DR sent me the Official Timing Belt Tensioner and I have been playing :-)
The object of my games is to make a cheap alternative Here are my results, please anyone feel free to comment  ( constructively that is)

Theory.............
Tension is easy to measure in a belt when access is available to the points of external force being applied. If we could somehow measure  the side load in the camshaft bearings for example and then divide it in two we would have a figure of belt tension. This we cannot do, so other methods are needed. The approach is based on the elastic properties of the belt material. As the tension is increased the belt stretches.

So if at a given tension the overall length of the belt is X mm. then if we increase the tension of the belt by adding a given extra belt length path then we can calculate the original tension ( assuming linearity in belt length to tension over this range)
So if we increase the distance by which the belt must travel , by such means as the 'wiggle' in the Official Tool, by a small percentage, dt (%), then the tension will increase by dt as well. Knowing dt will enable us to know the value of T, the actual belt tension.
Of course in practice the actual values of tension and wiggle are replaced by easy read number scale ( 0 to 10 in this case)

All we need is a tool which will 'deform' the straight section of belt by a fixed amount and then measure the force needed.

Assumptions
Introducing a wiggle in the belt ( or a twist as per the finger thumb method) has its results also dependant on certain belt material properties, properties that can vary with age, temperature, and possibly oil contamination and humidity. It is probably best to assume the Official Tool and all derivatives thereof are most accurate on a new belt

Experimentation

Well I did several things
:-)
First I ran a a finger thumb method test, setting tension by the Finger thumb 90 degree test method several times and recording the actual tensions with the Official Tool Results, not too accurate, ranging from 3.0 to 6.3 ( Book figure of 4.5 is correct). Whilst this method puts the tension ion the correct order of magnitude it is NOT at all accurate ( no surprises here)

Next I made a little device with telescopic tubes and springs and measuring rule, the idea was that by applying a known pressure I could adjust the tension such as to allow the belt to deflect a known amount After an hour of welding, grinding and making this thing, it was only a little more accurate then the finger thumb method. Results of same test range from 3.6 to 5.8, NO ***** good

Not to give up and realizing that the problem was not with the method but with the accuracy with which reading can be made I decided to go for a variation of the Thumb twist method I cut a section of clear acrylic about 1.5 inches wide and I guess 8 inches long ( just fits between belt at air pump and the radiator. I cut a slot so it would fit snuggly between the slots on the belt. then I set the tension with the Official Tool to 4.5. Sitting this acrylic on the belt in place of the Tool, it twisted the belt by its own weight about 1.2 inches down from the
horizontal. I then ground the far end of the acrylic to 1.2 inches. Now , by simply setting the level and marking with tape on the radiator, all I have to do is to slide my acrylic tool on the belt and adjust the tension until the top edge of the acrylic is in line with the level mark. Its own weight drops it that much. This is incredibly accurate, simple and cheap. I found repeatability excellent. Setting up wit this tool gave results of 4.4, to 4.6 every time, in fact it may be the Official Tool that is not very repeatable.

So, if anyone wants more detail, measurements or whatever, just ask.
This is a cheap easy accurate method to set tension, with greater readability than the dial gauge Official Tool
Anyone see any problems with this method?
I found the belt twist to be approx 10 degrees, so belt properties are kept to a minimum

Phil
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The belt properties have more effect the more the belt is distorted from its natural line Therefore the less the deflection caused by the measuring tool the less the belt properties have effect on the measurement.  This is what is wrong with the older factory tool, that twists the belt. My tool deflects the belt very little, but is long to 'magnify' this deflection and make it visible My tool easily measures the change in tension of the whole belt when another part of the belt is twisted with finger and thumb The Official Tool can see this too, but is seen as less of a change on the dial.  Therefore I conclude my tool is more accurate and is suitable for used belts as well as new  Ill be making a drawing today and making it available for any who want to see it
Phil
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Also what exactly is the official tool measuring, > tension around the whole path of the belt or just deflection between 2 points on that side only?
Here we go again , the tensioner directly tensions the timing belt from the crankshaft to the passenger side camshaft drive pulley . The rest of the belt is running over toothed gears , the tension changes as each camshaft rotates compressing and releasing valve springs . This is why the engine must be at TDC to measure tension . The crankshaft gear is pulling down from the drivers cam gear and turns the oil pump gear along the way . The farther away from the crank gear as you follow the belt up around the oil pump to the drivers cam around the water pump pulley to the passenger cam around the tension pulley then back to the crank pulley the more work the belt must perform ; the load accumulates . This is why the drivers side camshaft is the one that breaks when the belt is too tight , the passenger cam gear only
has the load of turning that camshaft the drivers cam has the load of turning both camshafts and the water pump as the belt is pulled down toward the crankshaft . Which also brings up the warning never to turn the belt backwards by turning the crankshaft backwards . The tensioner is designed to carry only the load of the loose side of the belt and not all the force needed to rotate the camshafts . Turning backwards can damage the cup shaped bi-metallic washers in the tensioner .  The tension at the water pump pulley driven by the smooth back side of the timing belt varies depending on the alternate pushing and pulling affect of compressing and releasing valve springs on each camshaft . The belt acts much like a bicycle chain with half of it tight the other half just going along for the ride . The fact that it
is a toothed belt running on toothed gears means the length of the belt from the crank gear all the way to the passenger cam gear cannot change without jumping teeth , the tensioner is playing with the part from the passenger cam to the crank . And most significantly we are measuring the tension of a non- running engine , I believe that when running there is little or no tension at the tensioner examining the idler pulley you will note that it does not touch the belt but is needed to keep the belt from hitting it's self as it runs up to the passenger cam then back down to the water pump .
For the belt to hit it must be loose . The later style tensioners have an internal baffle / damper to control flutter to keep the tension roller in
contact with the belt to keep the warning light from coming on . Has anyone run their engine with out any covers and observed what the belt is doing at higher rpm ?? maybe use a timing light for the stop motion to better see deflection and cyclical motions, please do not drive on the street this way .
Jim Bailey
928 International
jim@928intl.com
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Hello all,

