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  1. #1

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    Timing belt on 3.4 DOHC?

    The timing belt looks brand new on my 3.4 DOHC, thank goodness.

    However, I know I'll have to change it someday so I've been reading up on it for the hell of it. I'm still a bit foggy on some issues:

    1. Why isn't it a matter of marking all the pulleys, loosening the tensioner, removing the belt, and reinstalling the new one making sure the pulleys haven't moved?
    2. What's the actual function of the cam hold-down tool? If to keep the camshafts from moving, why couldn't you do this from the front without pulling the plenum and valve covers?

  2. #2

    Timing belt on 3.4 DOHC?

    Well, I guess it's possible. I've always done it with the cam tools, but you could mark the pulleys and just swap belts if you wanted to do it quickly.

    But, how do you know if the pulleys haven't moved on the cams? I've seen quite a few cars that had timing lines on the pulleys that were correct, but the cams had spun slightly and the engine was actually out of time. Only a cam hold down tool can tell you this. The cam hold down tool sits on the two cams in slots that are machined in the cams. When you tighten the cam hold down tool it centers the cams. This cannot be done from the front, without taking the valve covers off...because of the un-keyed cams.

    You ever drive a 3.4 that was so slow you actually had to look under the hood to see if it was really a 3.4? Most of the time, thats GM's wonderful un-keyed cam pulleys at work!

    So what the hell were they thinking? Why would they not key the cam pulleys? The only reason I can think of (and it keeps me from getting really mad about the un-keyed cams) is that they were going to do variable valve timing or some other kind of cool thing like that. Maybe there was a good mechanical reason not to key the cams.

    Don't get me wrong...I think there is a lot of excellent design work/theory in the 3.4. And for a engine that had to be servely de-nutted because of Hydramatics' inability to complete a simple task (design a transmission that would survive behind a 3.4 that made 275 HP), the engine does perform well. But sometimes, I also think the 3.4 is an engineers' wet dream gone wrong. And I've worked on a lot of them, so I think I can say things like that.

    Would I own another one? Yeah, why not.

    Jason

  3. #3

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    Timing belt on 3.4 DOHC?

    Yeah, I've always wondered why the cams are not keyed. That really makes very little sense.
    Even if the engine was supposed to have variable valve timing, it didn't make it to production with VVT, so why didn't they go ahead and key the production cams? That's one (of several) annoying things about the 3.4 DOHC!
    Is the shaft of the cam visible from the front? I'm not at home, otherwise I'd pull the access cover to look. If it is, I would think it would be possible on the very first belt change to use the hold-down tools, then use an engraver or some kind of attachment on a Dremel and make a mark on the shaft and a corresponding mark on the cam gear. Then for any subsequent belt change, you could tell at a glance whether it's necessary to tear the plenum and valve covers off or not.

  4. #4

    Timing belt on 3.4 DOHC?

    You'd have to pull the cam pulley bolt out completely, but then you could scribe a line on the cam and one on the pulley. That sounds like it work work great.

    Jason

  5. #5

    Timing belt on 3.4 DOHC?

    1. You can and many other people already have. One problem, after 60k some miles i can almost guarentee the cogs are not where they are supposed to be. It is easier just to retime the cams when you have that part of the engine apart anyway.

    2. The hold down holds the cams square so that you can TDC the crank and put the belt on correctly, it is not used to hold the cams while you loosen the cog bolts.

    Funny, I thank god, gm, and whoever else that the cams are not keyed.

  6. #6

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    Timing belt on 3.4 DOHC?

    Quote Originally Posted by brian89gp
    Funny, I thank god, gm, and whoever else that the cams are not keyed.
    Why?

  7. #7

    Timing belt on 3.4 DOHC?

    First, gm's stock timing is not the best. Second and most importantly if you change anything on the engine performance wise the 'ideal' cam timing changes. If it were keyed we all would have to buy $200 adjustable cam gears to do what we now can stock.

  8. #8

  9. #9

    Timing belt on 3.4 DOHC?

    its actually very simple to do and under stand once you have done or watched one done....
    if you have any questions, cunsult this.....
    http://www.60degreev6.com/index.php?p=pages&pid=27
    Im glad because you can custom time then to make way for more hp....
    ex.
    http://www.60degreev6.com/index.php?p=pages&pid=7

  10. #10

    Timing belt on 3.4 DOHC?

    Its not likely that the cam gear is going to spin on the cam. The problem is getting the flats lined up. If you scribe a mark after you time the cams, you still have to get the cams in that position on the next belt change. You could do it with some vise grips and a wrench but its not going to be fun getting them dead on.

    I too am very glad the cams are not keyed. I guess you could trade in your convertible for a honda if you want keyed cams.
    Ben
    WOT-Tech

  11. #11
    Anonymous
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    Timing belt on 3.4 DOHC?

    Quote Originally Posted by Vegeta
    I guess you could trade in your convertible for a honda if you want keyed cams.
    You said the "H" word.

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