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Old 06-23-2018, 09:07 PM
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AtomicDog AtomicDog is offline
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Location: phoenix, az
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Default Back Tap spark plug hole threads???

I inadvertently cross threaded the first few threads in the spark plug hole in my #1 cylinder. I stopped before I completely destroyed the threads. But I am afraid if I run a regular 18mm tap starting from the outside I may tap crooked because the first few threads are crooked. I saw online a method to tap from inside the cylinder with a back tap. This would start my tap on good threads and work out toward the bad treads. This seems like a good idea. Has anyone tried this method. If so was it successful?????
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Old 06-23-2018, 10:37 PM
rocketman1988 rocketman1988 is offline
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Default yes

I did it on a VW dune buggy engine...worked great. Put a little grease on the tool threads to capture any metal. Also, don't tighten the mandrel all the way, all at once. Tighten it a little, run the back tap, tighten some more, run the tap. Continue until mandrel is fully expanded.
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Old 06-23-2018, 11:21 PM
BobTurner BobTurner is online now
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Careful - the ‘threads’ are helicoil. If they’re damaged, you may need to remove them and replace with new.
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Old 06-24-2018, 12:33 AM
JumpNurse JumpNurse is offline
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Location: Gilbert, AZ (and missing TX)
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I would be careful about doing that. Admittedly, I have limited experience with this in aircraft engines, but I do have a lot in race cars. If it is more that one or two threads, I would consider just putting a helicoil in it. I don't even know if that is an accepted repair in aircraft engines. I would imagine that getting a tap straight from inside the jug would challenging. Bad plug threads are a pain to deal with and rear their heads at terrible times in my experience. I don't recall the thread pitch on AC spark plugs, but I do have a set of specialty spark plug taps from snap on. They have a bit more taper to try and grab the good threads if you want to try one. I live near 5AZ3 if you want to try.
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Old 06-24-2018, 07:01 AM
tgmillso tgmillso is offline
Join Date: Nov 2012
Location: Launceston, Tasmania, Australia
Posts: 633

Again, no experience with this on aircraft engines, but cross threaded a glow plug hole in an alloy VW head, just the first thread like you have. A mechanic friend of mine used a back tap and it worked a treat. Had that cylinder at TDC and stuffed some clean, soft rope in there to stop debris falling into it, ran the back tap and then sucked the hole with a vaccumme whilst pulling the rope out.
If I was in your shoes, I'd try the back tap first unless anyone can say conclusively why you shouldn't go down this path.
Good luck.

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Old 06-24-2018, 07:31 AM
Scott Hersha Scott Hersha is offline
Join Date: Jan 2005
Location: Cincinnati, OH
Posts: 922

I had to replace a helicoil in my RV6 engine several year ago when I did what you with cross threading it. I had my local A&P do it for me - while I ‘assisted’, to learn. He has done it several times and has the right tools. It only took about 15 minutes and just like new.
Cincinnati, OH/KHAO
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Old 06-24-2018, 08:43 AM
lr172 lr172 is offline
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Originally Posted by BobTurner View Post
Careful - the ‘threads’ are helicoil. If they’re damaged, you may need to remove them and replace with new.
I am unsure of what will happen when you run a tap through a helicoil, but my speculation is that it is not recommended. Surely, running a back tap is filled with risk. Research how a helicoil is inserted and this should be apparent . There is a procedure for removing helicoils. You could then install a new helicoil to address the damage. If you damage the outter threads, you have an expensive repair on your hands.
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Last edited by lr172 : 06-24-2018 at 02:53 PM.
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Old 06-24-2018, 02:17 PM
deek deek is offline
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Location: Flathead Lake Montana - 8S1
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I *would not* back tap (or forwards) through a helicoil spark plug thread! Spruce sells a plug chasing tool that can be used to clean the carbon out of the threads, but it isn't the same profile as a cutting tap. If it were mine: I'd try the AS thread chasing tool, and if that didn't de-bugger the top threads I'd find a local A&P with a helicoil kit and have him install a new one; it's a 15-20 minute job.

Helicoil thread forms are different than cut or rolled threads - you will very likely damage or unwind the helicoil if you run a 75% thread cutting tap through it.
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Last edited by deek : 06-24-2018 at 03:55 PM.
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Old 06-25-2018, 01:59 PM
444TX 444TX is offline
Join Date: Dec 2005
Posts: 114

I see no problem lightly back chasing the plug threads. You have control with the amount of engagement and it is not a tap. Use common sense and keep it lubed. If no luck then move on.

I have a 14mm that works well on the rare occasion it is needed on car repairs.

Post some pics.

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A&P, ASE certified master mechanic
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Old 06-26-2018, 12:52 PM
lr172 lr172 is offline
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Cross threading in native metal is different than cross threading a threaded insert. The insert is more bendable due to the way it is manufactured. It is more like spring steel. It is the spring tension that holds the insert in place. If you look closely, you will likely see that the threaded insert is bent and not deformed. A thread chaser can only remove damaged threads on native metal, by breaking it off or shaving it. It can't un-bend the threaded insert back into it's original shape and it likely to start unthreading it out of the cyl head. I would examine closely with a 10X glass. As mentioned, if the threaded insert is removed incorrectly, it can damage the outer threads in the cyl head. Also, if it is bent, it will likely grip the chase once you go through the bent area and you may not be able to get the chaser out without pulling the insert and damaging the outer thread. Damage here, will required welding up the hole and re-tapping, if that is even possible and guaranteed to be expensive.

They make tools to properly remove helicoils and this would be the best path for you. It would only take a few minutes for a mechanic with the right tools to do this job safely.

In respect for the previous poster, I agree that a chase can be attempted if the operator is very careful about not letting it get tight or bind. I only fear that someone inexperienced may go too far and get in trouble. I feel it would be easy to get past the point of no return. A good examination of the damage should provide guidance on the likelihood of a chase addressing the issue.

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Last edited by lr172 : 06-26-2018 at 01:14 PM.
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