Small Block Knock Mystery

Discussion in 'Engine Topic' started by Steven Simmons, Sep 12, 2019.

  1. Steven Simmons

    Steven Simmons New Member

    Messages:
    6
    Likes Received:
    1
    Joined:
    Sep 12, 2019
    Hello everyone. I have used this site plenty with my 1978 Camaro build so I finally bit the bullet and joined.

    Anyway here is my current predicament. I just finished putting a new top end on my small block (generic goodwrench 4 bolt main crate engine from the 90s, edelbrock aluminum heads, rpm air gap, comp cams 12 212 2, harland sharp roller rockers 1.5). I broke in the cam and it was purring like a happy cat with an exhaust leak.

    Here is where the trouble starts. The next day I started tuning it and fine tuning the timing. I got it running well, drove it around the block a few times but it was late and I didn't think my would neighbors enjoy the sounds of a small block breathing through flowmasters as much as I do late at night. The next morning I went to fire it up and it sounds like it has a nasty rod knock. My question is what could cause an engine to knock this badly after just sitting overnight when it was running great the night before? The engine cranks freely all the rocker arms move the same amount. There is no noise when cranking the engine. I am going to double check the timing in the next few days to eliminate that variable. Do you have any recommendations about things to try to make sure that it is in fact a rod knock before I pull the engine again? I didn't think that you could bend a rod just by cranking and idling. Any input from you guys would be great and I thank you in advance!

    (Also this is my first post If I posted this in the wrong section I am sorry.)
     
  2. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

    Messages:
    26,875
    Likes Received:
    904
    Joined:
    Feb 26, 2004
    Location:
    Bloomington, MN
    Check your flex plate or flywheel bolts to be sure they are tight.
    A cracked flex plate can also make a "knock" sound.
     
    Dave Nelson likes this.
  3. 1980RS

    1980RS Veteran Member

    Messages:
    3,147
    Likes Received:
    506
    Joined:
    Jun 17, 2006
    Location:
    MN
    Welcome to NastyZ/28. Here is what we used to do at the dealership to isolate that problem. Take a plug wire off each cyl. one at a time to see if the knock stops, if it does not after all 8 then do like Terry said and recheck the flex plate. You can also take the torque converter bolts out and slide the converter back and that should take the load off the engine and the knock is gone you have found your problem. Let's hope it the latter of the two.
     
  4. Steven Simmons

    Steven Simmons New Member

    Messages:
    6
    Likes Received:
    1
    Joined:
    Sep 12, 2019
    Thank you both for the reply's! I will try the disconnecting of the plug wires. I did a 4 speed m21 swap a few years ago and used lock tight when I torqued it on before I put the engine in a few days ago. So I doubt that it would have come loose. I will put it on jack stands and use the good old long screw driver in the ear trick and see if I cant hear it from bell housing. I will update this thread for the sake of anyone who ever has a similar problem. I also forgot to add in the original post that I had excellent oil pressure and temps during the whole break in and every time there after. I have 40lbs at cold idle with it knocking away. Again thank you all for the help. Worse comes to worse I will be doing a stroker kit a little sooner than me and my bank account really planned on!
     
  5. tom3

    tom3 Veteran Member

    Messages:
    11,850
    Likes Received:
    311
    Joined:
    Aug 1, 1999
    Location:
    ohio
    Any possible chance of something falling down in the intake that might have gotten on top of a piston? Doesn't take much.
     
  6. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

    Messages:
    10,201
    Likes Received:
    414
    Joined:
    Nov 4, 2004
    Location:
    Ringgold, GA
    You can also use a mechanic's stethoscope, listen around the valve covers, check if you have a rocker nut that has backed out. Do you have an oil psi gauge, any loss in psi?
     
  7. Steven Simmons

    Steven Simmons New Member

    Messages:
    6
    Likes Received:
    1
    Joined:
    Sep 12, 2019
    I don't think so. I haven't taken the carb off so it hasn't been "open" since the break in process. I would think that if something was going to get sucked in there it would have been during the break in. I am not going to rule anything out until I know for sure. Thank you Tom!
     
  8. Steven Simmons

    Steven Simmons New Member

    Messages:
    6
    Likes Received:
    1
    Joined:
    Sep 12, 2019
    I pulled both valve covers and everything seemed to be just fine. And it has around 40lbs when I start it up. I have been hesitant to run it for longer periods of time since I don't want to damage any of the new parts I just put on. I will give you guys an update when I work on it this Friday or Saturday. Thank you CorkE!
     
  9. Steven Simmons

    Steven Simmons New Member

    Messages:
    6
    Likes Received:
    1
    Joined:
    Sep 12, 2019
    Hello everyone!

    I went ahead and pulled the engine and found that I had a cracked piston and a chunk came loose and bounced around in the engine. I think that it was cracked before the new heads and just couldn't handle the higher compression. The head can be resurfaced since I caught it early enough. All the parts I just bought look to be in good shape to be reused since It only has really been ran long enough to break in the cam and go around the block twice. The engine is going to the machine shop this week to get everything sorted out. I want to sincerely thank all of you guys for the suggestions!
     
  10. 1980RS

    1980RS Veteran Member

    Messages:
    3,147
    Likes Received:
    506
    Joined:
    Jun 17, 2006
    Location:
    MN
    Hyper pistons? cast or forged?
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.