Spark Plug Thread Repair - Mercedes-Benz Forum
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post #1 of 17 (permalink) Old 03-14-2019, 08:37 AM Thread Starter
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Spark Plug Thread Repair

Greetings from the r107 subforum.

After committing the cardinal sin of using platinum spark plugs in my 84 380sl m116 engine, I have purchased a set of the correct NGK BP5ES plugs and Beru wires that I plan on replacing soon. To atone for my sin, I have a sacrificial BMW that I will be offering up to the Tristar gods.

Anywho, the damned number 8 spark plug hole has some partially galled threads that made it difficult to torque the spark plug down when I installed these older plugs a year ago. I got close to the required 29 ft-lbs, but not as good as I wanted. Knowing when I remove this spark plug, the threads will most likely be ruined on the way out.

In anticipation, I have purchased this OEM brand spark plug thread repair kit from the autozoo.

This thread repair kit is the one that threads into the larger whole and then is "hammered" into the hole, creating a permanent fix. There are little "tangs" at the end of the thread which prevent it from coming back out again.

From what I gather, this is the preferred way of repairing this issue.

I've watched several videos on how to do this properly. I have bearing grease which I will use to catch the shavings and some compressed air to blow the cylinder out to get any other shavings out.

My question is: is there enough aluminum for the entire insert to fully screw into the head without screwing too far into cylinder chamber? My fear is screwing this in and then having the spark plug head be too far in, making contact with the cylinder head. This was an issue on certain Ford motors outlined in this video here:

Edit: issue is at the 9:55 mark

Are there any other tips I should be reviewing? This is the first time I am doing this and would like to have any gotchas covered before I continue.

Thanks again gents.
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post #2 of 17 (permalink) Old 03-14-2019, 09:32 AM
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Typically, the threads that get damaged are the ones at the top of the spark plug threaded hole (makes sense).

This tool

https://www.ebay.com/p/Westward-1UBE...0991135&chn=ps

allows you to insert the tool into the hole and then cinch up the center expanding rod to catch the lower good threads. Then you back the tool out to straighten the upper bad threads. I didn't pay $100 for mine. Another option to think about. Cutting a complete new threaded hole using the technique you posted scares me to get all those cuttings in the cylinder - despite using grease to "catch" the cuttings. I wouldn't do it except as a last resort before taking he head off.
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Al Lydon
Atlanta, GA

1984 500SEL AMG -5.4L, Sebring exhaust, full suspension, 2.82 LSD, 8"/9" wheels, Recaros, door wood, Clarion rack stereo. Anthracite Grey/Black
1985 500SEC AMG Widebody, 199/278, 5.4L DOHC 32v, Sebring, suspension, fridge
1986 560SEC

Priors:
1990 560SEC AMG 6.0L, 149 Arctic White/Blue
1994 E500, 147 Polar White/Grey
1990 560SEC, 199 Blue Black/Black (Japan)
1991 560SEC, 040 Black/Palomino
1961 220SEb Coupe
1987 560SEC
1995 E420
1996 SL320
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post #3 of 17 (permalink) Old 03-14-2019, 10:17 AM
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Quote:
Originally Posted by alydon View Post
Typically, the threads that get damaged are the ones at the top of the spark plug threaded hole (makes sense).

This tool

https://www.ebay.com/p/Westward-1UBE...0991135&chn=ps

allows you to insert the tool into the hole and then cinch up the center expanding rod to catch the lower good threads. Then you back the tool out to straighten the upper bad threads. I didn't pay $100 for mine. Another option to think about. Cutting a complete new threaded hole using the technique you posted scares me to get all those cuttings in the cylinder - despite using grease to "catch" the cuttings. I wouldn't do it except as a last resort before taking he head off.
Alydon, here is an alternative at a better price. https://www.ebay.com/p/Powerbuilt-64...6042962&chn=ps
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post #4 of 17 (permalink) Old 03-14-2019, 10:30 AM
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I've repaired dozens of threads with Heli-coils. Never had a problem. The advantage as I see it is they remove very little of the original metal, and are easy to install. The only disadvantage I've ever heard of (never seen) is that the coil can pull out when you remove the plug some subsequent time. (I'm guessing those were not properly installed in the first place.) If it does, you have to notice it, and install another.

JMO...

John
"Real engines don't have spark plugs!"

Last edited by John350; 03-14-2019 at 10:35 AM.
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post #5 of 17 (permalink) Old 03-14-2019, 10:32 AM Thread Starter
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Alydon, here is an alternative at a better price. https://www.ebay.com/p/Powerbuilt-64...6042962&chn=ps
This looks like a MUCH better tool!

Thank you both for the suggestion and the better price. I just bought it. Hopefully it will get here by Saturday.

