#8 Spark Plug Wont Thread In All The Way

[Reynolds Rides]

Hey Guys,

I just bought a 94 ZR1 with 123,000 miles. I recently noticed that the #8 cylinder plug wire sits higher than the others. I have been able to determine that the #8 spark plug is not threading in all the way. I have removed and re-installed the plug about 3 times now with no apparent issues except for it not going in all the way (about 4 threads wont go in). There are no signs of cross threading or obstructions down the hole. I pulled the #4 cylinder plug to compare it to the #8 plug and they appear to be the same (all of the plugs currently in the car are Autolites) but when I tried to install the #8 plug into the #4 hole, it only went in a few threads. Also, the seller gave me the old set of spark plugs (AC Delcos) that he had removed and I tried to install one of those into the #4 hole and it would only go in a few threads as well. But, the #4 plug threads into the #4 hole all the way and without any issues. Hopefully that makes sense. Weird, right? The car seems to be running well but I would like to fix this. Any help or advice would be appreciated. -Jeff

[Jagdpanzer]

I've seen this problem before on #8 spark plug. Some time in past #8 plug wasn't properly tighten. Over time it started backing out. The exposed threads filled with hard carbon buildup to the point the plug will no longer screw all the way in. This can easily fixed by running spark plug thread chaser down the hole to clear out the carbon buildup from the lower threads. Pack the chaser flutes with dielectric tuneup grease to catch the debris from falling into the cylinder. Install a new set of spark plugs and you should be all set. Be sure thread chaser you get will fit down the spark plug hole along with a socket and extension.


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[Reynolds Rides]

In that scenario, couldnt I just soak the threads in sea foam to dissolve the carbon?

[DRM500RUBYZR-1]

I like Phil's answer a lot!
Very plausible explanation.

However, as suggested in the other forum, get a borescope and take a very careful look down that spark plug hole.

You are doing this blindfolded.

The scope will quickly allow you to determine if someone had attempted to install the wrong plug with a washer, or even two, and they remain down there and interfering.

The scope will also let you observe if there are deposits lodged in the threads as Phil has experienced.

Once you know the certain cause, solutions will narrow down to the correct set.

Those are expensive heads to be evaluating by speculation as opposed to easily obtained empirical evidence.

Good luck, keep us posted, and get a set of new plugs.
I am confident that you will have this corrected quickly once the true cause is known.
:cheers:
Marty

[jss06c6]

I like Phil's answer a lot!
Very plausible explanation.

However, as suggested in the other forum, get a borescope and take a very careful look down that spark plug hole.

You are doing this blindfolded.

The scope will quickly allow you to determine if someone had attempted to install the wrong plug with a washer, or even two, and they remain down there and interfering.

The scope will also let you observe if there are deposits lodged in the threads as Phil has experienced.

Once you know the certain cause, solutions will narrow down to the correct set.

Those are expensive heads to be evaluating by speculation as opposed to easily obtained empirical evidence.

Good luck, keep us posted, and get a set of new plugs.
I am confident that you will have this corrected quickly once the true cause is known.
:cheers:
MartySpot on Marty!

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[XfireZ51]

Thx Phil. I?ve experienced one particular plug hole, I think its #4 that appears harder to tighten down than others. However, I can?t say that the plug wite looks any higher than the rest of them. But given the number of miles on these heads, I may try the plug chaser thing on all of them anyway, couldn?t hurt to make certain.

[XfireZ51]

Phil,

The thread chasers at AZ and others takes a 13/16' socket, which won't fit our plug wells. What are u using for this and where are u getting it?

[WARP TEN]

Phil,

The thread chasers at AZ and others takes a 13/16' socket, which won't fit our plug wells. What are u using for this and where are u getting it?

While I have no experience in this Dom, I just Googled spark plug thread chasers and found a 14mm one at NAPA that used a 3/8" drive, not a socket. $7. Is 14mm the right size?--Bob

[XfireZ51]

While I have no experience in this Dom, I just Googled spark plug thread chasers and found a 14mm one at NAPA that used a 3/8" drive, not a socket. $7. Is 14mm the right size?--Bob

Yes Bob, 14mm is correct. However, checking the web I found that u can make ur own plug thread chaser by just cutting three grooves in a used spark plug, of which I have many.
Not sure why all plug chasers don?t come w 3/8? drives given all the OHC motors out there.

[Jagdpanzer]

Lisle 20020 Limited Access Spark Plug Chaser https://www.amazon.com/dp/B008MFR2IS/ref=cm_sw_r_em_api_glt_fabc_TRSDB2B4X7TR02VEA16R?_ encoding=UTF8&psc=1


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[1991 Corvette ZR-1]

Lisle 20020 Limited Access Spark Plug Chaser https://www.amazon.com/dp/B008MFR2IS/ref=cm_sw_r_em_api_glt_fabc_TRSDB2B4X7TR02VEA16R?_ encoding=UTF8&psc=1


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Thank you, sir. I bought one as a just in case. Great price.

[Reynolds Rides]

Problem solved. The lower threads were corroded so I chased them and now the spark plug goes in all the way and the car runs great. Thanks for your help. I made a YouTube video on the process. Check it out if you are interested:

https://youtu.be/RWaERjskaBk