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