Knock Question

[Jackson LeBlanc]

I have a 1987 4.2 L AMC 258 that my dad rebuilt 4 years ago. He broke the engine in slowly and after about a week the engine developed what sounded like a rod knock. There is also another knocking noise that happens when the clutch is engaged. When the clutch pedal is pressed the knock goes away. I isolated the rod knock to cylinder #1 I’m just not sure if it’s rod knock or piston slap but the knocking gets louder as the engine warms up and the oil gets more viscous. There isn’t any metal in the oil whenever I do changes I’m just concerned and I want to tear into the thing to fix it. I’ll record audio at some point and post it

 

[Chris]

I have a 1987 4.2 L AMC 258 that my dad rebuilt 4 years ago. He broke the engine in slowly and after about a week the engine developed what sounded like a rod knock. There is also another knocking noise that happens when the clutch is engaged. When the clutch pedal is pressed the knock goes away. I isolated the rod knock to cylinder #1 I’m just not sure if it’s rod knock or piston slap but the knocking gets louder as the engine warms up and the oil gets more viscous. There isn’t any metal in the oil whenever I do changes I’m just concerned and I want to tear into the thing to fix it. I’ll record audio at some point and post it

The fact that you hear a knock that improves with the clutch engaged could indicate that the noise isn't solely from the engine itself, as sometimes transmission or clutch components can add to the noise when they aren't properly aligned or lubricated.

The isolating of the knock to cylinder #1 is definitely helpful, but distinguishing between a rod knock and piston slap can be tricky without hearing it. Rod knock typically has a deep, consistent sound that correlates with RPMs, while piston slap usually presents itself as a more irregular sound and can sometimes be more pronounced when cold.

If there's no metal in the oil and regular oil changes are being done, that's a good sign. You might want to try these before tearing into it:

• Check the oil level and condition: Make sure you're using the correct viscosity oil based on your engine's needs.
• Check/replace the engine mounts: Sometimes bad mounts can transmit engine noise that sounds like a knock.
• Inspect the timing chain/belt: Make sure it's not too loose or worn which could contribute to noise.
• Listen for other sources: Use a mechanic's stethoscope or a long screwdriver to pinpoint the noise source more accurately.

If you decide to dig into it, documenting the process with audio and maybe video can help others assist you or even learn from your experience! Good luck, and keep us updated!