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Timing belt tension question

Discussion in 'Technical Q&A' started by blainewest, Sep 17, 2006.

  1. blainewest

    blainewest Formula Junior

    Aug 26, 2005
    721
    Kelowna, BC
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    Blaine W
    I was checking the oil in my 308gt4 and checked the timing belt tension with my hand on the "long" section of each of the belts. The rear belt is much "tighter" than the front belt. When I replaced the belts this spring I followed what I think is the correct procedure...install belt, release tensioner bearing spring, turn the motor over by hand 2 or 3 times then tighten the tensioner nut. By the way the tensioner mechanisms were in very good shape including the springs..no rust on them.

    The front belt in the long section has about 1/2" flex or maybe a little more. It does have signifigant resistance at about 45 degrees when twisted by hand again in the "long" section. I have put maybe 600 miles on the new belts.

    My question is should I be replacing my front tbelt? Or retensioning it (even though according to the manual this is a no-no)? Or should I be doing nothing? I recall that the old belts were also not uniform in tension with eachother. Can't recall if it was the front or rear which was looser though.
    Thanks, Blaine.
     
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  3. tuttebenne

    tuttebenne F1 Rookie

    Mar 26, 2003
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    Andy
    You have me thinking... My belts were changed four or five years ago and I didn't re-tension them. However I just did belts on my son's Porsche 944 and consulted a friend who has an Inde Porsche shop. He told me to tension the belt exactly as you did but followed that with a stern warning that the belt MUST be retensioned after 2000 miles. I know the cars are different but your comment regarding looseness and his recommendation to retension are difficult to ignore.

    I'll be interested in hearing other opinions

    Andy
     
  4. danno357

    danno357 Rookie

    Jun 1, 2005
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    redondo beach
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    Daniel J. Burley
    Did you check the front and rear belt tension with the crank position in the same spot? The reason I ask is, the rear belt may have all of the tension on the bearing due to cam position, causing it to seem like the belt is loose on the long side. And the front belt could have all of the tension on the long side. I almost confused myself (sorry about that!). I would turn the motor 90-180 degrees and check it again. The belt tension should be fine and not back out, but its always wise to check. Much easier to deal with than a 944!!!!!
     
  5. Watry

    Watry Karting

    Aug 8, 2005
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    Andrew Watry
    #4 Watry, Sep 18, 2006
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  6. tomberlin

    tomberlin Formula Junior
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    Apr 9, 2005
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    tom berlin
    I found the same thing after about 3k miles on a rebuild. Being aware that crank position matters when checking belt tension, I went ahead and snugged the one I percieved as loose. We'll see. Maybe I will get to rebuild it again.
    Tom
     
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  8. Glassman

    Glassman F1 World Champ
    Silver Subscribed

    I have yet to do a belt change all by myself, but I believe all the procedures agree that retensioning is not to be done. I would think the above post about engine load on the belt to be very relevent.
     
  9. pad

    pad Formula 3

    Sep 30, 2004
    1,409
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    Paul Delatush
    Do not retension. If you suspect something is wrong, replace the belts and the tensioners. Small price to pay for peace of mind. And if something is actually wrong, you just saved youself an engine.
     
  10. damaged

    damaged Karting
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    Mar 1, 2006
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    best way to check belt tension ( any car, any brand ) is to do the "twist" test.
    i.e get hold of the belt and twist it. You should be able to turn it about 90 degrees, much more and the belt is too loose, less and its too tight.
    Cam lobe position can of course affect tension on the "loose" side of belt, but i'm sure if you check the work shop manual there is a position that works for both banks, i.e 25deg before TDC, or 40deg after TDC etc. I will try to dig up a manual and see if i can find it for you.
     
  11. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    Do not retension the belts on a 3X8. The tension depends on where the cam is when the engine is shut off. If the lobes on a cam are past center, the cam wants to turn the crank. Hence, there appears to be too little tension in the belt at this time.

    The procedure on a 3X8 is different than it is on a 944. My 944 is my track car and I do the belt yearly. They are different! The belts are different.
     
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  13. Owens84QV

    Owens84QV F1 Rookie

    Oct 2, 2001
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    Greg
    Some of you were beginning to scare me...until today, I had never heard the topic of retensioning 3x8 belts, especially since I just did mine in February 2006.
     
  14. Rifledriver

    Rifledriver Two Time F1 World Champ

    Apr 29, 2004
    27,103
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    Brian Crall
    Do not retension belts. If the belts have come loose they were tensioned wrong in the first place. The twist test is OK on some cars but certainly not all. If you do that on a 348, 355 or 360 the belt and valve life would be measured on your wrist watch.

    Do it Ferrari's way. There is a different way for each engine. If you have and they are still loose or getting loose you have not followed all the steps correctly.
     
