timing placement.
You see, with the crank at TDC and the cams at the correct position, by loosening the sprockets you can slip the belt over them and it wont matter if they move a little, since the cams wont. Get it?

msvphoto

Thank you for (I think) clearing this up...but now I still wonder. I had a feeling it maybe something like that.

However, I'm still confused. On the I5 you must remove the cam sprocket to remove to the rear cover. On the V6 the rear covers are not in the way of replacing things like the water pump and thermostat. I realize the sprockets have no woodruff keys. However, if the crank and the cams are locked and the sprockets are not released, then it all comes down to plus or minus a tooth of the timing belt--not sure I quite get the fine tuning the cam timing part, since you can't time the three things (two cams and the crank pulley) to a tolerance any smaller than a belt tooth width anyway (or can you?). After all, if the three things were in time before, don't they stay that way assuming you get the new belt on right?

It was when the cam rotated while trying to break the sprocket free with the puller that I got scared. (i.e. what if the cam rotates at the same time the sprocket pops loose??? Are you screwed then, or what????) Since the sprocket was still pressed on the cam all I had to do was rotate the cam back to the lock-tool tdc position and put the lock tool back on. Should I have released the sprockets with the lock tool engaged? Are the tabs the lock tool engages affixed to the cams and not the sprockets?

I guess I really am I'm still confused. It wouldn't be a big deal to open it back up to reset the timing if I really blew it, but the car feels exactly the same as before and I can't really visualize how the timing could be any different than it was before.

I will say that the V6 TB job is a whole lot easier than the I5s I have done (still dreading the day I have to do the '83 Ur-Q TB--knowing how little room there is to work in there).

msvphoto

<<You see, with the crank at TDC and the cams at the correct position, by loosening the sprockets you can slip the belt over them and it wont matter if they move a little, since the cams wont. Get it?>>

Thank you for the further clarification. I can certainly see how it would be easier to make sure the tooth count between pulleys is setup right. I still wonder what happens if the cam moves while the sprockets are loose?? Are you then screwed?

4Driver4

You are assuming that the old belt and the new are exactly the same length, and that the idler and adjuster are the same diameter. Sure, they're pretty damn close, but there is some variation. When you free the sprockets, lock the crank, tighten the belt, then tighten the sprocket bolts, you are assuring that the timing is *exactly* right. Was this a huge problem with the old 5 cylinders? No. But did you ever put a timing belt on after machining a warped head? You could get cam timing close, but not perfect. You may have tried two different teeth, then gone with the closer one.

With the old key, you are right, the sprocket goes on the cam one way, and adjustment is done one tooth at a time. But with the V6, the sprocket can go onto the cam in an infinite number of positions.

Your assumption is correct WRT the locking plates; they fit onto the cams one way.

SoCal Avanti

Honestly, I don't know what exactly it would take to fix a mistake like that. My best guess would be that it depends on which cam moves. If it is on left (passenger) side, then you could use TDC at the crank and bottom most cam lobes for cylinder #1, but you can still be a few degrees off. For the right cam, I think you would need a degree wheel attached to the crank to find TDC of another cylinder besides #1. All in all, it sounds like it would suck.

4Driver4

It's not a big deal if the cams turn a little - just don't turn them too far or a valve or two will meet a piston. There are grooves machined into the cams that only allow the locking plates to fit on one way. If you turn a cam, just turn it back until the tool fits the plate.

One of the nice things about this is that you can break both sprockets free, put the tool on, then re-install the belt. You don't have to worry about spinning the cams into the right position before you put the belt on. Lock the the cam in one spot, then spin the sprocket to where ever you need it.

msvphoto

Thank you all for helping me better understand this.

Now that I know there are mating grooves in the cams (for the plates for the lock tool) I will certainly have much less fear (next time) when using a puller on the sprockets. I can see how allowing the sprockets to run free would make installing the belt easier for sure.

However, as far as timing goes, the tolerances of the idler and tensioner rollers play no part in the timing equation best I can tell. Manufacturing tolerance of the belt will, but given that the minimum "mechanical resolution" of adjustment is still one belt tooth I still don't understand how one could get any closer to the way the engine was before (timing-wise) if all else remains fixed. If the crank is locked, the cams are locked, and the belt is installed and tensioned correctly, how can you get any closer to correct timing?

Not trying to belabor the point, just trying to better understand.

Yes, I do recall the issues with shaved heads being 1/2 tooth off and needing to find the tooth that is "sweeter" so to speak. I can see how the infinite adjustment of the sprockets may help with heads off work, especially if the heads were shaved down.

Are the tolerances of the belt construction really that great?

SoCal Avanti

The truth of the matter is, if all you are doing is a simple belt change, and as long as the cam sprockets have never been loosened by anyone before you, you dont need to either.