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Timing belt conundrum

Discussion in '308/328' started by Irishman, Aug 9, 2019.

  1. yelcab

    yelcab F1 Veteran
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    Nov 29, 2001
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    One of the factors that make the cam timing in a Ferrari a pain in the ass if the index pin does not have a threaded hole (like a Porsche 911) to allow easy removal and insertion from one hole to the next. I have always struggled with that pin … until I recently found a solution from Superformance. Check out the index pin with a 4mm x 0.75mm threaded hole. That hole makes it a 15 second job to remove the pin and insert it into the next position.

    If you have done it before, have all the homemade tools to index the cam shafts, and armed with these pins, you can get all four cams checked and adjusted pretty quick.

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  2. johnk...

    johnk... F1 Veteran
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    So Just how much do belts stretch? I compared an old belt, on a car for 27k miles, to a new belt. Belt length tolerance for the 308 is 0.75mm. When I measure a brand new belt and compared it to the old belt, the old belt was actually shorter. So assuming the old belt was the shortest possible when new and the new belt the longest possible, to remain within the tolerances, the old belt stretched had to be less than the belt length tolerances. And that corresponded to about 1 degree of timing change.
     
  3. fatbillybob

    fatbillybob F1 World Champ
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    Not needed to remove or insert pin. I can do it easy with just fingers. There is a trick. Remind me at the next Stooggapolussa

    I have seen forced pins with witness marks on them. That is bad. I could also envision a hamfisted guy with the screw-in pins yank a pin then knock the cog off the land /stuff goes flying and you have to start all over again.
     
  4. yelcab

    yelcab F1 Veteran
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    There is a certain level of finesse. Porsche techs know this very well. You screw in the tool, continue twisting until the pin rotates and you pull out the pin. Advance / retard the crankshaft as needed, find the next hole aligned, and put the pin back in after you have loosened it from the tool a bit.
     
  5. derekw

    derekw Formula 3

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  6. yelcab

    yelcab F1 Veteran
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    How does it work Derek?
     
  7. derekw

    derekw Formula 3

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    Lock the cams and crank on exact timing, loosen the big cam bolts, turn the eccentric pins with a flat screwdriver to tighten belts on driven sides (not tensioner side), tighten big bolts and turn engine over a few times, recheck and adjust if necessary. You will need to choose a hole in the cam and/or pulley to use that is close to the timing you want as you will get just less than one pulley tooth (12 degrees) of adjustment.

    On my HTD/GT2 pulleys (pictured) the 10mm holes are 174 degrees apart to give more options but you could drill 10mm holes in your steel pulleys to be in the middle of the adjustment range you want.
     
  8. mwr4440

    mwr4440 Three Time F1 World Champ
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    John,

    I wish I could answer the question but there are certain things the Indy Mechs won't tell me and frankly I can't blame them. Even though the US entrusted me with nuclear weapons and all the secrets they entail, some other things just aren't discussed.

    Business is Business.

    I respect that.
     
  9. johnk...

    johnk... F1 Veteran
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    Jun 11, 2004
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    Yep, I did a lot of work for DOD back in the day. But I telling you I measured old stretched, well used against new unused and the old belts were shorter than the new. That says that the belt stretch had to be less than the belt length tolerances. Current belts on my 308 are 9 years old and 5k miles and still tensioned pretty much as new. The old belts I took off were 25 yrs old, 27k miles and they were still tensioned what I would call "normally". I don't think there is anything magical about my car. :) Perhaps they won't tell you be it's BS. :):)
     
  10. mwr4440

    mwr4440 Three Time F1 World Champ
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    9 years old ?????? :eek:

    Id CHANGE those 'War Horses.'

    Because of the EXTREMELY SMALL Radious of the Crank (Drive) Pully, those belts get pretty abused compared to about 110% of nearly all other applications in which they are used.


    I don't doubt what you report but consider that your measurments are from two products from 25yrs prior and 9yrs prior to today.

    Just a guess but I bet the manufacturer made several changes in their design since then.

    And maybe or maybe not, 'ALL for the Good.'

    The TECHs here beyotch about that in new replacement parts, frequently.
     
  11. derekw

    derekw Formula 3

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    These belts don’t usually fail due to stretch or issues with the tensile member (cords and backing.) It is usually the high stresses in the sharp tooth root of this old trapezoidal design. In this application I think they often fail due to failed plastic flanges on the cam pulleys, bad tensioner bearings, installation mistakes, lower drive bearing wearing and not keeping the lower pulley square, crap getting into the openings... lots to go wrong.
     
  12. conan

    conan Formula Junior

    Nov 13, 2011
    266
    I did some research awhile ago about the different cords used in timing belts. Fiber glass or aramid fiber seems to be common and with both materials, the elongation at break is about 3%. The great news is that the creep is very low in the 0,1% range. So, 1m gives about 1mm creep. Of course, the timing belt design may have slightly different characteristics, and with a safety margin on applied load on the belt, one can expect lower creep values.
     
  13. johnk...

    johnk... F1 Veteran
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    I'm going to repeat this. Regardless of the way belts were made and how they are made now, the tolerances on belt length would not change. These are set by ensuring that the belt pitch matches that of the pulleys. That said, the point is simple. The old, stretched, warn out belts were SHORTER then the new ones. I'm pretty sure the old belts didn't shrink. So the only conclusions that stretch is less than the manufacturing tolerance on belt length.

