Heli-coil advice requested - Mercedes-Benz Forum
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post #1 of 37 (permalink) Old 05-22-2019, 10:19 AM Thread Starter
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Heli-coil advice requested

The intake manifold bolt corroded into the block was totally uncooperative. I used every technique that I knew without success. I have drilled out the bolt and now I need to install a Heli-coil.

Is M8-1.25 the correct kit for one of those 40mm hex socket bolts? Do I need to drill out the current hole to a specific dimension before I start the tap?

Any and all additional comments will be appreciated.
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post #2 of 37 (permalink) Old 05-22-2019, 11:13 AM
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Yes, those are M8x1.25mm bolts. Follow the Helicoil instructions (drill first, then tap Helicoil, but be careful how deep you drill so you don't go through). You will need "blind hole" Helicoil taps.

I am not sure I understand what your problem actually is? Can't you just replace the bolts with new ones and clean the tapped holes? When drilling the old bolts out did you damage the existing tapped holes?
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post #3 of 37 (permalink) Old 05-22-2019, 11:58 AM Thread Starter
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Yes, those are M8x1.25mm bolts. Follow the Helicoil instructions (drill first, then tap Helicoil, but be careful how deep you drill so you don't go through). You will need "blind hole" Helicoil taps.

I am not sure I understand what your problem actually is? Can't you just replace the bolts with new ones and clean the tapped holes? When drilling the old bolts out did you damage the existing tapped holes?
Liviu165, when I drilled out the corroded bolt I was left with a smooth hole. My hole is slightly smaller than the original bolt but I am unable to get the old bolt material to come out of the existing threads. I tried several techniques but none of them were able to dislodge the old bolt material inside the threads. The old bolt metal seems "welded" into the threads.

Should I tap the current smooth hole for M8-1.25 and see what happens before drilling for the heli-coil?
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post #4 of 37 (permalink) Old 05-22-2019, 12:46 PM
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Should I tap the current smooth hole for M8-1.25 and see what happens before drilling for the heli-coil?
I would advise you to do that, put some oil in the holes first and let it sit overnight, hopefully it will help the remainder of the metal be removed later. You will need a blind hole M8x1.25mm tap. I think it will work but you have to be very patient with every hole.

If it would be me I'd try using a sharp 90 degree pick and "catch" the beginning of the steel threads left by the former bolts. I was successful to get the steel threads out after drilling the bolts. Later I applied a different technique: I slightly tap the bolt head first before taking it out. That dislodges the bolt a bit, the rest is easy.

Removing the old metal from the bolts is a much better approach, helicoiling should be your last resort. Helicoiling those holes will be another challenge, so try to remove what's left w/o damaging the aluminum threads. You'll thank me later.
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post #5 of 37 (permalink) Old 05-22-2019, 02:15 PM
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Good advice from liviu165. The only change we'd suggest is to use both a M8 x 1.25mm taper tap and a bottoming tap. Start with the taper tap, because it's easier to keep the tap straight in the hole so you don't accidentally start at an angle and cut into good threads. Once the taper tap hits the bottom of the hole, remove and then chase the lower threads with the bottoming tap.

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post #6 of 37 (permalink) Old 05-22-2019, 02:54 PM Thread Starter
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Good advice from liviu165. The only change we'd suggest is to use both a M8 x 1.25mm taper tap and a bottoming tap. Start with the taper tap, because it's easier to keep the tap straight in the hole so you don't accidentally start at an angle and cut into good threads. Once the taper tap hits the bottom of the hole, remove and then chase the lower threads with the bottoming tap.
I agree, Liviu165 is an excellent resource.

I will get a three piece, 8M x 1.25, tap set which has Taper:Plug:Bottoming taps. I'll put some oil in the hole this evening and let it sit until my tools arrive. Definitely worth a try.

Thanks to everyone providing input. I need all of the help that I can get.
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post #7 of 37 (permalink) Old 05-22-2019, 04:32 PM
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The steel thread material is harder than the aluminum head material. No way you can run a tap in there with the threads still in place and get good results. The tap will seek the softer aluminum and you'll end up with a loose coil of steel in the hole. Ideally you can use some penetrating oil and a sharp pick to unwind the steel from the hole, then you may be good to go, or you are in good shape to use the heli-coil. Otherwise, figure out what size drill the helicoil tap calls for and drill it out that big. With any luck all the steel will come out at the same time. If that doesn't work, you can use a different insert, Time-sert, for example, that uses a larger hole. Just make sure there's enough material so you don't strike water.
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post #8 of 37 (permalink) Old 05-22-2019, 05:21 PM
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Thank you, Tom.

I totally agree with post #5, very good advice.

John is right too in regards to re-tapping with M8x1.25: it may damage the aluminum threads which are softer, he is recommending (with better words - I wish English was my native language) the same thing: extracting the steel threads left in the hole by the original bolts with a sharp pick and penetrating oil.

One thing that is essential in your case for successfully uncoiling the steel threads left is that the hole through the bolt is as big as possible w/o touching the aluminum threads and is centered with the tapped hole. That weakens whatever steel is left in the hole and what you'd get out would look basically just like a steel insert. I've done it before but it was never easy.
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post #9 of 37 (permalink) Old 05-24-2019, 01:01 PM Thread Starter
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Update: I did not realize it at the time but I was not centered well enough in the bolt that I was drilling out. The hole that I drilled was too big for an M8-1.25 helicoil to get a grip. It was close, but not adequate.

Will a M10-1.25 helicoil fit in the bolt hole area that is adjacent to the number one injector? It is the second bolt from the front on the passenger side.
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post #10 of 37 (permalink) Old 05-24-2019, 01:57 PM
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3/8" hardware is about 9.5mm and Helicoils are readily available, if you need bigger than M8 but less than M10.
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