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Timing belt conundrum

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

  1. johnk...

    johnk... F1 Veteran
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    Jun 11, 2004
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    You can call the marks anything you like. If you replace a belt the change in the relative position of those marks will not change by more than +/- 0.5 degrees from where they were the last time the belts were changed. Whether the engine will be timed correctly, again, the caveat is, "if you start with a properly timed engine".

    People who do not accept that must believe that dimensions of metal parts change and that the manufactures are lying about tolerances on bet length, and tooth pitch.

    And here is another point the never get discussed much. Please note that in my post #21 I said the max error due to belt tolerance was +/-0.75*. That was in error. The tolerance for belt length is +/- 0.75mm which translates to a little more than +/- 0.5*. If you start with a correctly timed engine and the belt does stretch 2mm, then by the time you are preplacing the belts the time would be off more that 1 degree from where it was set, more than would occur if the tolerances of the old and new belts were at the extreme opposite limits. But belt stretch is irrelevant because all it tells you is where the timing is compared to where it was when that belt was new. Now, if belt stretch is an issue that effects timing adversely, why would you not compensate for it want replacing belts? If belt stretch were to account for a timing change of 1.5*, why not time the engine to be 0.75* advanced so that at the end of the belts life the engine would only be retarded by 0.75* as opposed to starting with time dead on and ending 1.5* retarded?

    Again, I repeat, the caveat is if the engine was timed correctly the last time the belts were changed. If you are confident that is the case, a belt sway is sufficient since the max change due to a belt swap is within the factory spec for timing. If you aren't confident, then check the timing and adjust if you feel necessary. But please stop trying to convince me that 1" drill makes a 2" hole.
     
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  2. Irishman

    Irishman F1 Rookie

    Oct 13, 2005
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    Thanks for all the good suggestions.

    I managed to get the belt installed, the tensioner set and the marks all lined up. The suggestion to install the tensioner assembly loose was easy and might have helped. I was all on board with trying to get the drive cog to jump a tooth in order to tighten the slack between it and the exhaust cam. However, I was not successful doing so. Maybe I didn't turn the crank far enough but I got uncomfortable turning the crank that way with the camshafts not moving.

    In the end, since the intake cam was the one wanting to jump the most I installed the belt with the intake one tooth in the opposite direction it wanted to jump. Sure enough after two crank revolutions it came back to TDC with both cams on the marks on the next try.

    I didn't mark cams and belts before disassembly. I also don't typically lock the cams. I don't know if it's strictly necessary but anytime I change the timing belts I also adjust the valves and change the cam seals. So the camshafts always come out of the car anytime I do this job.
     
  3. mike996

    mike996 F1 Veteran

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    "IF after Assembly AND the Valve Timing Process has been FULLY and CAREFULLY conducted, and ANY of the cam and cam cap marks align perfectly, that is just Italian Dumb Luck."

    If that is the case than every belt change would require a piston stop/degree wheel to find piston #1's TDC and then each cam would have to be degreed-in individually. Is that what the Ferrari mechs did at the dealer for each belt change?
     
  4. Hinecker

    Hinecker Formula Junior

    Mar 14, 2011
    273
    Hello all, the marks should align every second turn, two rotations of crank equals one rotation of cam.
    In my opinion, you are installing the belts without enough tension to begin with. The portion of the belt that goes directly from the lower drive sprocket to the cam is at tension when mounted.

    John.
     
  5. kcabpilot

    kcabpilot Formula Junior

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    As I said earlier, when you are on the PM 1-4 mark the #1 and #4 pistons are at TDC, the #2 and #3 are at BDC and all #5 through #8 are 90 degrees off and exactly midway between TDC and BDC. So you are in no danger of colliding pistons with valves until you have turned the crank nearly 90 degrees either way.
    As for marking the belts and sprockets, that's only useful if you have not removed the cam covers and cannot see the index marks.
     
  6. johnk...

    johnk... F1 Veteran
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    Some interesting numbers from Gates. A 1" wide timing belt will stretch at the rate of 1mm/1888 lbs. The same belt has a max tensile strength to failure of 760 lbs. So before the belt stretches 1 mm is would break.

