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View Full Version : Collapsing can tensioner through reverse rotate engine?



maxiz1
10-22-2018, 08:15 PM
Hi Guys,
I just wanted to share my observation. After re-installing the cam tensioner back in place, I was trying to align the the timing marks on the camshaft thus this involved a small back and forth adjustment to the engine rotation (by hand). I noticed that when I rotate the engine clockwise, the cam tensioner expanded. If the engine were to move ever slighty on the anti-clockwise direction, the tensioner collapsed by itself.

So my question, is this normal? If so, can I use this method instead of using the special tool 3366 to relieve the tension?

walky_talky20
10-23-2018, 05:41 AM
This is very normal.

The amount of force needed to rotate the intake camshaft is not zero. The valve springs are pretty heavy and there's a bunch-ton of them. Twelve, to be exact. Imagine the cylinder head is sitting on your coffee table with the everything installed, except the cam chain. Close your eyes, I'll wait... Now try to roll the intake camshaft. You are going to need a wrench or pliers or something. You cannot roll it easily by hand. So that is the force we are working with here. Also notice, that even when you used a wrench, the rotation is not smooth at all. It's very notchy as it finds various positions it wants to "snap" to.

Now let's compare that amount of force (the force needed to roll the intake cam against all of its valve springs), to the force applied by the single spring within the cam chain tensioner. It is a pretty tight spring in there, but it is no match for all those valve springs. No match at all. This is why it relies on engine oil pressure *in addition* to the internal spring. Otherwise it would be hopeless to keep the chain under control. As a side note, this is why your cam chain will rattle like a TDI if you have low oil pressure or if your CCT is worn out and hemorrhaging its available oil pressure.

Important thing to note* - When you are rotating the engine forwards, the chain is pulling across the stationary foot down below. But if you roll the engine backwards, the chain is pulling against the sprung foot. That is where you will get most of this spring-compressing behavior.

As for using that method to compress the tensioner (I assume for CCT or gasket service), the answer is: not really. While you can use the camshaft forces to temporarily compress the internal spring, you still need something to hold it in the compressed position while you try to do a thing. But it may help to get your 3366 tool started and installed.

maxiz1
10-23-2018, 08:18 PM
Walky - as always you never failed to impress me with your thorough response. Thanks man.

That also answered my lifetime quest for why the the engine would kind a like "give in" at a few points during the rotation by hand. Now I understand it is due to the shape of the cam lobes and the exerted force from the springs.

Also I found the cheap chinese made CCT tool does not fit well that I have to do a lot of trimming. But hey, for 6 bucks incl shipping what rights do I have to complain...

walky_talky20
10-24-2018, 06:47 AM
No problem. Glad I could shed some light.

As for your lifetime quest: Yes, part of the notchy-ness you feel when rotating an engine will come from the valve springs. But you will mostly experience it it when rotating a camshaft on it's own or as a pair. B6 3.0 owners, for example, know all about the cams wanting to "jump" to various positions if they try to do a timing belt job without locking the cams in place. But the main contributor to the uneven torque demand when rotating a full engine (bottom and top end together with timing belt) is the compression of air within the cylinders. When it gets tough to turn, you are feeling the compression stroke on one of the cylinders. When you enter the power stroke, the air pressure actually helps it roll forward. So you get this repeating sort of easy-hard effect. If you remove the spark plugs, it removes the compression factor altogether and it is a much easier time to roll the entire assembly. Then you'll feel just the fight from the valve springs.