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Heli-coil advice requested

2K views 38 replies 6 participants last post by  Tom Kaiser 
#1 ·
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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#2 ·
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?
 
#3 ·
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?
 
#5 ·
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.
 
#6 ·
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.
 
#7 ·
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.
 
#8 ·
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.
 
#9 ·
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.
 
#11 · (Edited)
If I understand correctly what you're saying, the drilling was so offset that even a M8x1.25 Helicoil will not have enough threads left on one side?

If I understood correctly, then I recommend you consider the following:
- Verify what size Helicoil you need that your hole will be able to be correctly tapped while keeping the center hole lined up with the hole in the intake manifold
- Verify if your cylinder head can accommodate that size hole without weakening and thus allowing for a crack to the occur future. If I understand correctly you are referring to the hole with the arrow, if so it looks you that there might be enough material, but that would need to be verified. Also investigate if the hole could go deeper.
- If you drill again in preparation for the next Helicoil, you may want to make some sort of sleeve on the drill bit, temporarily install the manifold, and use the bolt's hole passing hole through the manifold to hold your drill in place as you drill so that both holes will have same the center. Failure to have the same center may result in a crack in that area in the cylinder head in operation due to excessive stress.

Considering your situation, I suggest that you involve a good machinist to do it for you at home (friend, neighbour, etc.). If this operation will not succeed, then unfortunately I am afraid the cylinder head will have to come off and may have to be replaced. You don't want to go there, I promise you.

Perhaps other members have another solution.
 

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#12 ·
Liviu165, I called my local independent MB service shop and spoke with the gentleman that has helped me several times in the past. He knew exactly which bolt I had an issue with and said that I could try the M10 helicoil but no guarantee that I would be successful. It is the bolt with the arrow in your picture. He emphasized that the bolt must hold torque.

I plan to use the intake manifold as the template guide when I drill the M10 hole in the head. That said, I will need to get my intake manifold drilled out to M10 ahead of the job. I will look for a good machine shop for that job.

Since I am drilling an M10 hole then maybe I do not need to helicoil. Maybe just an M10 tap will do the job?

You are correct. If this M10 hole is not successful then things get ugly in a hurry. The local independent talked about pulling the heads and said most of the work has already been done. Then he went into great detail about all of the other tasks that should get done. By the end of the conversation I was exhausted.
 
#13 ·
Be careful what path you chose, once you tap for M10, you can't come back to Helicoil for M8 anymore. Same thing if you decide to Helicoil for M8 - you can't tap M10.

Basically you have only shot at this, a one way ticket trip. The guy you spoke with is correct in what he told you.

I don't know if just a M10 will do or not, but I know that if it does not hold up, things will get messy afterwards. I wish you the best, I don't know what else to tell you. I was hoping other members may have a better solution.
 
#14 ·
Here is what I would do. First, put the manifold on like Liviu suggested and make a sleeve for the M-8 size and drill it again to try and straighten the hole out. See what it looks like. Then I would drill both the manifold and head out to the M-10 helicoil size. Use the helicoil tap. Then get a helicoil for the correct hole size but with the M-8 bolt. Its a special helicoil but the do make oversize ones like that. You could also timesert the hole but that is a harder process. Trust me you do not want to take the head off. Good luck. If you have the helicoil kit, I might do all the holes
 
#17 ·
That's great question, it will come down to the drill size. The drilled hole (centered with the hole in the intake) will dictate if Timesert is more appropriate.

I have never used Timesert, but I would like to caution that Timesert requires a chamfered hole at the top and when having a gasket over a surface having Timesert installed, I think the sealing surface is reduced more than if Helicoil would have been used.
 
#18 ·
Liviu165 is right on the timesert but also it not only needs a chamfer but the surface of the thing you are doing needs to be countersunk with a special tool so the gasket will seal. For what you are doing I would stay with a helicoil. I am sure you have enough meat on the head to fix things. Keep us in the loop. Actually I think it was timesert that had the oversize insert, not helicoil.
 
#21 ·
I leave soon for a trip with my brother. That vehicle will now sit for a month.

I have enough time to investigate something I read about on my parts vehicle head. A saturated solution of Alum (ammonium aluminum sulfate) is supposed to dissolve ferrous bolts without damaging the aluminum head. I will try this on the same head bolt that also sheared on my parts vehicle. The head is sitting on my workbench so I only need to go to the grocery store to see if McCormick Alum has the correct chemical composition. If not then I will buy some off of eBay.