    A 308 belt has 96 teeth.
     
  7. Saabguy

    Saabguy Formula 3
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    John, you got that conversion waaaay off. 0.75 mm converts to 0.0295 inches, NOT .5 inches.

    Lester

     
  8. johnk...

    johnk... F1 Veteran
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    #33 johnk..., Aug 12, 2019
    Last edited: Aug 12, 2019
    0.5* = 0.5 degrees.

    Let me sum this up like this: If 3 or 5 or how ever many years ago the last belt change was done the timing was A, and the timing today with the old belt is B, where B changed because of belt stretch or whatever, and after putting a new belt on the timing is C, then C will (must) be closer to A than B if the belt stretch is greater than the tolerance in belt length. So if you believe belt stretch is significant and greater than the belt length tolerance, then the idea then swapping belts can mess up timing is a myth.

    If belt stretch is greater than the belt length tolerance then swapping belts will always restore the timing closer to what it was at the previous belt change.
     
  9. Saabguy

    Saabguy Formula 3
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    That makes a lot more sense.

    Lester

     
  10. Irishman

    Irishman F1 Rookie

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    Thanks, good to know as I wasn't sure how far I could go.
     
  11. derekw

    derekw Formula Junior
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    “As I said earlier, when you are on the PM 1-4 mark the #1 and #4 pistons are at TDC...”

    #1 or #4 will be at TDC exhaust stroke so a couple of teeth off can cause the valve to hit the piston
     
  12. kcabpilot

    kcabpilot Formula Junior

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    Which is why I lock the cams. You can rotate the crank either way from the PM 1-4 mark and not worry about interference on the 1-4 bank. You have to rotate close to 90 degrees before worrying about the 5-8 bank. Different people have their own methods, my method is to set the engine up at #1 TDC on firing stroke with all the cam marks lined up. That way there's no guessing as to where everything is at or what's going to happen when you turn the crank. Lock the cams at that point and you are safe to turn the crank quite a bit either way to make it easier to get the belt on in the proper position so that everything is aligned and all of the slack is pulled out of the long run.
     
  13. mike996

    mike996 F1 Veteran

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    Well, as you say, I guess we all have our own 'way.' I would never move the crankshaft once I am ready to pull the belts (whether I put everything at TDC or just marked the cam positions where the engine stopped. I only move the cams to achieve whatever slight movement is necessary to get the tooth to "drop" into place. There is no way to "accidentally" get a tooth or more off - one tooth off is a long way and is obvious whether you set up everything at TDC or just mark the cams' positions when the engine is shut off.

    If there is concern about bashing a piston into a valve/bending it, that risk is far greater by turning the crank than it is by turning the cam, simply because it takes so much more force to turn the crank. If you turn the crank you can easily hit a valve without even realizing it at first. Turning the cam it is almost impossible to not immediately realize it if a valve hits a piston - long before you could actually bend a valve.
     
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  14. kcabpilot

    kcabpilot Formula Junior

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    Those two conditions are not the same Mike. At TDC you know you are safe moving the crank either direction because all that is going to happen is #1 and #4 will move down and nothing else is anywhere near the valves. "where the engine stopped" is a complete unknown position. You can't safely move anything from that starting point.

    My only problem with turning the cams with #1 and #4 at TDC is that they are unstable and unpredictable, they can snap forward or backwards because of the uneven load of the valve springs to the off centered lobes. I am just more comfortable with them locked and knowing that there is no way for me to cause interference by turning the crank a large number of degrees either direction from that position. If you have had the heads off and see how the pistons move it becomes very clear, this is nothing like a normal cross plane V8. It's two four bangers 90 degrees apart.

    Anyway, just explaining my position, not here to tell anyone how to do it.
     
  15. mike996

    mike996 F1 Veteran

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    Well, I definitely agree that the cams can, and do, snap forward or backwards! ;)
     
  16. conan

    conan Karting

    Nov 13, 2011
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    At TDC with removed belts and the cams have moved to the nearest resting position, how close are the valves to the pistons?
     
