1982 Mirafiori 2.5l diesel

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1982 Mirafiori 2.5l diesel

darneoshiel

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1982 Mirafiori 2.5l diesel Timing belt

I recently fitted a new timing belt and need advice. Timing belt fitting....should the tensioner be locked tight or should tensioner be left loose to activate on the spring. I am getting different advice here so its unclear to me.
 
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Re: 1982 Mirafiori 2.5l diesel Timing belt

I'll see if I have Fiat Workshop Manual for any diesel engine on that era. (Update - just looked but not for the 2.3 but for the 1.6D then the tetioner is locked down)

HOWEVER! For Fiat petrol engines of the 80s (e.g. 1.6 and 2.0 L twin cams) then the belt tensioners was:

1) A smooth NON toothed sprung loaded pulley
2) You fit the belt, rotate the crank (smoothly) for several rotations
3) You then lock the tension down

Later Fiat tensioners which you don't lock down a toothed AND you have to adjust the tensioner into an operating "windows" indicated by a pointer which has to be aligned with a marker.

I'm pretty sure you have the early tensioners where the spring is designed to set a tension and then has to be locked off.
 
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Re: 1982 Mirafiori 2.5l diesel Timing belt

Later Fiat tensioners which you don't lock down a toothed AND you have to adjust the tensioner into an operating "windows" indicated by a pointer which has to be aligned with a marker.

I'm pretty sure you have the early tensioners where the spring is designed to set a tension and then has to be locked off.

Digging down into the unreliable morass which, these days, is my memory I seem to remember that there were a number of manufacturers who attempted to achieve a correct belt tension with the use of a spring on the belt tensioner. Wasn't the Ford Pinto like this? The idea was that you assemble the whole shebang - which included a powerful spring tensioning the tensioner pulley - but you left the tensioner locking nut about a half turn slack. The engine was then rotated D.O.R for a few revs which settled the belt and allowed the spring, which was exerting pressure on the tensioner, to set belt tension. Then the tensioner locking nut/bolt was tightened so locking the tensioner. This type of tensioner is quite different to the commonly found self adjusting tensioner often used today which has the ability to compensate for variations during the life of the belt. The big difference being that the spring on the earlier type is used only to achieve tension when fitting the belt. Once the locking bolt is tightened the spring ceases to have any effect. With the later type the spring continues to apply pressure to the tensioner during operation so can compensate for small changes in belt tension as the belt ages and settles. The big "give away" as to what type your's is would be that I would expect to find no lining up marks on the earlier type but usually the later type has a system of pointers (commonly two) which, as you tighten the tensioner, come into alignment to indicate that tension is correct.

Hope that's of some use?
 
Re: 1982 Mirafiori 2.5l diesel Timing belt

Having reflected (above) on the different spring tensioner types, it's set me to thinking about how much of an advance the newer self tensioning assemblies are.

When I was just starting out in our trade timing belts were almost unknown. Mostly cams were driven by gears or chains. Then, all of a sudden, in the early '70's, timing belts were everywhere! Ford Pinto's - in Cortina, Capri, Granada, etc. BL's "O" series - in just about everything from Marina to Princess 2 to Light delivery vans. All the major manufacturers seemed to think they were the greatest thing since sliced bread! In fact one of the very first I ever did was a Fiat 128. At that time the only application I'd seen them in use was when the American Commandos Drag racing team came over to Santa Pod and the big blown V8s used them for blower drives.

These earlier applications had fixed tensioners which left it up to you to apply the correct tension. Some, like Ford's Pinto, used a spring to try to take the uncertainty out of the equation. There were varying opinions on how tight the belt should be and it wasn't until some time had passed before tension checking tools became generally available. In the early days, before people became really familiar with them, it was not unusual to hear an over tensioned belt whining away gently in the car next to you at the traffic lights!

Of course, as we know well, over or under tensioning a belt will shorten it's life so, over the years, I've reflected on it in depth. I just love the newer self compensating spring loaded family of tensioners which most manufacturers now seem to be using, as they take all the guess work out of it. Where a "fixed" tensioner is involved I evolved my own way of doing it fairly early on and it has served me well over the years. Tensioning timing belts does seem to be a subject which often comes up so I hope it might be of interest for me to say here how I do it where a fixed tensioner is in use?