I still have M10 helicoil and the head from my parts vehicle as options for me.

I'll update the board sometime in June.
 
#19 ·
M8 helicoils require a 8.2 or 8.3mm drill, depending on the thread pitch. M8 Time-serts require a 13/32" (~10mm) drill. So, in order of material removed, M8 helicoil followed by 3/8" tap followed by M8 Time-sert. The advantage of the Time-sert is it keeps the hardware original. Disadvantage: the hole must be counterbored to get the insert flush with the surface of the head.
 
#24 · (Edited)
If you're cutting new threads, you won't need a helicoil. The only reasons to use the helicoil are to repair damaged threads, or to provide a better thread for a part that is regularly disassembled.

OBTW, I don't know how far off your drilling is, but, I will confess to having installed heli-coils in less than perfect holes, with acceptable results. I mean, it's not like the intake manifold is a high stress component... ;)
 
#25 ·
I agree with John in that this area is not critical in getting it perfectly correct, but I also went into the garage and looked at the M-10 and M-8 helicoil. I found you can actually double them if you want to get back to the original thread size and need to drill out to a larger size. That is drill it out bigger then put the large helicoil in then an 8 in the same hole which the larger helicoil is in.. I confess I have not tried this for durability, but no reason why it would not work.
 
#26 ·
I'm not sure how you determined that, but, absent data from Helicoil, I'd want to try it on a junk piece of aluminum first. I think it would be an incredible coincidence if the tap for an M8 Helicoil cut M10 threads (if I understand what you're saying.)
 
#28 ·
John, here is a picture of a M-10 x 125 helicoil and a M-8 x 125. The second picture show one being threaded into the larger one. Now I don’t know how strong it would be but if he is insistant on a standard size you could do this. Time sert also makes something called a large sert, which would work as well. When I did the heads I helicoiled all the bolt holes. The kit had lots in it and I figured I might as well when it was apart.
 

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#29 ·
I have ordered the M10-1.25 helicoil kit and a M10-1.25 40mm hex socket head stainless steel bolt. Also a 13/32" cobalt drill bit. The bit has arrived, the other parts are forthcoming. As mentioned earlier I will not get back to this project until late-June.

Inserting a M8-1.25 helicoil into a M10-1.25 helicoil should not be necessary for me since I intend to drill out the manifold bolt hole to M10 size.

I was not impressed with the Alum solution dissolving ferrous bolts. It takes way too long for that option to be seriously considered.

My parts vehicle right hand side head is sitting on my workbench and will be an option if the M10 helicoil fairs. If any of those bolts become problematic (as mentioned by Liviu165) then from my perspective it is game over.

I do have a question about that option. What would be needed to transfer my low mileage camshaft onto the high mileage head? Is there someplace that specializes in that task?
 
#31 ·
The helicoil changes size (gets smaller) when you thread it into the specially tapped hole. This is what helps keep them from unthreading when you remove the fastener. It is not reasonable to assume that, because one threads into the other on the bench that it will do the same thing in use. If that were the case, then the M8 helicoil tap would be exactly the same as a standard M10 tap, and it is not. If you don't believe me, screw the M8 helicoil onto an M8 bolt and you will see it is a very loose fit.

If you're planning to use M10 hardware, you can just drill and tap the original hole for M10; no need to go oversize then back to the M10 helicoil.
 
#36 ·
Liviu165, I put all of the pieces back into place since I will be gone for a month. They are not torqued but just lightly bolted into place. My memory requires this unnecessary step along with lots of notes and plastic baggies marked with specifics. When I return next month then I will take it apart again and get close ups. I am also recruiting a person with better mechanic skills than my own to help when I get back.

I want to thank everyone on this board who has contributed to my education. Yes, I make mistakes. Without this board I would not even try these tasks. I will keep trying until I am completely out of affordable options. There is a finite limit on how much money I will pour down this hole.
 
#39 ·
During reassembly of the lower and upper intake manifold pieces I decided to do the work in the engine bay. I loosened the EGR tube at the exhaust manifold connection and rotated the EGR tube upward in the engine bay while it pivoted at the EGR valve. I stuck a 2x4 block of wood under the bottom manifold piece for additional support. I hooked up the EGR tube to the bottom piece then attached the top manifold piece and bolted things together with three of the long bolts. This allowed me to check the eight gaskets for proper seal. The block of wood kept everything off of the new gaskets until I was ready to lower into place. A slight lift of the assembled intake manifold allowed me to easily remove the block of wood. It was easy to manage by myself.
 

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