Cam/ Timing Belt Questions Help Please ...

[Piglet]

Years ago I had a Renault5 Diesel, great car.
When I bought a belt for it from Renault it had the timing marks printed on the belt, I just rotated the engine until all pulley marks were visible then slipped the old one off.
The marks were just visible on the old belt so I was able to check they were the same before re-fitting.

I believe modern belts are Kevlar. I know Kevlar core rope stretches, but only a fraction of 1%.

[liteace]

Your old Ren 5 didnt have electronic, pilot main and post injection, so timing wasnt a big problem. The reason they have to be setup like they are is so they run 100%, the old trick of marking the pulleys, yes it works and the engine runs but not at 100% efficiency

[Piglet]

Your old Ren 5 didnt have electronic, pilot main and post injection

Happy days

[Boghopper]

Of course the belt will deflect and stretch that's why they have a spring loaded tensioner to compensate for the discrepency as they age. And that's why Ford tell you to replace them. If you put the timing pin in the block and bring the crank web onto it with no.1 at T.D.C. in many cases the cam locking bar won't fit because the belt has stretched and the cam timing has retarded. The straight run (load side, on the top) on the belt isn't as tight as it once was and the cam isn't being turned quite enough to allow the bar to fit.

I had this happen to me last weekend on my Connect. The only way to advance the cam is to remove the belt with the crank pin still locking the crank and just ease the cam around a fraction to bring the cam to its rightful position, which is timed up with the crank, then you put the new belt back on and tension it. If you cannot be bothered locking the crank, then when you lock the cam off with the locking bar the crank timing will have advanced by whatever the cam was retarded by. You will just transfer the error from the cam to the crank. Why do you think Ford go to the bother of casting timing holes into the block that align with the crank web, because they were bored and had nothing better to do? No matter how many times you want to try and avoid this logic, it is the only way to correctly time the crank to the cam and correct any wear 60,000 miles has put on the belt. I also draw the cam sprocket off so as to allow the tensioner and belt to find their prime position before I adjust the tensioner and retighten the cam sprocket. And let me just say my van feels a great deal nippier than it did. It is eleven years old with 63,000 on the clock and it is the first time it has had the cam belt changed.

[axxeman]

I'm not sure about the belt stretch vs new belt topic. The belts have fairly coarsely spaced teeth, and from what I have seen, it would be damn near impossible to be one tooth wrong. The error would be massive and the thing would not start. The new belt, after being placed where the old belt came off, will correct any stretch error by pulling the crank back to where it should be if the cam is locked and the crank is floating. Probably less than half a mil too.

Also a van "feeling nippier" isn't a particularly scientific test. I always "feel" that mine is driving better after I have done something to it - even down to inflating the tyres back to where they ought to be! The mind is a powerful tool and, psychologically, we like to pat ourselves on the back for a job well done. Human nature ...

Greg

[Piglet]

All true
BUT
If the old belt has not been re-timed since first fitted then a new belt will bring the timing back to what it should be.
Subject only to manufacturing tolerances of new belts.

No-one makes a fuss about their timing going off as the belt ages. Which clearly it must.