Spark plug gap closed fresh rebuild

[Cdog_96]

Uh oh, fresh rebuilt tbi 350 just changing the plugs and noticed #2 plug gap closed. Engine also runs rough under load, bad timing maybe ? What am I in for ? Any help appreciated thanks !

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[nvrenuf]

Are you sure that’s the correct STYLE of plug for your heads?

 

[Cdog_96]

Are you sure that’s the correct STYLE of plug for your heads?

Others maintained their gap, these are just stock tbi heads

 

[Rickf]

Get one of those economical (cheep) scopes and take a look at the top of the piston.

 

[Tonimus]

Dollars to doughnuts that either you aren't supposed to use extended reach plugs, or you have something riding around in the cylinder.

 

[Snoots]

That should be about an R45TS with a gap of .045

 

[gmbellew]

That should be about an R45TS with a gap of .045

I thought 350TBI (at least my 1990 burb) is CR43TS at 0.035 gap?

 

[nvrenuf]

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

 

[Ricko1966]

Put in another plug,correct plug,of course,maybe someone dropped that one prior to installation and never even looked at it.. If a new plug does the same thing. Start it with that plug out,run it a minute,then put a plug in. If that plug closes too,that head needs to come off to see what's hitting the plug.Dont waste time trying to diagnose it through the plug hole the heads going to have to come off anyway to fix it. I've had plugs close up like that on other engines due to rod bearings. IDK if plug location on sbc will allow that or not. But that's one thing I'd be looking at. Incorrect rod bearing,wrong piston,wrong connecting rod letting one piston park to high in the hole.if it's doing this at cranking speed,as in never started,I wouldn't suspect a rod bearing,because there wouldn't be the momentum needed at cranking speed.

 

[TotalyHucked]

Man that plug's F'd. Like @nvrenuf said, it bent the strap down hard enough to pull the porcelain to one side. I'd start with a cheap borescope, if you don't see anything, take all the other plugs out and put a fresh one in that hole. Put a bar on the crank bolt and turn the motor over by hand to see if it ever stops/gets stuck. If it does, take that plug out and then put the bore scope in that cylinder. It could be that that one got dropped before install but I find that hard to believe.

Who did the motor? Definitely need to figure out what happened with that plug first. If nothing is found, throw another plug in it and then check the timing before driving it again. Is it hard to start when hot? Could also be a combo of timing issues/advance issues and carb issues

 

[Grit dog]

Well it would run rough because it was missing on the smushed plug cyl imo.
There’s been some speculation since OP didn’t say what “fresh built” means. Appears it wasn’t from hand cranking the engine over as the plug has been run for a bit (ceramic looks brown) which suggests it didn’t smash upon first roll over of fresh engine.
And the background in the pic looks like a fully assembled ready to drive engine.
OP also didn’t say if they’ve replaced the plug and tested again.
Before doing anything, I’d spend the $3 and slap a new plug in it and see.
Need better description from the OP. Did he install the plugs and/or build the engine or has he no knowledge of the condition of that plug previously and someone else closed up the motor.

 

[Cdog_96]

Well it would run rough because it was missing on the smushed plug cyl imo.
There’s been some speculation since OP didn’t say what “fresh built” means. Appears it wasn’t from hand cranking the engine over as the plug has been run for a bit (ceramic looks brown) which suggests it didn’t smash upon first roll over of fresh engine.
And the background in the pic looks like a fully assembled ready to drive engine.
OP also didn’t say if they’ve replaced the plug and tested again.
Before doing anything, I’d spend the $3 and slap a new plug in it and see.
Need better description from the OP. Did he install the plugs and/or build the engine or has he no knowledge of the condition of that plug previously and someone else closed up the motor.

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.

 

[gmbellew]

I'm not an engine builder, but I am not sure how the timing being off will smash a plug like that.

 

[Grit dog]

So it’s possible (maybe not probable) that you smashed the plug on installation? Ran smooth, changed plugs, ran rough. And it didn’t smash the plug first time.
Gonna say slap a new plug in it and see what happens.

 

[scrap--metal]

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).

Get one of those economical (cheep) scopes and take a look at the top of the piston.

I'd start with a cheap borescope

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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