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  1. #1
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    Default chevy 350 interference heads?

    I got a '97 suburban with the 5.7 vortec. The timing chain cover is leaking, and I figured with almost 200k, I would change the timing set as well while I am in there. I don't believe it is an "interference" motor, but thought I would check just so I know ahead of time since I am sure that somewhere in this process something will get moved and I don't want to damage valves. Or any other advice is appreciated.

  2. #2
    Super Member 94BULLITT's Avatar
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    Default Re: chevy 350 interference heads?

    Yes it is a interference engine.
    Roger

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  3. #3
    Elite Member Dmace's Avatar
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    Technically they are not an interference engine. Lose a chain at high RPMs and the valves could float and make contact but at low RPMs they will not touch.

    According to the manual, setting the timing requires a scan tool:
    Setting Timing
    Camshaft Retard Offset Test
    The ignition timing cannot be adjusted. The distributor may need adjusting to prevent crossfire. To insure proper alignment of the distributor, perform the following:

    With the ignition OFF, install a scan tool to the DLC.
    Start the engine and bring to normal operating temperature.

    Important
    Cam Retard Offset reading will not be accurate below 1000 RPM

    Increase engine speed to 1000 RPM.
    Monitor the Cam Retard Offset.
    If the Cam Retard indicates a value of 0 2, the distributor is properly adjusted.
    If the Cam Retard does not indicate 0 2, the distributor must be adjusted.

    Adjusting Camshaft Retard Offset
    With the engine OFF, slightly loosen the distributor hold down bolt.
    Important: Cam Retard Offset reading will not be accurate below 1000 RPM


    Start the engine and raise engine speed to 1000 RPM.
    Using a scan tool monitor Cam Retard Offset.
    Rotate the distributor as follows:
    To compensate for a negative reading, rotate the distributor in the counterclockwise direction.
    To compensate for a positive reading, rotate the distributor in the clockwise direction.
    Repeat step 4 until 0 2 is obtained.
    Turn the ignition OFF.
    Tighten the distributor hold-down bolt to 3Nm(25 lb. ft.).
    Start the engine, raise engine speed to 1000 RPM and recheck Camshaft Retard Offset.

    But Ignition timing is easily set at the distributor by finding TDC in cylinder 1 then lining up the distributor mark with #8.
    Last edited by Dmace; 02-24-2013 at 07:33 AM.
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  4. #4
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    Default Re: chevy 350 interference heads?

    Quote Originally Posted by Dmace View Post
    Technically they are not an interference engine. Lose a chain at high RPMs and the valves could float and make contact but at low RPMs they will not touch.
    This is not correct. They are absolutely an interference engine.

    Do not rotate the crank when the chain is off. You should pull the plugs so that there is no residual compression, although unlikely. Bring the motor to TDC on #1 before you pull the chain off. You may have to rock the crank back and forth a few degrees to get the sprocket and chain back on. Put the balancer in the oven at 250 for 15 minutes before you try to put it back on. Take it straight from the oven ( disregard wife ) , lube the snout with oil, and slide it on as far as you can before it sticks. Use the balance bolt with a stack of lubed washers to pull it on the rest of the way. Do not hammer the blancer on. Do not apply too much torque to the balancer bolt, as it WILL strip the crank snout and ruin your day.
    Tim.

  5. #5
    Gold Member firefighter jim's Avatar
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    Default Re: chevy 350 interference heads?

    Quote Originally Posted by tcartwri View Post
    This is not correct. They are absolutely an interference engine.

    Do not rotate the crank when the chain is off. You should pull the plugs so that there is no residual compression, although unlikely. Bring the motor to TDC on #1 before you pull the chain off. You may have to rock the crank back and forth a few degrees to get the sprocket and chain back on. Put the balancer in the oven at 250 for 15 minutes before you try to put it back on. Take it straight from the oven ( disregard wife ) , lube the snout with oil, and slide it on as far as you can before it sticks. Use the balance bolt with a stack of lubed washers to pull it on the rest of the way. Do not hammer the blancer on. Do not apply too much torque to the balancer bolt, as it WILL strip the crank snout and ruin your day.
    Ditto................interference engine

  6. #6
    Super Star Member Diamondpilot's Avatar
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    Default

    If the chain is not broken I would just make reference marks, take old off, install new.

    Chris

  7. #7
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    Default Re: chevy 350 interference heads?

    thanks for the responses guys, I was finding conflicting stories when I did a google search.

  8. #8
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    Default Re: chevy 350 interference heads?

    So who do you believe, and what proof have they shown you? Googling anything from "Interference engine list" to "Vortec 5.7 valve interference" to "Vortec 350 valve-to-piston clearance" shows nothing....if what we're looking for is whether or not a stock 5.7 Vortec is an interference engine or not.

    There are thousands of posts about engines that have over-revved and had the pistons and valves kiss each other, or carboned-up guides causing valves to hang open and hit the pistons, or improper combinations of heads/pistons used together, or modified engines with higher-lift camshafts and whatnot causing interference, but none of those things constitutes what is defined as being an "interference engine".

    I've seen 4.3 Vortecs assembled with the camshafts out of time, and no....the pistons and valves did not contact each other. The 4.3 is essentially a 5.7 minus two cylinders, so damage should have occurred if it were indeed an interference engine.

    Those that have said the Vortecs are interference engines, (in stock form), have yet to show where their information is coming from. Anyone got a link to share or a scan of a service manual?

  9. #9
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    Default Re: chevy 350 interference heads?

    I would say that it is just safer to assume that it is and treat it accordingly. If they had said "it is not an interference engine, it doesn't matter what you do" then I would be more skeptical. It could be that I will pull the cover off and the chain slack will be within the tolerance and in that case I may not even change it, but I doubt it. Reading the responses makes me think that I will just line up the timing marks for installation before removal and then just not move them when I put the new ones on, I just know that sometimes things don't go the way we plan. BTW, I do have a GM service manual and it doesn't include any cautions about rotating the crank with the timing chain off, but still thought it was a good thing to ask.

    But yes, I know what you are saying about relying people you don't know for information. There isn't anyway of knowing if they are right or not, but then again, I have found "professional" mechanics that don't know what they are talking about so there is really nobody you can trust. And aside from me being cheap, is a big part of the reason that I do my own work on things. It is how we learn, and with enough research is really the only way you know that it was done right.

  10. #10
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    Default Re: chevy 350 interference heads?

    The term Vortec refers simply to the shape of the combustion chamber. It is still the same basic motor that came out in 55. A stock motor can have the cam timing out by +- 20 deg and still not get contact. Increase the duration of the cam and things get critical. Lift actually has little to do with it. There are dozens if not hundreds of rebuilder manuals on the SBC alone. Ford another hundred if you're inclined to look it up. Myself, I've been racing them for more than 30 years.

    Stock cams have valve lift on the order of .380~.400". Running clearance is ~ .100". Break the chain or rotate the motor without a chain and you have at minimum .250" of intereference.
    Tim.

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