Spark plug gap closed fresh rebuild

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Bextreme04

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Sorry for lack of info, this was professionally rebuilt by local shop just a stock tbi 350, I changed all the plugs after break in and this didn't happen. I went to drive it and it ran like crap under load in gear, as changing plugs again this is what I found. I am hoping it is timing because I have had trouble timing this engine.

That gap doesn't close without a significant physical force. It isn't timing and it isn't detonation. Either you pushed really hard on that plug while putting it in and closed that up or something bounced around in that cylinder and did that.
 

scrap--metal

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The spark plugs from my engine look like they were smashed differently than yours... I still had these 2 riding in my truck.

If you're lucky, your plug was bent before installation.

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Cdog_96

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The number 6 plug in my K10 has closed up like that on 3 occasions. Each time a peice of the piston bounces around and closes the spark plug gap causing the engine to run poorly (compared the baseline ). I replce the spark plug and it keeps going.

That's a high mileage 305, and not a fresh rebuild from a professional.

I bet something went down the intake, something was forgotten inside during the rebuild, or maybe detonation during break-in might've knocked part of the top ring land off the #2 cylinder (you mentioned trouble timing).



A borescope is definitely the proper tool to start with here.

Below is a borescope image of the problematic piston in my k10. Notice the missing edge on the left side.

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Does it go into spark plug hole ?
 

Max Headroom

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Some thing to consider:
The plugs screw into the head at an angle to piston top travel. If the insulator on the plug is actually bent and the gap is closed, it is likely that the piston top has hit the plug. In a stock application with stock timing, the plug shouldn't come in contact with the piston. The cam timing should prevent this. If you replace the bent plug and the engine still runs rough it is entirely possible that you havee jumped time at the timing chain and could bend more stuff.
What ever the issue is, I wouldn't start that engine to run till I knew absolutely what went wrong and fixed it. You could take off a valve cover on the bent plug side, put a socket and breaker bar on the end of the crank and rotate the engine slowly in the correct direction.If the chain jumped a couple of teeth. The engine should refuse to rotate after some initial movement. Moving the distributor to "re-time" the engine will not tell you if you have a physical interference issue. If you have full movement turning with the crank end & socket, buy shorter plugs.
 

Bextreme04

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Some thing to consider:
The plugs screw into the head at an angle to piston top travel. If the insulator on the plug is actually bent and the gap is closed, it is likely that the piston top has hit the plug. In a stock application with stock timing, the plug shouldn't come in contact with the piston. The cam timing should prevent this. If you replace the bent plug and the engine still runs rough it is entirely possible that you havee jumped time at the timing chain and could bend more stuff.
What ever the issue is, I wouldn't start that engine to run till I knew absolutely what went wrong and fixed it. You could take off a valve cover on the bent plug side, put a socket and breaker bar on the end of the crank and rotate the engine slowly in the correct direction.If the chain jumped a couple of teeth. The engine should refuse to rotate after some initial movement. Moving the distributor to "re-time" the engine will not tell you if you have a physical interference issue. If you have full movement turning with the crank end & socket, buy shorter plugs.
You are thinking of intake valve contact. The spark plug does not move and has nothing to do with timing. If the piston was going to make contact with the spark plug, it would do so at EVERY rotation of the crank, even with no cam installed at all.

I have never seen a stock truck SBC with a small enough quench area for there to be any concern that the spark plug is going to make contact with the piston.
 

Cdog_96

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Some thing to consider:
The plugs screw into the head at an angle to piston top travel. If the insulator on the plug is actually bent and the gap is closed, it is likely that the piston top has hit the plug. In a stock application with stock timing, the plug shouldn't come in contact with the piston. The cam timing should prevent this. If you replace the bent plug and the engine still runs rough it is entirely possible that you havee jumped time at the timing chain and could bend more stuff.
What ever the issue is, I wouldn't start that engine to run till I knew absolutely what went wrong and fixed it. You could take off a valve cover on the bent plug side, put a socket and breaker bar on the end of the crank and rotate the engine slowly in the correct direction.If the chain jumped a couple of teeth. The engine should refuse to rotate after some initial movement. Moving the distributor to "re-time" the engine will not tell you if you have a physical interference issue. If you have full movement turning with the crank end & socket, buy shorter plugs.
Motor turns over fine by hand, I bought a bore scope and looked in there and saw a few black pieces very small and couldn't tell what it was. Could it be carbon ? The cylinder walls look pretty clean but seemed like the crosshatch looked a little deep. Didn't notice any gouges or chunks missing off piston or piston rings. I rotated it a little each time and checked. Would popping a valve cover off tell me anything?
 

nvrenuf

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Pulling the cover won’t show anything relevant to inside the cylinder.

At this point, I think I’d install a new plug and roll the motor over a few times by hand to see what you feel and then check the plug.
 

Nardulli

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I would not run that engine until you diagnose the problem. Make sure that plug is the same as all the others. Pull the other plugs and do a compression test on that cylinder. If you have a borescope that would be something I would do first. if it is the correct plug, and like all the others, then clearly you have something making physical contact no matter what, that’s not good nor is it repairable without pulling that head off. ignition timing would not have any impact on this problem. A high lift excessive duration cam might, but you say it’s a stock build. In any event, this is on the engine builder. tearing things apart, might void the warranty, assuming you have one. Doing a compression check would not void a warranty so that’s worth doing, as well as the borescope. Anything else I would leave to the engine builder at his expense.
 

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