Collapsed spark plug and knock!!
 

My '79 recently developed a slight knock in one of the cylinders. It also started missing, so I took all the plugs out and found #3 electrode collapsed onto the center electrode. I replaced all the plugs and started the engine back up. By shorting out all the plugs one at a time I found that #3 stopped knocking when shorted out. I then removed #3 plug and it was fine. However, the nature of the knock seems to point to a bad rod bearing. Engine only has 10K miles after thorough rebuild.

I'll pull the caps off #3 &4 rods later today to see what the crank and shells look like. Anyone else ever experienced something like this? It's impossible for the piston to hit the plug, so go figure.

Al

Could be something fell into that cylinder you could take it to a garage with a scope and they could look in the cylinder and see.

Do you think it could be timing? If the cap had wandered a little bit then you may have a case where you're entering pre-detonation on that cylinder, soon to be followed by the others. Since a rod knock is a mechanical problem, it would exist regardless of whether the spark plug operated or not. The fact it stops when you short the plug points to an ignition / timing problem. It seems reasonable to me that a mild case of pre-detonation could deform plugs and sound like a knock.



MajD

A typical rod knock will go away when the throttle is snapped and be the loudest as the engine slows down after the snap. It also tends to be quiet when first started and get louder after the engine runs for a minute or two.

Has oil pressure changed?

Rod knocks increase with engine rpm sitting in park or neutral. Disconnecting or grounding the spark will greatly lower the intensity of the knock.

You are correct, and that's what it's been doing. Shorting out the spark makes the knock disappear altogether. I removed the #3 connecting rod, and bearing from the crank. Bearings and crank journal look like new. Tomorrow, I'll Plastigage the clearance between a new bearing and journal to see if the clearance meets spec. It's a conundrum. If it's not this maybe a wrist pin??? We'll see.

The only time I've seen a plug electrode bent it was hit by a piston. The piston top also had a hole in it. If the bottom end checks out for clearance and torque, the next place I'd look is the wrist pin. If you do that, leave the pan off so you can pull the head and #3 piston.
I'm hoping this is something that you caught in time and did no damage.
Best of luck and please let us know what happened.

I've had stock cast pistons let a chunk go in the area between the top ring and the top of the piston. The plug would close when it happens. Ran one for a while with the missing piece (didn't know why the plug gap closed) till I pulled the head. These days a scope could find that problem pretty quickly.

I like the idea of borescoping the CC. Thanks for the ideas.

Al

A piston slap will also diminish when the spark is grounded, but if I remember correctly, it goes away slightly when accelerated.
Again, going by memory, I don't believe I could get a wrist pin to stop making noise when shorting the plug.
If you pulled the rod cap and it looks good, I would think about looking down the cylinder. Cheap scopes can be purchased that will connect via USB to a laptop. I picked up a $50 one years ago and it works OK, not totally great. I am sure newer one work better.
It really sucks when a serious engine noise ruins your day, I really hope it's something simple.

Wrist pin.

That would be my next guess. Changing the rod bearing didn't do it. I'll need the garage space fairly soon, so I'll put it away for the season and pull the motor out next spring.

Thanks for all the inputs

I had a sbc 350 that a rod stretched and if I turned it 7000 rpms it would immediately start skipping. It was just barely pecking the spark plug shut. 6800 and it wouldn't do it. Didn't notice any knocking though when it did it. Have had a piston skirt to break off and it would knock like crazy.

To close this thread out, which might help someone else. I did borescope the cylinder earlier this spring and found a chunk of piston missing. I think the intake valve hit it, because I had a broken valve spring, too. Ended up pulling the motor and replacing it with a 383 roller, 430 HP 450 lbs. torque with aluminum heads. Cost about the same as rebuilding the original motor. Numbers matching? Naw, but who cares?

Thanks for the update. :cheers: