Timing belt stretch?

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LFY
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Timing belt stretch?

Post by LFY »

Hello everyone.
Recently, I replaced my timing belt, water pump and tensioner. The only thing I didn't change was idler bearing because it had done only 10K miles. I drove my car for 100 miles and noticed some knocking noise coming from timing belt cover. After pinpointing the source of the noise with a rubber hose, I found that the idler bearing has failed. So, I had to remove the belt again to replace it. (The noise disappeared after changing the idler bearing)
Now, this is where the story begins.
This time (I mean the 2nd time) I noticed that installing the belt on the car was easier than when the belt was brand new.
I mean I needed less force to install the belt on the tensioner pulley.
Does it mean that a timing belt experiences an initial stretch when it is first installed on the car?
I also noticed an interesting point in Haynes manual which said: According to Peugeot, a belt is considered an old belt even after 1 hour of use! Only one hour!

Now, what I learn from this experience is that:
It is better to re-adjust the tension after a few miles of driving with a brand new belt to take up the minimal slack which is caused by the tiny amount of belt stretch.
I know this stretch is negligible and probably won't cause any noticeable change in engine timing but with proper tension, the belt lasts longer.

What do you think about this?
The car is a Peugeot 406 1.8 16V (XU7JP4 LFY)
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GiveMeABreak
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Re: Timing belt stretch?

Post by GiveMeABreak »

On some of these 1.8s special tensioning equipment is required when replacing the belt. I think it's referred to as a 'SEEM C.TRONIC 105.5'
seem.PNG
Details here:

viewtopic.php?p=183978#p183978
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LFY
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Re: Timing belt stretch?

Post by LFY »

You are right but this equipment is used for the early versions which were equipped with the manual tensioner.
My car is the latest version with the automatic spring loaded tensioner and doesn't need any special tools.
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white exec
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Re: Timing belt stretch?

Post by white exec »

Brand new belts tend to be quite stiff, and after a bit of running in do become more flexible.
There would be a very small amount of stretching for a new belt, but that wouldn't be significant, and wouldn't affect the tooth-to-tooth pitch of the belt.
Provided there's no visible 'thrash' of the belt at driving revs, all should be well if it was correctly tensioned when fitted.
Re-fitting a belt (which hasn't got near the end of its life) shouldn't be a problem, and will just serve to re-tension it, which does no harm.
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LFY
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Re: Timing belt stretch?

Post by LFY »

white exec wrote:
15 Sep 2021, 11:53
Brand new belts tend to be quite stiff, and after a bit of running in do become more flexible.
There would be a very small amount of stretching for a new belt, but that wouldn't be significant, and wouldn't affect the tooth-to-tooth pitch of the belt.
Provided there's no visible 'thrash' of the belt at driving revs, all should be well if it was correctly tensioned when fitted.
Re-fitting a belt (which hasn't got near the end of its life) shouldn't be a problem, and will just serve to re-tension it, which does no harm.
Thanks for your answer Mr. Chris! I always enjoy reading your posts which are written like an expert.

So, do you think the reason for the easier installing of the belt for the 2nd time was that it had become more flexible (and not stretched)?
Do you think it is a good idea to remove, re-install and re-tension a new belt after a few miles of driving to achieve a more precise timing alignment?

I think the bold phrase needs more explanation.
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white exec
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Re: Timing belt stretch?

Post by white exec »

I think very few people would bother to re-tension a toothed timing belt after fitting a new one. Dealer workshops certainly wouldn't. If you were doing the job yourself, I suppose you could do it — it would do no harm, and would leave the belt at the specified tension.

Timing belts are also fairly 'heavy' things, not lightweight, and tend not to thrash (vibrate) as much as ribbed Auxiliary (alternator/AC/steering) belts, which can also have quite long runs between pulleys, and are more lightweight. Aux belts do stretch over time, and can benefit from having their tension checked.

Fitting of new timing belts usually includes the instruction to fit the belt, position the tensioner, then turn the engine over by hand at least twice. This is not just to check that timing hadn't accidentally been lost, but also to cater for the new belt settling into its path after an initial bit of stretch.

Have found the same with a new (or re-fitted) Aux belt. Fitting then re-tensioning has usually reduced visible belt thrash.