Belt Tensioning - Lucky?!?!?

Icey1174 · 07-23-2009, 12:12 PM

Belt Tensioning - Lucky?!?!?

Over the years I have read over and over and over again about the need to properly tension your timing belts and then again to recheck them.

2 1/2 years ago I put my belts on and tensioned them snug with no slack. They were not wrenched on so tight to stretch them, but sung that they would not jump a tooth. I did not check the tension with any measuring devise.

I have also read different posts where people using two different tension measuring tools get very different readings. Throughout my professional career I have taken a couple classes on Metrology, the study of measurement. We learned the science of measurement and learned how to properly calibrate and measure with dozens of different measuring devices. Two different people can take the same caliper or micrometer, measure the same standard, and get two different readings. Although the readings will not be miles apart, they can make a huge difference. The proper way to measure is to calibrate the tool to a known standard yourself then measure your sample. People do not calibrate all the time, that's why they have their own tools. With each person calibrating the same measuring devise on their own they should get the same measurement on the sample.

Are all these novice users of this tool measuring the tensioning properly? Is this tensioning accuracy more so a big scare because of the damage it can do? Is it a big scare because it is a known failure point and Porsche put out such a very tight set of specs to virtually ensure zero failures? Has the belt technology and manufacturing process been refined enough over the years to produce a high quality belt with high quality standards that the very accurate tensioning does not have to be so accurate anymore? Or was this Porsche's answer to potential warranty claims to save the company money?

I agree that it is always better to be safe then sorry. Just looking for some input from people that have experience with metrology, engineering, and quality control. I am also looking for input from anyone that has done their own belts and their experience with using the tool or if they flew by the seat of their pants as I have done. I doubt many will be brave enough to admit here they did not tension with the tool. And maybe I am the one lucky man on Earth!

Thanks guys!

krystar · 07-23-2009, 12:58 PM

i'll be brave and say i've tensioned w/o the tool

Icey1174 · 07-23-2009, 01:00 PM

Krystar - any other input on your thoughts of the exact tension "scare?"

Thanks for the honest input. I will continue to tension mine by hand but I will adhere to the three year interval.

speedracing944 · speedracing944's Garage

I use P9201 and calibrate it before each use using the calibration bar supplied. When I take readings I take 3-4 separate readings on each belt each time I change the tension. There are many sources of error which contribute to the total uncertainty in the belt tensioning. You can use partial derivatives to calculate the total uncertainty which exists if one chose to do so.

Speedy

mattdavis11 · 07-23-2009, 02:30 PM

Never had a problem since I quit using the P9201. Here is the tool I use...

cjpacitto · 07-24-2009, 08:34 PM

Got belts on tonight but not tensioning till tomorrow. Will be using both my hand and a krikit.

Here are my thoughts on your questions (btw, I am a professional engineer)....

Calibration is very important. I run a testing lab and have plenty of experiecne with the difference a proper calibration can make in a test result. Most tools are close even if not calibrated. In this case the difference in the results is noticeable but minimal. Occasionally, a tool that has not been calibrated can be WAY off making a huge difference.

Different users can certinaly get different results from the same tool, but those variations are multiplied when one (or both) of the users are not skilled at using the tool. There are plenty of things you can do with a simple torque wrench to get it to read differently on the same bolt. Why would a tensioning tool be any different?

The belt has a job to do. So long as it is doing the job the tension really does not matter. As soon as the belt stops doing its job the tension becomes very important (-:

Having the belts too tight will result in accelerated wear of the bearings in the rollers.

The belts can be a little too tight or a little too loose with no adverse effects. If the belts are way too tight or way too loose the effects can be bad. The belts will stretch over time so starting off with the correct tension (or maybe just a tad tight) will probably help keep the belts within the acceptable range for a longer period of time.

When I removed my old belts I was a bit surprised by the excessive amount of slack they had developed.

My new belts are the Gates ones with kevlar. Supposedly they "virtually eliminate belt stretch" We'll see. I wonder if I should still use the "new belt" tension spec or if the "used belt" tension spec would be more appropriate? Maybe I should go 1/2 way between the two?

cvriv.charles · 07-24-2009, 09:01 PM

I used my hand,..... even though I had the 600 tensioner. Is it still 600 beans?

Icey1174 · 07-24-2009, 10:01 PM

Thanks for the professional input CJP. I will be using the same belt the next time and I will again tension by hand.

cjpacitto · 08:01 PM

Oh this is great. I carefully and painstakingly tension my belts using the krikit. I get them both dead nuts on where I want them. Once I'm happy I go to put the "extra bits" in. You know, the cam belt idler roller, the balance belt idler, and the metal guide that goes on the water pump. Guess what? Can't get the metal guide on because the lower stud for it is directly under the spring tensioner arm. Screwed with it for 20 minutes... it just wasn't going to happen. So I loosen up the nuts on the spring tensioner, move it out of the way, and get the metal guide and it's nuts in place.

Now that the metal guide is in, it is not possible to use the krikit to tension the cam belt because there is no longer enough room. Tried several times but no luck... could not get valid readings. So, after the best of intentions to do it the "right" way I ended up with no option but tensioning it by hand.

Oh well, should be fine. The engine startted fine and runs great.

btw, the krikit tool was fairly easy to use and gave pretty consistent results when used properly. It was finicky though if you did not apply the pressure to it in the right direction and would give you inaccurate readings .

