Spark plug gap closed fresh rebuild

Bextreme04 · Sep 19, 2024

Cdog_96 said:
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.

Snoots · Sep 19, 2024

nvrenuf said:
Note that it didn’t just close the gap, the porcelain is knocked off center.

Good eye John!

scrap--metal · Sep 19, 2024

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 · Sep 19, 2024

scrap--metal said:
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 · Sep 19, 2024

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 · Sep 19, 2024

Max Headroom said:
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 · Sep 19, 2024

Max Headroom said:
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 · Sep 19, 2024

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 · Sep 19, 2024

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.

Cdog_96 · Sep 19, 2024

Nardulli said:
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.

I didn't notice any chunks hanging around in the cylinder with the bore scope. There was small pock marks on top of the piston and some carbon flakes I believe. What should cranking compression be and do I just remove that plug for the compression test ?

Max Headroom · Sep 19, 2024

Nardulli said:
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.

It may not affect the plug issue but if the cam jumped time it would definitely make it run poorly. I agree that a compression check on all cylinders is a good idea. I still think there is a possibility the piston may have hit a valve and pushed it into the plug. For this to happen would mean definite damage to the valve (and maybe cause small black pieces in the bore). Pulling the valve cover a gently rolling the engine over would let you see if one of the valves wasn't moving as you roll it over.

nvrenuf · Sep 19, 2024

Cdog_96 said:
What should cranking compression be and do I just remove that plug for the compression test ?

A compression tester will screw into the spark plug holes. Removing all of the plugs will make it easy and faster to test.

The actual compression reading will vary depending on the compression ratio from engine to engine based on the parts used, just look for the readings to all be very similar. If you find a hole that’s more than 10%(iirc) different there’s a problem.

Craig Nedrow · Sep 23, 2024

Sounds to me like the head was milled to much, or like everyone’s saying, something in the cylinder. Timing would not bend a spark plug unless timing was so far advanced that that motor was detonating and the detonating bent it. I did it on my 345 IH dump truck.

Tonimus · Sep 24, 2024

Cdog_96 said:
There was small pock marks on top of the piston and some carbon flakes I believe. What should cranking compression be and do I just remove that plug for the compression test ?

What did those look like? It's entirely possible whatever was banging around in there did its thing and went out the exhaust valve. You should do a relative compression test. Pull them all and check. Anything over 120 is "enough".

Grit dog · Sep 24, 2024

Cdog_96 said:
I didn't notice any chunks hanging around in the cylinder with the bore scope. There was small pock marks on top of the piston and some carbon flakes I believe. What should cranking compression be and do I just remove that plug for the compression test ?

Bigger question is, was the rebuilder you or a shop? If a shop I’d consider delivering them a truck with an engine and a bent spark plug.

Spark plug gap closed fresh rebuild

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