  15. Verell

    Verell F1 Veteran
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    May 5, 2001
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    A key trick to get tension right on the 3x8 engines is to make sure that the crank position is such that the long belt section is very tight when you tighten the tensioner down. This ensures that all the slack in the belt is taken up by the tensioner.

    If you're using cam locks, then you just need to use the crank to keep that belt section tight against the cam lock's resistance while tightening the tensioner down.

    If you're not using cam locks, then you're working against valve resistance & the crank position will be different for each bank.

    When measuring tension, you want it to be the other way, no slack on the tensioner side, all the slack in the long belt section.
     
  16. tuttebenne

    tuttebenne F1 Rookie

    Mar 26, 2003
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    Thanks Verell, this is simple and makes sense.

    Have you ever encountered having to re-tension a belt(s)?
     
  17. blainewest

    blainewest Formula Junior

    Aug 26, 2005
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    Blaine W
    Wow. Thanks for all the response guys. I've decided, for peace of mind, to simply replace the belt. It just seems to loose to me. I've got no plans for this Saturday anyway. If things go as expected maybe go cruising Sat. night. THanks again, Blaine.
     
  18. Artvonne

    Artvonne F1 Veteran

    Oct 29, 2004
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    Obviously many many of these cars have gone many years and many thousands of miles without the belts ever being touched, replaced, or re-adjusted. The OWM explains that further adjustments will only lead to further stretching of the belt and its life expectancy being shortened. But.....

    It has been reported that under racing conditions the belts can stretch so much they need to be replaced each race, with the F-40 reportedly requiring some type of stronger belt using kevlar. Then there is one other "fact" that has never been fully discussed. Every Ferrari V-8, from the GT4 through at least the QV cars, have an opening in the timing belt cover to access the tensioner bearing bolt. That IS its only purpose, to loosen and adjust the bearing with the cover in situ. If the belts were fully meant only to be adjusted upon installation, there would not be any need to access the tensioner through the cover. While Ferrari may indeed specify no further adjustment, I wonder if there may not be the possible need of retensioning after the motor is fully brought up to temperture?
     
  19. tvine

    tvine Formula Junior

    Jul 19, 2006
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    Tom Vine
    As I have mentioned in a previous post to another thread...based on maintenance records, the original timing belts on my '79 GTB were re-tentioned once by the previous owner before being changed. These belts are not currently on the car but they did not fail before being changed. Again, based on maintenance records, these belts ran for 48,000 miles over 13 years before being replaced.

    Do not infer any recommendation in this, just one datapoint.
     
  20. f355spider

    f355spider F1 World Champ
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    May 29, 2001
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    You were lucky.
     
  21. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    The bolts give one access to the condition of the bearing. An experienced person can listen with a stethoscope probe on the retaining bolt and tell how the bearing is doing.
     
  22. Rifledriver

    Rifledriver Two Time F1 World Champ

    Apr 29, 2004
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    That is a handy feature but is not what it is intended for. In the early days of belts it was felt proper to adjust them as a routine. It was also felt that they could go much longer than what is recommended now. Times and service recommendations change due to the knowledge that comes from experience but once a part is designed and goes into production it stays.

    The new cars no longer have that hole.
     
  23. fatbillybob

    fatbillybob Two Time F1 World Champ
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    Aug 10, 2002
    20,334
    socal
    Do you own a steger tensiometer claibrated in Ferrari units as opposed to porsche units?
     
  24. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    Did I say that was the only purpose of the access?
     
  25. Verell

    Verell F1 Veteran
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    May 5, 2001
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    If I remember correctly, the F40 used a single long belt like the 348, even a small % elongation becomes significant over a long length. It's a totally different system.

    BTW, when smg2 researched the 3x8 belts, we were surprised to learn that they used fibreglass cord! You don't get much stronger than that. Prior to that the understanding (going back to '01 or so in the old fchat!) was that they used kevlar cord.
     
  26. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    Wish I had some old ones lying around. Just threw some away last month. I'd cut one up if I had one.

    By the way, the coefficient of thermal expansion for glass is about 4.7d-06 in/in-deg F. The same for aluminum is about 13.d-06 in/in-deg F. So the belt tightens as block warms. Lead, zinc, and magnesium are about the only metals that expand more than aluminum when heated.
     
  27. blainewest

    blainewest Formula Junior

    Aug 26, 2005
    721
    Kelowna, BC
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    Blaine W
    This appears to be spot on... I ran the car today and got it to operating temp and shut it off and then felt the tension on the long side of each belt. There is a very noticeable difference in tension when warm...much tighter.
     
  28. Verell

    Verell F1 Veteran
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    May 5, 2001
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    This is pretty well known/mentioned in the archives.
     

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