    I've posted this before. Which is new and which is 25 years, 27k miles old?

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  14. derekw

    derekw Formula 3

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    It would be interesting to lock the distributor advance, point a timing light at a belt with a white line marked, and see if and how much the timing changes at different rpms.
     
  15. johnk...

    johnk... F1 Veteran
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    I was looki
    That would not work because there are 96 teeth on the belt and 30 on the cam pulleys. So the cam rotates 3.2 time for one cycle of the belt.

    However, based on the data I have for a Gates belt for a 308 if the tension segment of the belt went from zero lbs tension to the max allowable tension that segment of the belt would stretch about 1 mm, or roughly 1.25 degrees of cam rotation. But since the belt is pre-tensioned it would not be at zero tension, and the belt is never operated at max allowable tension, so the change in tension would be less and the stretch less. Figure 1/2 that amount, or less, as a good estimate. So maybe timing would change by 1 degree max (at the crank).

    You could mark the cam pulley, but could you detect a 1/2 degree movement?
     
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  16. Brian Harper

    Brian Harper F1 Rookie
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    I think you are chasing the wrong measurement. The length of the belt doesn't matter, the inside diameter matters.
     
  17. johnk...

    johnk... F1 Veteran
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    The pulley diameter does enter the picture to find the change in angle. The change in the length of the belt on the tension side, between points a and b in the figure below is what matters though. If that section of the belt were to stretch than the only way to take up the slack is to retard the cams relative to the crank. That is to say, if the cam drive pulley were held at a fixed position, the cam pulleys would have to be rotated clockwise to take up the slack. The amount of slack and the pitch diameter of the cam pulleys yields the degrees of rotation necessary as, angle = (Stretch/ diameter) x 360. Perhaps that is what you are referring to? The pitch diameter of the cam pulley is about 286 mm so 1mm stretch is about 1.25 degrees of cam rotation.

    [​IMG]
     
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  18. kcabpilot

    kcabpilot Formula Junior

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    Seems as though there is an awful lot of over-thinking going on here :)
     
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  19. Casino Square

    Casino Square Formula 3

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    Love the double belt comparison that JohnK provides. Hard to argue with that evidence.
     
  20. johnk...

    johnk... F1 Veteran
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    Or perhaps just "thinking" it with actually manufacture's data and specifications rather than repeating the myths and rumors.
     
  21. conan

    conan Formula Junior

    Nov 13, 2011
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    I think we all contribute pretty well to the issue with too large variations in belt length. It is clear that a stretch beyond what the teeth vs pulley can manage would cause big problems.

    Creep is probably small and factory variations likewise. Nevertheless, the factory tolerances could very well lead to a new belt being longer than an old worn belt.

    Another factor is engine block temperature. Aluminum expands and would lead to a different crank vs cam distance. I guess this is also in the millimeter range between low and high temp. Of course, the engine has a certain low temp range when operating, but still, installation at 20 degC vs operating at 90 degC gives a slight difference in timing.
     
  22. mwr4440

    mwr4440 Three Time F1 World Champ
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    #72 mwr4440, Sep 16, 2019
    Last edited: Sep 16, 2019
    Resurecting this discussion.

    Why we DON'T Trust the 'Assembly Marks,' to show us the Actual 'Timing.'

    Here is my (2V, FI) motor. Intake Cam 1-4.

    Properly Assembled, UN-TIMED.

    That Cam Mark* is 6° OUT on the Crank (w/Degree Wheel), from TDC.

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    * - For Point of Reference: The CAM Mark is the Rectangle on the Left; The CAM Cap Mark is the 'Arrowhead' or 'Pyramid' shaped 'Notch' on the Right.
     
  23. mwr4440

    mwr4440 Three Time F1 World Champ
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    PROs,

    Got a question or two.

    My '81 GTSi is a "NORTH AMERICAN" designated car, NOT a "US" car.

    Or in reality, Is It?


    In the Mondi 8/QV Workshop Manual in the section on Timing, it shows the valve timing specs for "US" cars and "BASE" cars. The INTAKE Valve Specs are the same, but the EXHAUST Valve specs are different by 4 Degrees.


    My QUESTIONS:

    1) Would my car originally built for and imported into Vancouver, Canada, have left the factory timed for the "US" specs or the "BASE" specs?

    2) If it would be considered a "US" spec'ed car, would my timing the EXHAUST for the "BASE" specs gain me anything?
     
  24. derekw

    derekw Formula 3

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    That's not too bad-- one of mine was 7 cam degrees off (14 crank degrees) based on the timing marks. I never checked actual timing of my old cams (using lift and lobe peaks) but suspect that would have added even more variation.
     
  25. mike996

    mike996 F1 Veteran

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    #75 mike996, Sep 17, 2019
    Last edited: Sep 17, 2019
    If the "assembly marks" are 14 degrees off and were consistently unreliable, how did the dealer set the timing before handing the car to the buyer? Did they use a degree wheel/piston stop too check/set timing on delivery/every time a car came in for a belt change? I find this difficult to believe. ;)
     

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