Spark plug threads stripped. Very strange

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AdminJeff
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Spark plug threads stripped. Very strange

Post by AdminJeff » Thu May 09, 2019 12:17 am

On the last tour had all kinds of problems that could have put me on the trouble truck. I avoided all of them. Without getting into details, one issue had me regapping the points and plugs. Upon trying to to Remove the spark plug from #2, it failed to budge. Literally no movement with a 18” wrech and all my massive biceps effort! Recall that I had literally just rebuilt the motor a week ago and installed all plugs by hand with no issues. Also know that I’ve replaced no fewer than several hundred spark plugs in my automotive Porsche racing career. Not once have I had an issue threading a spark plug into its orfice. And those plugs have fine threads, not 1/2” course pipe threads that are all but impossible to “screw” up. But today I was determined to remove the plug in question no matter the consequences..

The plug in question required the maximum setting on my air impact wrench to extract it. The bottom of the threads were totally destroyed. I’ve never seen this happen on a spark plug before. Ever.

$72 later and an hour delayed on my daily drive by using a proper 1/2” tapered thread tapping tool and I’m back on the road but WTF?

More info: stock iron head. No mods. No torque wrench on the plugs, just standard wratchet tight. All other plugs came out with no issues. No anti-siege used previously. Now used on all plugs.

Confused,

AdminJeff
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wayne sheldon
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Re: Spark plug threads stripped. Very strange

Post by wayne sheldon » Thu May 09, 2019 4:44 am

Just a few thoughts. Any chance the spark plug hole in that head had been re-tapped in the past? The damage to the two lowest threads looks like what i ran into on a non-Ford many years ago when an incorrect plug had been used in a car I had bought. The depth of the plug ran slightly beyond the threads and exposed those threads to the burning pressures of the firing cylinder. It apparently did not take long to "burn the threads enough that when I went to remove it, the galled threads bound going up and out through the head. A couple cylinders gave me some trouble, one was so bad that I was afraid I had ruined the head. Fortunately, a re-tap and proper fitting plugs fixed it okay.
You should carefully measure and check that plug fit, and probably the other three also. Model Ts sometimes develop funky threads, and won't go all the way in as they should. Sometimes when people have troubles that way, they re-tap and wind up with problems the other way.
IF ( my famous big IF again) the plug is threading in a bit far? As long as it isn't too close to the rising piston, and otherwise fits okay? I would carefully remove the two or three bottom threads with the bench grinder to avoid future seizing of the plug. Alternate fix, try a different brand of plugs. Maybe Champions or something won't go in so far?

Some concern about ill fitting parts causing future problems? If the head has been over tapped in one hole? Removing a couple threads is a good fix (as long as it doesn't reach too far down!). But the potential is there for you or someone else in the future to mix up the plugs and end up with the same problem you have. If the head has been over tapped? The only best fix is another head. A Helicoil might be okay, but I am uncomfortable with them in that application (although I have about 50,000 miles on one in my 2001 Expedition!). A trick I use to keep track of such marginal patch jobs is a dash of red or yellow paint. Put some paint around the base of the plug and the depression in the head. Every time you open the hood, you will remember that issue. If you need to clean and/or gap your plugs? You will have a ready reminder to put the modified plug back in the hole that needs it. Then whenever there is an added reason to pull the head (again!), replace it then at your convenience. Besides, it makes an interesting conversation piece in the tour parking lot chit-chats when you have your hood open and somebody asks about the odd paint spots.


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Re: Spark plug threads stripped. Very strange

Post by Autie » Thu May 09, 2019 8:05 am

Thinking back, when inserting the spark plug, you said it went in easy by hand. Do you remember if the plug took more revolutions with even the slightest resistance towards the end with your socket to fully tighten than the others? When you tapped the threads after the removal of the damaged plug, did the tap start off easy and then towards the the bottom, start having even the slightest resistance? Where I am going with this is, the damage on the plug occurred only on the bottom few threads tells me that is where the issue was on the threads of the head. Did you reuse your old head during the rebuild or replace it with a different head? After retapping, installing the new plug and then running the motor for a short while, try removing the new plug to see if you have the same issue but I am guessing that after running the tap through, you have rectified the problem. Wish you all the luck.