  17. johnk...

    johnk... F1 Veteran
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    No very close for a QV.
     
  18. Lawrence Coppari

    Lawrence Coppari Formula 3

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    Turning the crank when a piston is near TDC moves the piston very little which means there is a huge mechanical advantage at that point of crank rotation. A small amount of torque on the crank creates a great force the piston can exert on a valve possibly bending it. Turning a camshaft by hand does not have the same mechanical advantage unless you use a cheater bar 5 feet long or so.
     
  19. kcabpilot

    kcabpilot Formula Junior

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    #44 kcabpilot, Aug 15, 2019 at 10:34 AM
    Last edited: Aug 15, 2019 at 10:42 AM
    No way to answer that without knowing what "nearest resting position" is.

    Let's not take any of this out of context. With all of the marks aligned and the cams locked and belts removed turning the crank a few degrees one way or the other creates no danger of interference. Regardless of mechanical advantage, or lack of, the intent here is to have no contact of any sort between a valve and a piston.
     
  20. johnk...

    johnk... F1 Veteran
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    #45 johnk..., Aug 15, 2019 at 11:24 AM
    Last edited: Aug 15, 2019 at 11:31 AM
    Agreed, and as stated earlier, with the crank at TDC and the cams at the marks the only cylinders that could have vales any where near the piston is #1 and#4 where the valves are in overlap at the top of the exhaust stroke. Now, from experience, I know that in a 308 QV the valves never get very close to the pistons as I have experience with a broken belt in which the engine kept running. Only 4 valves were bent, 2 exhaust and 2 intake, and looking at the valve with the naked eye you would be hard pressed to see how much the valves were bent without a reference to a straight valve.

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    Looking further, as noted in another post, bank 2 pistons will all be at about 1/2 stroke, so no problems there. On bank 1, one cylinder will be at the top of the compression stroke with valves closed. One will be at the top of the exhaust stroke with the valves in overlap. The other two cylinders, 2 and 3, will be at BTD with one just finishing the intake stroke and the other will be just starting the exhaust stroke. So the only real issue is the cylinder at TDC with the valves at overlap.
     
  21. kcabpilot

    kcabpilot Formula Junior

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    Unlike Alfa Romeos the Ferrari cams have only one index mark therefore if the cam marks are aligned #1 is TDC on compression and #4 is TDC on exhaust. Although the #4 valves may not be completely closed they are barely off the seats as the cam lobes at that position are pointed inwards towards each other. If a belt were to fail, what subsequently happens depends completely upon what position the cams stopped at. When you are installing a cam on the head you will notice that there is a position (maybe two) where it lies fairly flat and you are able to put all of the caps on and start the nuts. In that position none of the valves will be completely open when the journal caps are torqued down. If the cam is in a position where the valves are completely open there will be severe interference between valve and piston and subsequent severe damage but this is a position that an unrestrained cam doesn't particularly like to stay in. It tends to want to snap to what you might call a "resting position" where all of the loads from the valve springs are in somewhat of a balance and none of the valves are fully open.
     
  22. steved033

    steved033 Formula 3
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    you can really say "it doesn't matter how many teeth the belt has"

    sjd
     
  23. Hinecker

    Hinecker Formula Junior

    Mar 14, 2011
    273
    Morning Steve,
    Exactly, as long as the tensioner can pick up the slack. The number of teeth only indicate the length, if it was longer it could be loose if shorter it would be too tight.
    The ratio is defined by pulley diameter.
    Keep in mind that this engine has two reductions, one inside (with gears) and one outside with belts... at the end it's still 2 to 1 reduction ratio, as in all 4 stroke engines.

    Hope this helps.

    John.
     
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  24. conan

    conan Karting

    Nov 13, 2011
    235
    Maybe well known, but it is always important to make comparative measurements with the same setup before and after a change of something.

    In the belt case, one important setup parameter is the tension on each belt section. If one measured the positions of the cams with the longest belt section at say 150N, then it has to be the same tension when checking positions after changing the belt. Of course, it depends on what accuracy is needed ..
     

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