27 replies to this topic

[pavel79]

I instaled new chain this winter and have the same marks position - about 1mm do not match. 

Camchain on your foto looks like new :-)

[TKH]

Someone with more knowledge will come along soon, but when I did my valves things were a little off and that's only 27k. You will get stretch in the chain and the tensioner will help take that up. If it's running sweet and there's no slack I think it should be fine, but if you are in doubt, especially if it's not had a change in 120k then change it. What does anyone else think?

[fixitsan]

Rather then starting a new thread....

In need of some advice. I'm checking my valve clearance and found the cam chain sprockets a bit off.

They appear to be off for just 1 mm on the markings.

What do you reckon: worn cam chain or normal wear?
The engine is running pretty smoothly. All the valves are with in tolerance.

Just 125.000 km on the clock.

 

 

 

on the assumption that the timing marks are absolutely perfectly lined up with where they should be relative to the cam (ideal manufacturing), and knowing that the crank makes 2 rotations for every rotation which a cam makes, and, the length of worn chain between the crank and exhaust cam sprocket is over twice as long as the bit between the two cam sprockets, , then the amount of rotation the crank has to make to get the inlet marks to line up is a few degrees of crank rotation. or in other words all the cam timing is out, i suspect if you look at the crank timing marks you'll see it more visibly (or drop a 'feeler', a paper drinking straw down the spark plug hole to feel when the piston is at TDC)

 

As the chain becomes worn the valve timing becomes more and more retarded (both inlet and exhaust of course), and the inlet retards a little bit more than the exhaust, due to the existence of several worn links between the exhaust sprocket and the inlet sprocket.

 

These changes are subtle and develop so slowly that you don't really notice. In reality, unless you are ruinning a very high spec engine the question is, do these small changes of a few degrees make much difference ? Also, there is a cam position sensor to consider, it's output is also retarded relative to the crank position due to chain wear.

 

I changed the chain on my 900 at 100,000miles. I think my backside dyno noticed a difference, it ran a little bit smoother, it sounded more alive....these are just subjective observations though.

 

When I tuned cars, we used to buy vernier adjustable cam pulleys, marked in fractions of degrees, which allows you to advance or retard each cam independently of one another. A Sunday afternoon spent trying to establish if there was a big difference between standard timing and advancing the inlet timing by 2.5 degrees was common. It might matter more when you run a 'hot' cam which has timing windows extending closer to the edges of what are feasibly the maximum limits of opening and closing times.

 

Back then the car i tuned a lot was the Austin Mini, and perhaps the vernier pulleys/sprockets made some difference because the cam drive, was chain (which wore) and the alignment of the sprocket teeth to the keyway on the cam was different on most engines by a few degrees. The cam profile was so 'agricultural' that it didn't really matter if you were a few degrees out here or there. That's why I started by saying that it was all based on the assumption that the TDM cam sprockets are accurately made within a tight tolerance

Edited by fixitsan, 10 March 2019 - 01:29 pm.

[Studley Ramrod]

Nothing at all to worry about imho.  Looks like there might be some slight chain stretch but I've seen far worse and they didn't seem to have any noticeable effect on the ehgines.

[PeterM91]

Nothing at all to worry about imho.  Looks like there might be some slight chain stretch but I've seen far worse and they didn't seem to have any noticeable effect on the ehgines.

Thanks for your response. I was hoping to hear that!

The same applies to Pavel's response.

I'll just leave it for now. Get back in 42000km for the next checkup.

 

 

 

on the assumption that the timing marks are absolutely perfectly lined up with where they should be relative to the cam (ideal manufacturing), and knowing that the crank makes 2 rotations for every rotation which a cam makes, and, the length of worn chain between the crank and exhaust cam sprocket is over twice as long as the bit between the two cam sprockets, , then the amount of rotation the crank has to make to get the inlet marks to line up is a few degrees of crank rotation. or in other words all the cam timing is out, i suspect if you look at the crank timing marks you'll see it more visibly (or drop a 'feeler', a paper drinking straw down the spark plug hole to feel when the piston is at TDC)

 

As the chain becomes worn the valve timing becomes more and more retarded (both inlet and exhaust of course), and the inlet retards a little bit more than the exhaust, due to the existence of several worn links between the exhaust sprocket and the inlet sprocket.

I to was wondering weather the timing marks on the cam sprockets are accurate. Doing a bit of algebra:

When the offset is 1mm at TDC on the cam sprockets, with a radius of 30mm, the rotating angle appears to be 1.9 deg. (Calculator: https://planetcalc.com/1421/)

Considering 2 rotations of the crankshaft to the camshaft makes: aprox. 4 deg. of crank rotation.

 

On my passat TDI you always lock the camshaft and position the non fixed cam wheel towards the crankshaft at TDC when changing the timing belt.

You probably know what I mean. That's a pitty it can't be done with the TDM.

 

When I position the marks on the cam sprockets towards the cylinder head, they are spot on. Any stretching of the cam chain between the inlet and outlet is not noticeable.

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[fixitsan]

Nothing at all to worry about imho.  Looks like there might be some slight chain stretch but I've seen far worse and they didn't seem to have any noticeable effect on the ehgines.

 

 

Why couldn't i Be so succinct !

 

 

In general a chain can lengthen and usually does wear equally by a tiny amount at each link, but I've heard of chains breaking because one link suffered substantially more wear than the others, but yeah, it's probably a rarity on TDM. But, BMW, who have a nightmare time with timing chains snapping on some models, when most links appear to be in good condition some of them have deteriorated substantially more, but that's probably quite a different example, possibly related to oil starvation

[mh1848]

After unsuccessfully trying to time the engine after shim adjustments - it became apparent after much scratching of heads and other well trained fettlers also trying their hand - i came to the conclusion that the cam chain was stretched - new chain and no timing issues -

 

my supposition was that with a bike laying up without the engine being turned over for example over winter or a longer lay up was that one side of chain is under more tension than the other and that his contributes to the stretch - the more often laid up perhaps also influencing it

Edited by mh1848, 12 March 2019 - 05:32 pm.

[dandywarhol]

After unsuccessfully trying to time the engine after shim adjustments - it became apparent after much scratching of heads and other well trained fettlers also trying their hand - i came to the conclusion that the cam chain was stretched - new chain and no timing issues -

 

my supposition was that with a bike laying up without the engine being turned over for example over winter or a longer lay up was that one side of chain is under more tension than the other and that his contributes to the stretch - the more often laid up perhaps also influencing it

 

Chains don't stretch, the pins wear