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Re: Spark plug threads stripped. Very strange

Post by Allan » Thu May 09, 2019 8:43 am

Re-tapping a tapered thread can only result in deeper penetration of the spark plug. That's OK if there are no clearance problems, but may result in galled threads on the plug as Wayne suggested. A sure fix which will work until the head needs pulling/replacement is to fit 14mm adaptors and run modern plugs.
Hope this helps.
Allan from down under.

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Re: Spark plug threads stripped. Very strange

Post by AdminJeff » Thu May 09, 2019 9:43 am

allan wrote:
Thu May 09, 2019 8:43 am
Re-tapping a tapered thread can only result in deeper penetration of the spark plug. That's OK if there are no clearance problems, but may result in galled threads on the plug as Wayne suggested. A sure fix which will work until the head needs pulling/replacement is to fit 14mm adaptors and run modern plugs.
Hope this helps.
Allan from down under.
All super interesting points. After I retapped the hole, which started off easy and then near the bottom I felt some resistance, the new plug fit exactly the same depth as all the others. This is only indicative of alll the plugs being at the same depth. I’ll also check how much they protrude into the head. It’s a little but hasn’t interfered with the piston yet.

I like the idea of grinding off the lower threads and the best idea I think is the 14mm adapters until I replace the head with a high compression unit - I’m still searching for a used one that I can afford as the wife controls the checkbook these days....

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Les Schubert
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Re: Spark plug threads stripped. Very strange

Post by Les Schubert » Thu May 09, 2019 10:34 am

Heli Coil makes a repair kit for 1/2” npt threads. I own the installation kit and have saved a number of heads by using these as required over the years. They seal great and plugs install just fine. Yes, the head does have to be off the engine

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Re: Spark plug threads stripped. Very strange

Post by Mark Gregush » Thu May 09, 2019 10:38 am

Is the head aluminum and did you try taking out the plugs while hot?
I know the voices aren't real but damn they have some good ideas! :roll:

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Re: Spark plug threads stripped. Very strange

Post by Autie » Thu May 09, 2019 2:06 pm

If tapping the thread eliminated the problem, let a sleeping dog lay until you replace with a different head. Also, what good would grinding off the lower threads accomplish if the replacement plug is fitting properly and has no more issues? >>>>IF<<<< and only IF there is still an issue, why create more work for yourself? If the issue arises again, Allens fix would be a good solution. ......"If it ain't broke (anymore), don't fix it"


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Re: Spark plug threads stripped. Very strange

Post by Scott_Conger » Thu May 09, 2019 2:27 pm

Completely off-topic other than the brand of plug...I cannot get my stem-wind '13 to even "pop" much less start, with Autolites. Champion "X" plugs in the car and it lights right off. Previous owner complained that it was ornery and difficult to start. No kidding! I wouldn't have an autolite in a single T that I own.
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Re: Spark plug threads stripped. Very strange

Post by AdminJeff » Thu May 09, 2019 10:53 pm

Scott_conger wrote:
Thu May 09, 2019 2:27 pm
Completely off-topic other than the brand of plug...I cannot get my stem-wind '13 to even "pop" much less start, with Autolites. Champion "X" plugs in the car and it lights right off. Previous owner complained that it was ornery and difficult to start. No kidding! I wouldn't have an autolite in a single T that I own.

Interesting. I’m curious why? Perhaps different resistance value in the plug.

What’s the part number of the correct Champion plugs?

