# Spark Plug damage



## OLDBOB (Jan 27, 2013)

What would cause the tip of the spark plug to be pushed into the center electrode shorting it out. There is 3/8 of an inch of clearance between the plug and the piston when the piston is at top dead center. The motor has been running fine and then after a run od about 30 minutes to the Paradise hole the engine was only running on 3 cylinders. I changed the plugs and when coming in to the Pensacola Inlet the engine again lost the top cylinder. Each time the plug was damaged. Nissan 90 2 stoke. I'm lost! :thumbdown:


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## dsaito (Feb 3, 2014)

You have less than 3/8" clearance :wallbash:


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## sealark (Sep 27, 2007)

Is the entire tip crushed down or just the arm that extends over the electrode. In any case I haven't a clue what could be causing it.


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## Inn Deep (Oct 3, 2007)

If you have the right size spark plugs then things don't sound good


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## kanaka (Oct 3, 2007)

Sounds like a stretched con rod.


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## Pushwave (Oct 6, 2009)

Sounds like a bearing on the piston arm or crankshaft might be bad...


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## OLDBOB (Jan 27, 2013)

*Spark Plug Damage*



dsaito said:


> You have less than 3/8" clearance :wallbash:


 Your right, but I was just comparing by eye the depth of the spark plug and the length of the threaded rod into the cylinder when the piston was at top dead center. Also, when using a bore scope, there is no sign that the top of the piston is touching anything.


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## Pushwave (Oct 6, 2009)

Try a different brand of spark plug, who knows maybe it's a bad batch. If nothing is hitting it and it happens after the engine is warmed up. Never heard of that happening if nothing is hitting it.


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## OLDBOB (Jan 27, 2013)

Pushwave said:


> Try a different brand of spark plug, who knows maybe it's a bad batch. If nothing is hitting it and it happens after the engine is warmed up. Never heard of that happening if nothing is hitting it.


I never heard of it either, that's why I'm perplexed. If the entire top was crushed into the plug it would definitely mean some thing was hitting, but it just bent over closing the gap. And twice in less than an hours running time sure makes it a mystery.


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## Emerald Ghost (Mar 11, 2008)

Did this problem occur shortly after you replaced spark plugs ?
Make sure you are following the book on purchasing the correct replacement plugs.


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## Spoolin Up (May 3, 2011)

Are u looking at the scope, while rotating the engine? Plug has to be too long 

Sent from my SPH-L710T using Tapatalk


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## OLDBOB (Jan 27, 2013)

*Spark plug damage*



Emerald Ghost said:


> Did this problem occur shortly after you replaced spark plugs ?
> Make sure you are following the book on purchasing the correct replacement plugs.


 Same plug for years, Champion L-78C, .040" gap per Nissan manual. The first plug to get damaged was in for about 30 hours. Second plug to get damaged ( the replacement for the first ) lasted about 20 to 25 minutes.


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## OLDBOB (Jan 27, 2013)

Bigdaddy's said:


> Are u looking at the scope, while rotating the engine? Plug has to be too long
> 
> Sent from my SPH-L710T using Tapatalk


 Plug is factory specified, and they don't come any shorter than the Champion L-78C


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## sealark (Sep 27, 2007)

A long shot, Maybe carbon buildup on the one piston?


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## Emerald Ghost (Mar 11, 2008)

Good thought Sealark.


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## Mac1528 (Mar 24, 2012)

I've only seen one bent wrist pin in my life and that was in a BA engine that turned lots of Rs. Possible?


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## OLDBOB (Jan 27, 2013)

*Spark Plug Damage*



sealark said:


> A long shot, Maybe carbon buildup on the one piston?


 I thought of that also, but the depth gauge (threaded rod) leads me to believe that there is a lot of clearance between the plug and the piston. Also when I look in the cylinder with the bore scope there are no marks on top of the piston. It is only happening on the top cylinder. It seems to me that if the piston were hitting the plug it would do it on the first revolution, and not after running for 20 minutes and that the top would be totally pushed down not bent to close the gap.


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## sealark (Sep 27, 2007)

You got a weird one for sure. Go over to Wills Marine and ask Tony.


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## OLDBOB (Jan 27, 2013)

*Damaged Spark Plug*

I looked at everything that I and all of you have thought of. I'm going to run it again over the weekend and if it happens again I'll tear it down. There may be a new motor in my future. If I do find out what is the cause I'll let you all know what happened because of your help. Thanks to all. :thumbsup:


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## Mac1528 (Mar 24, 2012)

Good luck Bob! I hope you find the gremlin.


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## SteveFL (Aug 2, 2010)

Probably a dumb thought but did you verify the spark plug crush washer was there?


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## DAWGONIT (Jan 16, 2009)

is there room to use a spark plug extender? used one all the time on my old chevy.
let us know what you find.


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## OLDBOB (Jan 27, 2013)

SteveFL said:


> Probably a dumb thought but did you verify the spark plug crush washer was there?


 I sure did, thanks


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## OLDBOB (Jan 27, 2013)

DAWGONIT said:


> is there room to use a spark plug extender? used one all the time on my old chevy.
> let us know what you find.


 I don't know what an extender does, but if it holds the plug from going in like say a thicker crush washer I would try it. Thanks


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## OLDBOB (Jan 27, 2013)

*Spark Plug Damage*

Well I did my last checks today. I used a micrometer depth gauge along with the 14 mm thread for a plug. I accurately measured the length of the spark plug from the crush washer to the tip of the ground electrode - the one that bends over so you can set the gap. The difference is .185". That's a lot of clearance. I put the plugs back in and ran the engine in the driveway at 3000 rpm for 25 minutes. I took the plug out and the clearance was still .040". I'll put the boat in the water and run it for about 3 hours steady unless something happens. If something causes the plug gap to close I'll have a 200 lb. anchor and a new motor. Enough is enough!!!!!!


