1980 308 GTSi 2v Cam Belt Change | FerrariChat

1980 308 GTSi 2v Cam Belt Change

Discussion in '308/328' started by pbonaguro, Mar 29, 2025.

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  1. pbonaguro

    pbonaguro Rookie

    Sep 30, 2010
    26
    Need some advice. I'm a decent mechanic have a couple race cars and a few collector cars. I've been studying up for a year or so on how to do a cam belt change on my 1980 308 GTSi, using a few different resources including birdman's procedure and cam locks. 100% sure I did everything right in terms of locking cams and flywheel at TDC, went way overboard in marking the belts and each pulley in several places, rebuilt the tensioners with new bearings and put new belts on. Everything seemed perfect up until this point.

    Here is where things went bad today and heartbreak is setting in. Removed cam locks and unlocked flywheel. Kept the tension screws loose so the spring was constantly pushing against the belt. Rotated the engine 2 full revolutions keeping and eye on the tensioners to see where max tension was needed and prepared to lock down tensioners. But, at that point I checked my marks on the belts and pulleys and they were way off. Turned engine clockwise with tensioner springs pushing against the belt.

    What could possibly have caused this? Is it possible that I may have bent valves by turning the engine over by hand and with it seeming to get out of time?

    If there is anyone that would be willing to chat and talk me off the ledge I would appreciate it. I'm in the Chicago area and there are a few Indi ferrari shops I will reach out to next week, but wanted to ask the forum if I'm missing something?
     
  2. Dockboy

    Dockboy Formula Junior
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    You have to rotate the crank 16 times to get back to the same marks;)
     
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  3. pbonaguro

    pbonaguro Rookie

    Sep 30, 2010
    26
    If that’s all it is, I’m going to buy you a gift. Seriously? I was trying to do that math, but didn’t want to keep turning.

    I was thinking that it seemed logical that my marks would line up every 2 rotations?
     
  4. smg2

    smg2 F1 World Champ
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    Umm 4 times actually. That should return to start point. 16 would work but just be more rotations.
     
  5. ProvaMo

    ProvaMo Formula Junior

    Jun 29, 2004
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    PM sent...
     
  6. pbonaguro

    pbonaguro Rookie

    Sep 30, 2010
    26
    Smg2, you just dashed my hopes. Is it 4 or 16. I think I turned 3 times and stopped
     
  7. 26street

    26street Formula Junior

    Jan 30, 2021
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    Mark k
    if i am reading this correctly you are looking at marks you made on the belt (before you got the tension on the belt)
    if that's right then check the following,

    my sure the marks on crank and cam gears are lining up every time cylinder #1 is on TDC compression stroke, the marks on the belt will not line up for a few turns the crank rotates 2 to 1 of the cams so if the marks line up every 2 times (on crank) on the pulleys you are good.

    you can only see one cam marking through the oil fill cap and that's the one you use when you know the belts are right and locking the cams for a fast belt change if you are not sure of the cams you will need to remove the valve covers and line up all the marks which is the safest way,
     
  8. smg2

    smg2 F1 World Champ
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    I'm referring to getting the camshaft tick marks to line up again.
    Belt marks? That's one that I have not measured and would be based on the two gears and their ratio and the crank rotations relationship. No idea off the top of my head.
    Best to use the cam tick marks.
     
  9. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Jan 11, 2001
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    Can someone please post the number of teeth on a 308 cambelt? TIA.
     
  10. johnk...

    johnk... F1 World Champ
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    Jun 11, 2004
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    Cam mark line up every 2 crank rotations.
    Cams turn 1/2 turn for each crank turn.
     
  11. pbonaguro

    pbonaguro Rookie

    Sep 30, 2010
    26
    Thank you all for the replies. I went back to check and TDC on the Flywheel lines up with TDC on the cam through the oil fill hole as I photographed before changing the belts.

    for the life of me I can’t figure out if/when/if ever the marks on the belts would ever line up with the marked teeth on the pulleys again? I think it’s a math problem having something to do with the number of teeth on the belt and the number of teeth on the pulleys. Turned the engine by hand 20+ times and no dice. Teeth on belt belt never line up again with teeth on pulleys where I originally marked? Any thoughts?