So - Having fitted all the parts you are renewing and with all relevant timing marks etc lined up correctly (engine cold) move the tensioner until the belt is tensioned much as you would with a fan belt - by which I mean tightened to the extent that it does not "flap" but also is not "bow string" tight. This stage is not all that critical but you don't want slack which might allow the belt to jump teeth. Now double check your timing is correct by looking again at your timing references (locking tools or, maybe, simply marks on pulleys - It'll depend on your engine) and if all checks out turn the engine by hand (socket on crank nut usually) for at least two complete revs to check no valves are contacting pistons. This should be done with plugs out so the engine turns easily and smoothly so you can feel if any contact takes place. Also do it slowly or the weight and momentum of the flywheel will bend the valve before you can stop the crank rotating!

If everything checks out start the engine and warm it up to full working temperature. You need to get it hot to the point of the fan cutting in as you want the whole engine, as a unit, to be fully up to temperature and that will not be achieved by simply watching the temp gauge hit "normal". The reason this is important is because with heat the engine will expand, especially so with aluminium components, so the crank, cam and water pump/aux shaft centres will move slightly away from each other compared to when they are cold. The problem here is that timing belts are not "stretchable". They can't be to work properly as any stretch would cause variations in cam timing! So if you fully tighten a timing belt when the engine is cold it's going to be too tight when hot!

So now, with your nice hot engine, (and key out of the ignition!) get your socket back onto the crank sprocket and just slightly turn the crank DOR until there is absolutely no slack observed on the driving side of the belt runs (usually crank to water pump/aux shaft and then on up to the cam sprocket). Any slack will now be on the return side from the cam sprocket to the crank and you should now adjust the tensioner so that this slack is completely removed. Moderately firm finger pressure is all that's required. You are not trying to get the belt "bow string" tight, just to completely remove any slackness. Specialist checking tools are available which are used when the engine is cold. I've used these and been surprised at how little actual tension they apply. When you've got it right you will find that when the engine is completely cold there is just the smallest "touch" of slack detectable in the longer runs of the belt. Of course on more modern engines with the spring loaded type tensioner you won't see this as the slack is always taken up by the tensioner. It's very interesting to look at the tensioner pointers and compare them when cold (you should have lined them up when fitted so they overlap - if in doubt google for an image.) and when hot. I've always noticed they are significantly differently placed when hot which seems to me to validate the theory that the Crank, Cam and Aux/water pump centres move as the engine heats up?

If I'm doing a belt on one of our own cars, (or a close friend where I can easily subsequently get my hands on it) and it's got the "fixed" type tensioner, I like to check it out after a few thousand miles (or, say, a couple of months) after fitting. It's very common to find the belt tension has gone a bit "slack". My belief is this is not due to the belt stretching but rather to settling and compaction of the driving face of the belt. A small adjustment might be beneficial. However when I was doing this in the workshop the car just had to survive with the original setting and we didn't have customers lining up to complain. There is a well known way to roughly check tension where the belt is gripped between thumb and forefinger, on it's longest run, and considered to be approximately correct if you can twist the belt through 90 degrees. I don't think too much of this as it is too variable due to different length of belt run on different engines and widely varying finger strength of different people.

Anyone else got their own favorite way of doing it or want to criticize/add to what I've detailed above?
regards to all
Jock
 
Re: 1982 Mirafiori 2.5l diesel Timing belt

No real criticism from me. I've lost count of the number of cam belt changes I've done.

Before I forget it was FIAT who were the first car manufacturer to use a cam belt for crank/valve timing in a production car. It was Aurelio Lampredi and his Fiat Twin Cam engine in the Fiat 124 that did this. The engine was notable for monoblock technology, belt driven camshafts and its aluminium alloy head.

I've never had to re-adjust cam belt tension between it's cold and hot running state.