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## kanaka (Oct 3, 2007)

Did a compression check yet?


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## OLDBOB (Jan 27, 2013)

*Damaged spark plug*



kanaka said:


> Did a compression check yet?


 Yes, I had 150 lbs on the cylinder that the plug problem, and 140 lbs on the
other two cylinders. This is high compared to the factory spec of 120 lbs, but I attribute that to carbon buildup.

It ran in the driveway on ear muffs for 25 minutes today, so I'll give it a good workout on Sunday. If I still have a problem I'll make it an anchor. It's not worth putting any money in to it.


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## RMS (Dec 9, 2008)

You can use spark plug index washers to reduce the depth of the plug in the cylinder head to gain more clearance above the piston. It's done in performance applications all the time.


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## OLDBOB (Jan 27, 2013)

RMS said:


> You can use spark plug index washers to reduce the depth of the plug in the cylinder head to gain more clearance above the piston. It's done in performance applications all the time.


 With .185" of clearance between the plug and the piston I don't think that will do me any good


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## RMS (Dec 9, 2008)

Measuring clearance at room temp versus WOT operating temp could be a considerable difference. Perhaps you should measure the other cylinders to establish a benchmark.
Also, wear (slack) in the wrist pin and rod bearing would not be detected in your measurement.

If there is no mark on the piston then other causes should be considered. 

Could you have enough carbon build up to increase compression ratio to cause pre-detonation that is melting the tip causing it to collapse?

Do you have a lean fuel condition in that cylinder that is overheating the tip causing it to collapse?

Could it be a combo of both?

A specific evaluation of the the plug tip would be critical to analyze. In other words is it a mechanical force or a heat induced force?

I have no specific knowledge of your particular motor model, just thinking of possibilities without the benefit of actual hands on observation.


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## OLDBOB (Jan 27, 2013)

RMS said:


> Measuring clearance at room temp versus WOT operating temp could be a considerable difference. Perhaps you should measure the other cylinders to establish a benchmark.
> Also, wear (slack) in the wrist pin and rod bearing would not be detected in your measurement.
> 
> If there is no mark on the piston then other causes should be considered.
> ...


 I do appreciate your comments, they are very analytical. Thanks


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## kanaka (Oct 3, 2007)

Really off the wall here but since you have a bore scope, look around real good in the cylinder in question and see if there's a ball of carbon rolling round in there.

BTW, when was the last time you did a decarbon job on the motor?

And, if there is "slack" in the wrist pin or rod big end, I'm sure the OP would hear a knocking sound.


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## OLDBOB (Jan 27, 2013)

kanaka said:


> Really off the wall here but since you have a bore scope, look around real good in the cylinder in question and see if there's a ball of carbon rolling round in there.
> 
> BTW, when was the last time you did a decarbon job on the motor?
> 
> And, if there is "slack" in the wrist pin or rod big end, I'm sure the OP would hear a knocking sound.


 Firstly, I have not done a de-carbon job, and you may have hit on something with the carbon ball, however it would need to be really large to have not been blown out the exhaust port.
When you talk about de-carbon, are you referring to the spray can treatment, or removing the cylinder head? If it gives me a problem again tomorrow I'll pull the head.


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## kanaka (Oct 3, 2007)

Here. If you do this on the muffs, make sure you, your house, etc...are upwind.

http://www.thehulltruth.com/boating-forum/149269-dunks-decarb-method.html#b

http://forums.floridasportsman.com/archive/index.php/t-625.html

Gotta add: If you do this and there's a lot of buildup especially in the exhaust area of the motor, it'll be dripping a black goo out of the prop area. Put a drip pan there.


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## OLDBOB (Jan 27, 2013)

kanaka said:


> Here. If you do this on the muffs, make sure you, your house, etc...are upwind.
> 
> http://www.thehulltruth.com/boating-forum/149269-dunks-decarb-method.html#b
> 
> ...


 I'll do this right away and see what happens. When I look in the cylinder the carbon buildup doesn't look thick as in some spots I can see the piston. The head could be covered. It can't hurt, but only help. Thanks


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## OLDBOB (Jan 27, 2013)

Did the de-carbon job today, but as I was finishing up the 3rd 15 minutes of running the Seafoam mix through the motor suddenly stopped. It has seized up. I followed the directions and mix to the letter. It's not worth rebuilding so now I need your help in finding a new motor. My budget is really tight.


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## kanaka (Oct 3, 2007)

That sucked. Sounds like the carbon started dissolving and a ring hung.

There's this http://okaloosa.craigslist.org/bpo/5073187067.html

What's your $$ range?

https://pensacola.craigslist.org/bpo/5075596255.html


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## OLDBOB (Jan 27, 2013)

kanaka said:


> That sucked. Sounds like the carbon started dissolving and a ring hung.
> 
> There's this http://okaloosa.craigslist.org/bpo/5073187067.html
> 
> ...


 I'm sure that it was bound to happen, but you are at least 99.9% correct that is loosened things up and instant stop.
My budget at this point is for a new motor. I don't want to fool around.
Since I made the original post I think that my mind is made up to go new.
Your pretty savvy, who would you recommend?


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## kanaka (Oct 3, 2007)

You got PM.


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