Mercedes-Benz Forum banner

1 - 20 of 39 Posts

·
Premium Member
1989 SEL 560
Joined
·
561 Posts
Discussion Starter #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.
 

Attachments

·
Premium Member
1991 560SEC
Joined
·
2,643 Posts
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?
 

·
Premium Member
1989 SEL 560
Joined
·
561 Posts
Discussion Starter #3
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?
 

·
Premium Member
1991 560SEC
Joined
·
2,643 Posts
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.
 

·
Registered
Too Many to List
Joined
·
168 Posts
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.
 

·
Premium Member
1989 SEL 560
Joined
·
561 Posts
Discussion Starter #6
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.
 

·
Outstanding Contributor
1990 350SDL, "Grandpa's Roadster" Project Car
Joined
·
3,010 Posts
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.
 

·
Premium Member
1991 560SEC
Joined
·
2,643 Posts
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.
 

·
Premium Member
1989 SEL 560
Joined
·
561 Posts
Discussion Starter #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.
 

·
Premium Member
1991 560SEC
Joined
·
2,643 Posts
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.
 

Attachments

·
Premium Member
1989 SEL 560
Joined
·
561 Posts
Discussion Starter #12
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.
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.
 

·
Premium Member
1991 560SEC
Joined
·
2,643 Posts
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.
 

·
Premium Member
1999 500SL, 1988 SEC
Joined
·
1,068 Posts
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
 

·
Premium Member
1991 560SEC
Joined
·
2,643 Posts
The local independent talked about pulling the heads and said most of the work has already been done.
I am not sure I agree with this part. When you put the heads back you may hear that Helicoil inserts are needed in the block, and then the real fun starts (expensive too!), not to mention additional issues that can occur if they mess the valve adjustments. Just ask member Stutz, he can write on novel on that.

Pulling the heads off is the last thing you want to do and chances are that unless the mechanic is very careful and picky when working on your engine, your engine will never run again like before. For that reason avoid that at all costs and use it only as a last resort.

After thinking some more on your issue, here is what I suggest you can try: Put the intake manifold back and drill with the appropriate drill size for Helicoil for M8x1.25 . Keep the drill bit centered with the hole in the manifold (a sleeve will be required). When done examine the hole, if it cleaned up all around then go with M8x1.25 Helicoil. If it did not, go to the next size up, drilling only. Do not tap Helicoil unless your hole came up clean. Don't hesitate to consider the appropriate drill for the 3/8" Helicoil if the one for Helicoil M8 did not work, you want to go with the smaller size that will do the job in order not to weaken the cylinder head.
 

·
Premium Member
1991 560SEC
Joined
·
2,643 Posts
Would M8 Timesert be a bit larger in diameter if the hole is ruined for M8 helicoil? Never used neither of them.
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.
 

·
Premium Member
1999 500SL, 1988 SEC
Joined
·
1,068 Posts
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.
 

·
Outstanding Contributor
1990 350SDL, "Grandpa's Roadster" Project Car
Joined
·
3,010 Posts
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.
 

·
Premium Member
1999 500SL, 1988 SEC
Joined
·
1,068 Posts
Whatever way you go PM me since I believe I have both kits and extra helicoils and time serts. I experimented to make sure I got the right length for all the head bolts including the manifold.
 
1 - 20 of 39 Posts
Top