VII · 07-26-2009, 11:26 PM

As far as I know you cannot use the kricket on 87+ cars because of the spring tensioner. I plan on doing my belts soon which means my only likely means of tensioning is my hand? What parameters did you abide by to tension by hand there cjpacitto? I have read things here and there but not a whole lot about 87+ spring tensioner belt tensioning techniques.

cvriv.charles · 07-27-2009, 12:24 AM

LOL. Im done with the tensioning tool. If you do it by hand and your off a bit,... it wont make a difference. I think the most important thing to remember is the check and recheck with a newer belt. retension. Thats all.

Isnt there a belt twisting rule too? Belt shold be able to twist no more then so and so. If it cant then your at about the right tension?

emoore924 · 07-27-2009, 09:17 AM

It would be interesting to have someone (several people) tension by hand, then go back with the tool and measure whether what they'd done was in spec or not...

cvriv.charles · 03:36 PM

Quote:

Originally Posted by emoore924

It would be interesting to have someone (several people) tension by hand, then go back with the tool and measure whether what they'd done was in spec or not...

No it wouldnt. LOL. It would,... but I could assure you that with a precision tool, every single one of them would be way off. As another member here said,... the tool was probably an insurance policy because someone down the line said,... "You have the entire engine timed with a rubber belt?"

Seriously,... to me thats kind of like manufacturing the short block out of cheese. Something so delicate with such an important job.

"But,... you have to use this type of cheese because it's firmer and holds together better." In the end,... it's all cheese. Something delicate with an important job.

speedracing944 · speedracing944's Garage

Quote:

Originally Posted by emoore924

It would be interesting to have someone (several people) tension by hand, then go back with the tool and measure whether what they'd done was in spec or not...

We can always do this at the Indiana get together. We can pic an expert belt twister and put him/ her to the test to see if they can beat the P9201. We should wager bets on how close the person is. Any belt twisting experts going to the Indiana GTG in August?

Speedy

mattdavis11 · 03:00 PM

How do you beat a 9201? I can't think of a single reason I would want to be on scale with a 9201. You could never convince me who is right, wrong, close or far off. When you loose all but 9 teeth on one side of the balance belt after using the tool, you'd have the same attitude towards it I do. It doesn't really matter what it says, 2.7, 3.2, 3.5, 4.0 or whatever, the hand keeps the belt intact, the tool doesn't.

It's a rubber belt, you don't need a p9201. Porsche has a lot of followers convinced that you need rocket science equipment to do a simple job. I think it's funny.

cvriv.charles · 07-27-2009, 03:35 PM

Quote:

Originally Posted by mattdavis11

How do you beat a 9201? I can't think of a single reason I would want to be on scale with a 9201. You could never convince me who is right, wrong, close or far off. When you loose all but 9 teeth on one side of the balance belt after using the tool, you'd have the same attitude towards it I do. It doesn't really matter what it says, 2.7, 3.2, 3.5, 4.0 or whatever, the hand keeps the belt intact, the tool doesn't.

It's a rubber belt, you don't need a p9201. Porsche has a lot of followers convinced that you need rocket science equipment to do a simple job. I think it's funny.

LOL. It's funny because you see this kind of thing everywhere!

speedracing944 · speedracing944's Garage

Quote:

Originally Posted by cvriv.charles

No it wouldnt. LOL. It would,... but I could assure you that with a precision tool, every single one of them would be way off.

Are you claiming that with multiple P9201s they will get readings which are way off from each other? Same belt, different P9201s = vastly different readings?

Speedy

John_AZ · 09:09 AM

Where to begin???

The Contitech belt is all that is needed if you adjust it properly and change it every 2 years. $11.75 from Pelican is cheap INSURANCE. Even if you only drive the car a few thousand miles a year the belt is subject to your leaky, oily, coolant dripping, front of engine.

The Kevlar belt is $100 plus. Spend it on a shop tension if you are worried.

cjpacitto said "a tight belt is bad on the rollers (btw, I am a professional engineer)".
The worst thing is the WP bearing being pressed by the overtightened cam belt. And remember, we tension when the engine is cold. The engine expands slightly as it warms up and further tightens the belt on the WP and burns out the bearing.

I do not own the P9201. My test is the "feel" of resistance between the cam belt and the WP pully movement by grasping and turning the WP pully. It has to move with mild force. Reason?- again the engine will expand when at operating temperature The feel is learned just like the 90 degree method.

There are good sites explaining the use of the P9201, the Krikut, and the www.Arnnworx.com tension tool, P920x.

Here is a link to make your own DIY tension tool to add to the list of confusion.
http://www.ehadesign.com/~dsmith/timing_belt.htm

GL
John_AZ
1988 924S 60K + 1987 924S 113K
1979 924 + 1977.5 924 + 1970 914 1.7 + MG midget +++===gone, past tutors

ballysdad · 07-27-2009, 05:53 PM

There had to be some sort of measurement and tool for the Porsche mechanics when this car was under warantee to go by.

JonA · 07-27-2009, 05:56 PM

I have the Arnnworx tool and use it for setting the tension and so far the tool has agreed with my intuition. When done it feels right, the 90 deg twist is there and it has to hold the water pump pulley just tight enough so you cannot spin it easily by hand. I don't worry about it too much after that.
I'm guessin that belt failures are more a function of belt age and neglect than improper initial tension setting.
On my 924 there was no tool specified to tighten the belt. It was all seat-of-the-pants and there were never any problems. The engine in my boat is an old ohc volvo and that belt has never been a problem eithor. With the non-inteference heads a problem would not have ruined my day but my point is maybe we all get a little too crazy with this.

Jon