In the off chance that my backtap doesn't work, does anyone have advice relating to my first post above? Knowing my luck, i'll have to resort to it anyway.
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post #6 of 17 (permalink) Old 03-14-2019, 10:48 AM
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That Powerbuilt Back-tap will clean up slightly damaged original threads, but won't restore any strength if sections of the original thread have been cut or torn out. Another vote for the Heli-Coil spark plug repair kit, if any threads are missing.
https://www.stanleyengineeredfasteni...thread-inserts
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post #7 of 17 (permalink) Old 03-14-2019, 11:13 AM
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Helicoils are great repairs, no question. For $16, though, the OP can try the tool he bought. If that doesn't fix the threads (straighten them out sufficiently), then he can always fall back to helicoils. But even helicoils will drop some cuttings no matter how careful you are. Most of the debris would get sucked out through the exhaust, and the metal is aluminum, plus Murphy's Law applies to car repairs with great frequency. Finally, getting a helicoil kit on #8 or #4 is NOT easy to accomplish given the very tight work area with the head on the block!

If I had no choice but to helicoil the head, I would take the head off without hesitation.

Al Lydon
Atlanta, GA

1984 500SEL AMG -5.4L, Sebring exhaust, full suspension, 2.82 LSD, 8"/9" wheels, Recaros, door wood, Clarion rack stereo. Anthracite Grey/Black
1985 500SEC AMG Widebody, 199/278, 5.4L DOHC 32v, Sebring, suspension, fridge
1986 560SEC

Priors:
1990 560SEC AMG 6.0L, 149 Arctic White/Blue
1994 E500, 147 Polar White/Grey
1990 560SEC, 199 Blue Black/Black (Japan)
1991 560SEC, 040 Black/Palomino
1961 220SEb Coupe
1987 560SEC
1995 E420
1996 SL320

Last edited by alydon; 03-14-2019 at 11:15 AM.
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post #8 of 17 (permalink) Old 03-14-2019, 01:39 PM
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Quote:
Originally Posted by okyoureabeast View Post
This looks like a MUCH better tool!

Thank you both for the suggestion and the better price. I just bought it. Hopefully it will get here by Saturday.

In the off chance that my backtap doesn't work, does anyone have advice relating to my first post above? Knowing my luck, i'll have to resort to it anyway.
I recently bought this exact tool, anticipating problems next time I pull the right rear plug. I'd really appreciate it if you took some pics of using it on your car.

I hope it works and that you don't have to use a helicoil or other repair.

As to advice if it doesn't work, I would probably find a sleeve that I could slip over an installed plug and cut it to length to indicate how far the plug tip is from the head surface. Then you could use this sleeve to set the depth to which you install the new threads on the bad hole. Hope that makes sense.
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post #9 of 17 (permalink) Old 03-14-2019, 02:13 PM Thread Starter
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Originally Posted by luckymike View Post
I recently bought this exact tool, anticipating problems next time I pull the right rear plug. I'd really appreciate it if you took some pics of using it on your car.

I hope it works and that you don't have to use a helicoil or other repair.

As to advice if it doesn't work, I would probably find a sleeve that I could slip over an installed plug and cut it to length to indicate how far the plug tip is from the head surface. Then you could use this sleeve to set the depth to which you install the new threads on the bad hole. Hope that makes sense.
Can you elaborate on this some more? Apologies, i'm having trouble visualizing it.

I guess I could figure out the length with that tool I just bought if it doesn't work. Honestly, I get the impression that there's enough aluminum in there that the whole plug rests against. Those German engineers wouldn't be like the Americans and provide less because of heat diffusion. Or would they?
@AutohausAZ

Thanks for the heads up on that tool. Can you guys get order 1619063 out hopefully today? I put it in Tuesday night and it's still listed as processing. I'd like to get my new cap and rotor on as well!
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post #10 of 17 (permalink) Old 03-14-2019, 06:25 PM
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Quote:
Originally Posted by okyoureabeast View Post
Can you elaborate on this some more? Apologies, i'm having trouble visualizing it.

I guess I could figure out the length with that tool I just bought if it doesn't work. Honestly, I get the impression that there's enough aluminum in there that the whole plug rests against. Those German engineers wouldn't be like the Americans and provide less because of heat diffusion. Or would they?
@AutohausAZ

Thanks for the heads up on that tool. Can you guys get order 1619063 out hopefully today? I put it in Tuesday night and it's still listed as processing. I'd like to get my new cap and rotor on as well!
The concern is installing the plug too far into the head, right? So it is necessary to measure from the surface of the head, where the plug seats, to the tip of the plug. I would find a metal or plastic tube a little longer than the plug and just large enough to slip over one of your installed plugs. Trim it until it's as long as the plug sticking out of the head. So one end of this tube is resting on the head and the other is cut off even with the tip of the plug. This would give you an idea of how much plug is sticking out of the head, or in other words, how much plug is in the head.

Place this sleeve on the hole of the damaged threads when inserting your new plug. When you've installed the plug until the tip of it is even with the top of your sleeve, you've matched the depth in the head of the other plugs.

Man, this is hard without pics but I don't have any sleeve material to use for pics. Hope this gets you closer to understanding what I mean.
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