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Re: Spark plug threads stripped. Very strange

Post by Scott_Conger » Thu May 09, 2019 11:03 pm

Ask the Forum "Pretty Please" for a set of original old Champion X plugs and I'll bet a set appears. Of the modern equivalent one-piece and the spendy two-piece plugs (which came with defective double threads at no extra cost for several years), I don't care for either of them. The original two-piece vintage plugs are the right length (position in combustion chamber) and run forever. Over the years that's what I've gravitated to and could not be happier. The old set in our '19 has many thousands of miles over 7 years and have not been out of the head in that entire time.
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Re: Spark plug threads stripped. Very strange

Post by AdminJeff » Fri May 10, 2019 8:54 am

Scott_conger wrote:
Thu May 09, 2019 11:03 pm
Ask the Forum "Pretty Please" for a set of original old Champion X plugs and I'll bet a set appears.
Pretty Please Mother May I?

I’d gladly keep administering these forums for 4 plugs :D

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Re: Spark plug threads stripped. Very strange

Post by Tbz4u » Fri May 10, 2019 2:11 pm

Thats $72 bucks you could have put toward a real head Z or pruis.. I would gave you a stock head . I have stacks of stock ones! BUDDY FOR WHAT YOU AND PATRICK DO FOR THIS SITE IS PRICELESS. KEEPING OUR OLD AGE PORN UP! :D :lol:

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Re: Spark plug threads stripped. Very strange

Post by AdminJeff » Fri May 10, 2019 6:31 pm

Tbz4u wrote:
Fri May 10, 2019 2:11 pm
BUDDY FOR WHAT YOU AND PATRICK DO FOR THIS SITE IS PRICELESS. KEEPING OUR OLD AGE PORN UP! :D :lol:
It’s Chris actually and that totally cracked me up!!

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Re: Spark plug threads stripped. Very strange

Post by TonyB » Sat May 11, 2019 2:58 pm

Jeff, I believe that the head has been reworked before you purchased it and the threads “cleaned” up. This allows the plug threads to extend into the combustion chamber and makes removal of the plugs very difficult due to the carbon build upon the bottom of the threaded area. You cleaning it up again will make the problem worse.
You need a new head.
If you don’t have local friends who can donate one (I have seven but I’m 500 miles away) then grind the threads from the bottom of the threaded area of all four plugs. I would not do this but if you need a fix, it’s the quickest and cheapest solution.
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Re: Spark plug threads stripped. Very strange

Post by AdminJeff » Sat May 11, 2019 5:03 pm

TonyB wrote:
Sat May 11, 2019 2:58 pm
You need a new head.its the quickest and cheapest solution.
Funny my wife tells me this often!

Another member offered me a Z head which is on my list as soon as I can afford it.

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Re: Spark plug threads stripped. Very strange

Post by John Warren » Sun May 12, 2019 10:03 am

Jeff, I don't think that the plug threads were compromised by the plug going in to far, your plug was galled when you installed it. Maybe hot and dry had something to do with it. Be sure and use something on your threads. Your clean up job, as long as it is not leaking will be fine.
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Re: Spark plug threads stripped. Very strange

Post by Ruxstel24 » Sun May 12, 2019 10:17 am

I agree with John W here, but removing did it more than a damaged thread to start with. Most common on aluminum to steel threads, but I've seen cast/steel, steel/steel do the same. If dry threads catch together upon removing, a piece can get stuck on the fastener/plug and gall the whole thread up. Pipe threads are a little more forgiving do to the taper, straight threads generally requires an instert.

(Japanese fine threads and a bit of rust is a booger)

I used a wee bit of pipe dope on mine in a aluminum head and they seal well, no problem removing.

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Re: Spark plug threads stripped. Very strange

Post by GrandpaFord » Mon May 13, 2019 1:20 pm

Use a Teflon tape or anti-seize compound. If you use the thin white Teflon tape, use several wraps. The yellow or pink stuff is thicker and works better on the plug threads. The copper based anti-seize compound works the best for me.

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