knock knock

[billfish]

My 51 ford 8ba has developed a knocking noise over the past few months. It is noticeable when cold but seems to disappear after warm up. By accident I found that if i unplug #5 spark plug wire while engine is running the knock stops. I'm thinking this is a pretty good clue as to what the problem is, wrist pin or rod bearing maybe. Does anyone here have an idea that might help identify the source of the noise. Any help would be appreciated.

[Mike B]

That's what I was always told (pulling a wire) Bill, but, one that disappears (knock) when warmed up, might just be piston slop from a worn bore.

[Capt Kirk]

Did you pull the wire when it was cold? I don't have much experience with a flathead since I got my car with a fresh engine. I do however have a lot of experience rebuilding Jeep 4.0 engines (as a hobby) and what you're describing sounds exactly like a worn out 4.0. With these jeep engines, the clatter (almost like a diesel) is piston slap. My daughters jeep was doing exactly what you're describing and when I rebuilt her engine there was one piston that I could make that noise with my finger rocking the piston in the bore. If left unattended too long, a piston skirt will break and not long after the piston ends up in the pan and the wrist pin commences to open up the coolant jacket. I've seen each of these stages during these builds. My daughters engine ran fine, the clatter stopped when warmed up, bearings and wrist pin were in great condition. I caught it in time. A bad rod bearing presents itself in a different way than what you're explaining. Not sure about wrist pins in a flathead. Jeeps are pressed into the rod and float on the piston.

[jhb]

On my '40 V8 the exact same thing you describe was happening. It turned out to be the rod bearings and has been fine ever since they were replaced. The reason the noise stops after a wire is pulled is because there is no 'explosion' to rattle the loose bearing on the crank. Plastigage will confirm the diagnosis. Good luck, let us know what you find out.

[billfish]

The other day I dumped a bottle of Lucas heavy duty oil stabilizer in the crankcase and the knock or piston slap was gone immediately. I know this is just a band-aid fix but maybe it will buy me some time.

[flatjack9]

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I doubt that the oil stabilizer would stop piston slap. Bearings - yes.

[Roger W Gerow]

I once experienced a knock when engine was cold. After it warmed up the knock went away. Turned out to be a collapsed piston. 59AB engine.

[Capt Kirk]

A rod knock will quiet during acceleration and heavy deceleration. It will present itself when things are "slacked up"...between accelerating and decelerating and it won't go away. Piston slap (in my experience) will present itself when an engine is cold and it'll tend to diminish as things get warmed up. A worn rod bearing won't. I'm guessing piston slap and in the case of a Jeep 4.0, the piston will go away and things get a lot more expensive to fix.