If you go to the 928 International site and look in the accessory area there is a new product called "Aftermarket Belt Tension Tool". Those of you that know me know that I can't stand taking my car to somebody when there is a  way to accomplish a maintenance job myself. Those of you that really know me know that's an understatement ;) That's how this tool was born. If interested use the link below.

http://www.mailordercentral.com/928intl/products.asp?dept=102

It's a faithful but updated copy of the original cam belt tensioning tool.
The construction is entirely stainless steel so it can't rust or go out of calibration. The tool is elegantly simple and comes with detailed
instructions with clear pictures featuring my 79 WIP and also an S4.

Thanks to Mark Anderson for helping to get this going and for putting the finished product in the virtual catalogue.

Regards,

Jay Kempf
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Jay,

I just bought one of your \$50 t-belt tension gauge tools from 928 International.  Tried it out this weekend on a friend's '87 that had just
reached 2000 miles after having a new belt put on.  As might be expected, the reading was a bit low, just outside the lower range.  So he adjusted the tensioner about 1/4 turn and that placed the gauge right in the middle of the range.

After using that other, high-dollar tool a few times I've got to say I like yours a lot better.  It's nice to have a tool that:
1) Gives consistent readings from one try to the next
2) Gives consistent readings when being tried by two different people
3) Doesn't cause a sore back from hovering over the engine trying to get the thing perfectly level, not touching anything, etc.

Simplicity is good.

Well Done,

+Greg Nichols
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Wow Greg,

Thanks,

I was away from the computer for a few days (visiting Scott Mohr and family, thanks for the hospitality) so this response is a little late.

Keep us all updated as the belt settles. The great thing about having a tool is you can check it every 1k miles if you want.

A lot of people asked about using the new tool on an S4 and certainly that was done for about a year before the tool was introduced but independent verification is welcome and would give people good feedback. I am biased of course so my opinion only counts like any manufacturers opinion that wants you to buy their stuff.

Jay Kempf

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I set all of the belts I do to the top of the edge on the Kempf tool, this should cross to the 9201 tool and be at about 5.0

My Kempf tool, when the window is closed, is 5.3 on the 9201 tool. 5.0 is about 80%.
I set new belts to "closed" (5.3) and retension to 80% (5.0).

I agree with Mr Merlin for 32V cars. Setting your belt in the center of the Kempf tool "notch" or "window" works but it's not 5.0 according to my calibrated 9201.

I've checked the Kempf tool with the 9201 tool about a dozen different times. On MY Kempf tool, 90% of the window covered is 5.3 on my friend's 9201 tool. I put a paint line on my Kempf tool at that point, and always reset the tension to that line.

My Kempf tool looks like yours. Mine is calibrated against the Porsche 9201 tool to the high side so far that hardly any of the window shows (for a 5.0 setting). In the center of the window is about 3.0...which works ok on older non-32V cars, since the tool was designed using one of them. However, If I understand you right, and your tool indicator doesn't even reach the window. I'd guess that you're about to throw a belt.
Better watch it. That would be way too loose....If I understand what you're describing correctly, you could be walking soon and facing a huge repair bill. If my belt were that loose, I'd either adjust it correctly before driving or have it tailored until i could.

Way to measure belt with Kempf is to use 9th free opening counted from cam gear. This is right at edge against center cover and on top of tensioner housing.

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calibration is easy with the Kempf tool!!
I just did a test with a 5lb weight and some speaker wire. no disassembly necessary. Just feed the wire through the Kempf tool and hand the weight. 5lbs goes to full scale.
check out you Kempf tool !!!
Mark Kibort

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Here is a "universal" tension check tool. no experience with this however....