On static tensioners (be they sprung loaded and then locked or adjusted by other methods) I verify the tension both hot and cold not only by the 90 degree belt twist but also by the dynamic revving and slowing of the engine. Here I'm looking for *excess* to *excessive* "whip" on either side of the tensioner AND on the pulled long run side from camshaft to crank sprocket. Here it is now the natural engine dynamics that are putting the belt under both pulling/drive tension and relaxation tension states.

If there is absolutely no whip or minimal whip then the belt is potentially too tight. This often confuses people as they fear that the belt could jump a tooth or more.

For a belt to jump a tooth the belt has to basically ride up in tooth height over the full contact/wrap/engagement with either the cam sprocket(s) or crank sprocket. In rough numbers a single tooth is say 3mm. So if there was only a one tooth sprocket/drive then the belt would need to be 3mm too long/loose. At the crank sprocket (where we have the smallest wrap) then we are are taking around 5 teeth. Roughly you can multiply number the smallest number of engaged teeth by the tooth depth. So for 3mm and 5 teeth there needs to be getting on for 15mm of slack before a jump can occur.

Now for those that have fitted cams belts 15mm is quite some length and often all the clearance you get before any tensioning by the tension is done.

So it is absolutely natural and normal for a modest amount of belt whip. This is readily verified with a modern dynamic tensioner system. Even though the tensioner is modulating up and down the belt will still display a small to modest about of whip.
 
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Re: 1982 Mirafiori 2.5l diesel Timing belt

And I've just done an alternative validation of my tooth skipping measurements.

Fiat CAM belt teeth are 3.5mm. Typical wrap is 50% of the crank pulley.

So applying simple geometry of radius change.

Change in belt length for a tooth jump is:

L = (2*pi*r2 - 2*pi*r1)/2 = 2*pi*(r2-r1)/2
L = pi*3.5
L = 11mm (10.997)

So not quite the approximate 15mm I crudely estimated/mentioned but 11mm is still a significant length of belt.
 
Re: 1982 Mirafiori 2.5l diesel Timing belt

A wee comment on my video above of the supercharged engine. I've never seen a tensioner on a timing belt which runs on the inside of the belt however they commonly run them on the inside of these blower drives - strange?

Also, regarding how much slack you need in the belt before it can jump teeth, I've often come across a belt which had so little slack available that it is really quite difficult to fit it round the sprockets. Sometimes you've got to leave the tensioner off and fit the belt first then fiddle the tensioner into place. I now find myself wondering if this is intentional to make "tooth jumping" (is there such a term?) an unlikely possibility?
 
Re: 1982 Mirafiori 2.5l diesel Timing belt

Pugglt Auld Jock's links to those videos actually show how "tolerant" the cam belt drive system is to variations in both setting and wear. Look at those belt whip and waggles.

To my knowledge cam belt failures are in most/all cases due to manufacturing faults, belt crimping/abuse, over tensioning, or external contamination, debris etc. entering the normal operation of the cam belt drive chain.

Good designs should include tolerance and foreseen abuse. This is a fundamental engineering good practice.
 
Re: 1982 Mirafiori 2.5l diesel Timing belt

Just came across this which I thought might be of interest:



Absolutely no use to me as my mobile 'phone is "steam driven" - just about manages calls and texts! The downside to it might be that those of us obsessed with perfection could spend hours trying to adjust it till it hits the recommended frequency exactly? Then, 10 minutes later, it'll be different? - Wonder where you find out what the frequency for your actual engine is?

Oh dear, Thinking about pursuing perfection my brain has gone off at a tangent again. When I worked with tyres we had some absolute state of the art balancers (as we were working with racing tyres). One day we decided to try balancing a wheel/tyre assembly, dismount it from the machine, remount it and check the balance again. We tried this several times - and I've tried it a few times during my life on different machines - and it never comes out with a perfect result the second time? I've come to the conclusion that it's the inability of the machine to remount the wheel in exactly the same relationship to it's hub which is the problem. ie manufacturing tolerance. Anyone else tried this? I'm mildly surprised at this outcome.

Anyone recognize the engine? That exhaust manifold is quite distinctive looking isn't it? The belt teeth are quite widely spaced, like the teeth on our old MK1 Panda, and the tensioner is obviously not of the "automatic" type, so I'm guessing it's probably an older model?
 
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