Q&A / Tech

Mailbag: Tracking Down the Source of Engine Knock

Q: I built a 350 engine using parts from Summit Racing. At about 3,000 miles the engine would make a clacking noise at startup, but only when the engine hadn’t been run in a while. When the oil pressure came up (60-70 pounds), the noise stopped in a second or two.

At about 3,500-3,800 miles, I started hearing a knocking noise when the engine was warmed up. It seemed to come from the right side of the engine, so I adjusted the lifters on that side, but the noise was still there. While I had the valve cover off, I stethoscoped the rocker studs while the engine was running. The knock was audible through each one.

I couldn’t hear the noise through the stethoscope through any other part (fuel pump, water pump, intake manifold, etc.), which led me to think it was a bad connecting rod. With the engine running, I pulled the #2 cylinder ignition wire and the knock stopped (I could still hear it through the stethoscope). I removed the rod cap on the #2 cylinder. The bearing looked okay, but I haven’t Plastigaged it yet. There was about a teaspoon of sludge in the oil pan directly under the oil pan pickup. What should I do to fix the problem?

A: We’d encourage you to check out our original post on engine noises, but you were smart to check the connecting rod. Both the rod and the crank journal it rides on need immediate attention. Have the crank journal measured with a micrometer to see if it is within factory tolerances, and then have the connecting rod’s big end checked to make sure it is round and within spec. If you installed new rod bolts when you rebuilt the engine, you may have to have the rod resized.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.

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  1. randy oehlert says:

    did you really build an engine without checking clearances?
    check you’re thrust bearing and all mains for clearance.
    did you install new cam and cam bearings?
    don’t sound like a rod bearing to me.
    rod bearings most of the time knock louder when you kill the cylinder…(pulled the plug wire) because the rod doesn’t have as much pressure pushing down the piston which keeps it tighter on the rod journal.
    good luck..hope it turns out simple fix.

  2. Sound like a wrist pin or broken skirt.

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