    You guys have been fantastic. Thanks for all of your comments.
     
  12. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    If you can count the number of teeth on one of your old belts and post it, I can check the math to see if SMG's 16 crank rotations is correct, or some other number, or no number ;).
     
  13. pbonaguro

    pbonaguro Rookie

    Sep 30, 2010
    26
    Thank you all for the replies. I went back to check and TDC on the Flywheel lines up with TDC on the cam through the oil fill hole as I photographed before changing the belts.

    for the life of me I can’t figure out if/when/if ever the marks on the belts would ever line up with the marked teeth on the pulleys again? I think it’s a math problem having something to do with the number of teeth on the belt and the number of teeth on the pulleys. Turned the engine by hand 20+ times and no dice. Teeth on belt belt never line up again with teeth on pulleys where I originally marked? Any thoughts?

    You guys have been fantastic. Thanks for all of your comments.
     
  14. smg2

    smg2 F1 World Champ
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    That'd be an interesting math problem. though it wasn't me on the 16 rotations.

    I used to have the tooth count.. that was a long time ago.
     
  15. pbonaguro

    pbonaguro Rookie

    Sep 30, 2010
    26
    I’m very intent on understanding when the belt would come back to the original marks. I’ll get back to with number of teeth on belt and pulleys.
     
  16. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    I've got the cam pulley number (30) -- just need the belt number of teeth.
     
  17. smg2

    smg2 F1 World Champ
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    There's a funny ratio between the gears and crank. The pulley ratio between drive and cam is 1.5. when I went to the 8mm gt tooth, cam tooth count is 36 lower drive is 24 to keep that 1.5 ratio due to the crank& drive gear ratio.
     
  18. smg2

    smg2 F1 World Champ
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    Ok dug up a belt, Ferrari 111334 dayco 096r254...
    96 teeth 25mm wide
     
  19. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Yes, that "funny ratio" is the crankshaft gear having 27T and the gears on the cam drive sprockets being 36T giving a 1.333... ratio:
    1.333... x 1.5 = 2
     
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  20. Dockboy

    Dockboy Formula Junior
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    #20 Dockboy, Mar 29, 2025
    Last edited: Mar 29, 2025
    Dayco cam belt 94095 101x254 has 101 teeth
     
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  21. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    That's a 308QV belt -- which, I believe, is shorter than a 308-2V belt (so fewer teeth?), but it's good data. However, this could explain some confusion about this subject as the number of crankshaft rotations for a mark on the cam pulley and a mark on the cam belt to realign may be (is probably) different for 308QV vs 308-2V. But marking the cam pulley and cam belt seems a bit crazy to me -- much better (at PM1-4) to put a mark on the cam pulley and a corresponding mark on the (fixed) cylinder head somewhere. It doesn't have to be uber-precise -- you're looking to just not be off a whole tooth (12 degrees) -- JMO.
     
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  22. smg2

    smg2 F1 World Champ
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    That be the one.
    Yeah the QV cam centerline, i.e. distance between cams is shorter as they changed the intake and exhaust valve angles.
     
  23. smg2

    smg2 F1 World Champ
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    Yeah that was in the back of my mind, it's been nearly 20yrs since I did all the design dev on the new pulleys.
    Thanks!
     
  24. pbonaguro

    pbonaguro Rookie

    Sep 30, 2010
    26
    Again guys, thanks for all of your time and thoughts. If I could do it all over again I would have paid more attention marking the cams. I think the reason birdmans procedure has you mark the belt is to just be sure the new belt goes on identical to the one that comes off. You still have to lock the cams and crank which I did. But by marking the belts, the crazy question came up of when will/how many rotations for belt marks and pulley teeth to come back to original position. I did many/20+ turns and never lined up. Crazy.., sorry for the math/engineering question.
     
  25. smg2

    smg2 F1 World Champ
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    But see here's the thing.. for SteveM it's going to be an itch he has to scratch..
    So we eagerly await is calcs..:D
     

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