Setting Timing Belt Tension At Tdc

I'm confused now so I'm going to ask more or less the same question Cream asked at the start...if a cylinder's valves are both closed when it's at TDC (end of 'squeeze' stroke or end of 'blow' stroke) why does it matter which TDC stroke you use to measure the tension of that cylinder's belt? Its cams will surely both be unloaded for both?

 

when your teaching you have to show the right way first. only experience will tell you what you can get way with.
i know mechanics that get right stressed doing t.belts? when its one of the easiest jobs you can do. no dramas

 

But...but...if a piston is at TDC how can the valves be open? There's no room. If there was then we wouldn't have to worry about bent valves

 

you get a bit of overlap. look at some more vids

 

At top dead Center on the firing stroke the valves will be closed and the cam lobes are at ' rabbit ears ' facing away from the rockers / buckets / Desmo actuating arms
On the other top dead ( on the rock ) the valves are just open , normally on car cylinder heads as a rough guide the inlet will be opening by about 1 mm and the exhaust will be still open by 1 mm but closing .

So at the smaller openings on the Rock the springs are not under a lot of tension because the valves are only just open .

Now I haven't worked on Desmo engines but I hear that the spring weight is not as much as say a over head cam and bucket set up
Derek I see what you are saying about the trying to turn the cams against the springs but is that were the cams are at a lot of lift therefore more Spring weight or do you mean the cylinder that is firing the cams will be un loaded but you can still feel the spring weight .

What I'm getting at the folk who have timed it in the wrong position say with a tension guage there has been no difference between the the vertical and horizontal cylinders maybe because spring weight is so slight ' on the rock '

 

WTF are people talking about with 'spring tension' on a Ducati engine?

It's nothing more than Mother-in-Law's hairclips..................nothing like an 850 Guzzi engine....

Look, FFS sake, the distance between the cam and the camshaft pulleys doesn't change as the engine is rotated............so how does the tension change?

The only time that might only occur is when the engine heats up......

Stop effing about and just adjust them..........otherwise you will end up adjusting them by taking the atmospheric temperature into consideration as well.

Or whether your missus is wearing black knickers instead of pale pink ones.....

 

exactly what I found aswell, this was also using the ducati tension gauge which ive now sold, wont work with the testestretta

 

When all the timing marks line up, the vertical inlet cam pulley is applying a force to the belt. The drive pulley is effectively locked so the belt tightens on one side and slackens on the other. If the inlet pulley wants to rotate clockwise it will increase the tension on the section of the belt you're tryna play Stairway To Heaven on and you'll hit F-sharp instead of F.

 

Thanks for explaining that.

 

Very good Chris good picture of rabbit ears , maybe gazelle ears or what ever , but remember if the inlet and ex manifolds are in an opposite orientation the rabbit ears face towards each other .
I'm going to see if I can find the 20 page long postings on tightening a bleed nipple just to give balance to this one

 

The pulley isn't really applying a force.........there is only a tiny spring pressure from the rocker......the pulley can be moved with one finger..........It isn't anywhere near the force applied by conventional valve springs.

........if it is rotating clockwise, then that's making the adjuster side of the belt 'looser' and the other side may be a tiny bit tighter but not enough to cause a problem with adjustment bearing in mind how tough the belts are...........however, with all the timing marks lined up, the tendency is for the pulley to turn slightly anti-clockwise.

There is much more tightening of the belt by the engine heating up and expanding........which is why some people adjust the vertical cylinder belt slightly slacker as it runs hotter than the horizontal cylinder.

.............as I said, Grandma's hairclips.

 

Let's try to clarify.
On the power stroke tdc, the piston has come up, compressing the air/fuel mixture, ready for the plug to ignite it and, once ignited, the expanding gasses then send the piston back down the cylinder.
For this, both exhaust and inlet valves need to be firmly closed, so there is no load on the cams and rockers.
On the other tdc, the piston has just finished pushing the spent exhaust gasses out of the cylinder through the exhaust valves and is about to pull the new mixture of air and fuel into the cylinder through the inlet valves.
A very basic engine might have both exhaust and inlet valves closed at this point with the exhaust valve having closed and the inlet valve yet to open.
However, our engines are anything but basic and use what is called valve overlap to enhance performance.
This means that the inlet valves open before the exhaust valves close. Therefore at this tdc, there is load on the cams and rockers because both valves are slightly open, although not open enough to contact the piston.
Therefore, it is recommended to take your measurements on the power stroke tdc, when the valves are closed and everything is unloaded.
Whether this will actually make any difference to your in-service belt tension is another matter.
@Mr.R doesn't appear to think it will and he is not inexperienced in these matters.

 

I had a feeling it wanted to rotate anti-clockwise rather than clockwise, so my thinking was flawed anyway. Gonna stop worrying about it now...

 

Maybe you're thinking of mechanical watches, where you were told to turn the hands forwards to adjust them, never back...

 

Rather than help this thread devolve even further it's probably best to just part-quote Derek's post (which I 100% agree with) i.e. the tension of a timing belt on a 4valve measured between the two camshaft pulleys when the inlet cam is in a position where it's trying to close both valves is definitely different to the tension of the same belt measured in the same place when all 4 valves on that cylinder are closed. As already said, the helper springs on two valves are quite powerful and almost defeat the argument for lower parasitic losses that were made and evident on early desmo engine designs. How much of a difference re: above is another matter. I don't bother to set up 2valve belts on each TDC firing. It would be good to get an input from raceteams/Ducati service specialists on above - pretty sure I already know the answer.

 

well. there's a good chance i have fitted more belts than most.easy over a thousand. albeit on cars with proper valve springs bigger pulleys and slower revving engines. but with much higher intervals 50k used to be the norm nearer 100k+ now. in the last 10 years or so vernier pulleys are becoming the norm so following procedure is becoming more essential (get it wrong and you will get phase errors at best) but pre tensioned pulleys do make it easier.
but previous to that the only special tool you would need was a bottle of tipex and the ability to recognize about 70-90 DEg of twist. although it is essential to know the correct procedure, certainly i have learned a thing or two on this thread but unless you ARE a race team and are looking for every 0.0001% of performance i wouldn't get to stressed. an over tight belt will often howl. a slack belt can cause erratic tick over. both will cause premature wear.
if its any consolation. i still know mechanics that get stressed when fitting belts.
you want try fitting a belt to a 24v v6 along with water pump in 3" of space then watch the owner red line it as he/she drives away knowing that they wont lift the bonnet again unless the screen was bottle runs dry.
NOW THAT IS STRESSFUL.
touching wood right now.