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  #1  
Old 09-29-2009, 12:25 PM
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Glow plug reaming, but what about cleaning the threads?

After seeing a few threads about stuck or difficult glow plugs (mainly OM606 - does this affect other OM60x engines, as much?), I've wondered a couple things.

All the reamers that I've seen clean the immediate chamber around the glow plug (see this post here for pictures - the second is a cleaner tap - discussed later in this post).

However, with stuck glow plugs, it would seem that the immediate problem is carbon in the threads, not the carbon in the area that would be removed by the reamer (which would insulate the glow plugs while operating). Enter repair taps - also known as thread cleaning taps. They aren't designed to cut threads, but clean them. Here's an example.

Now, my real questions are...

Has anyone tried this?
With what engine?
What are the thread sizes and pitches for the various engines (I can't find any of this on the forum)?


I would assume a procedure would be something like this:
  1. Get engine nice and hot (see post #10)
  2. Remove glow plugs
  3. Ream chamber - remove carbon with a shop vac or with grease on the reamer to catch the carbon.
  4. Clean threads (possibly with a little grease or shop vac - again, carbon)
  5. Replace GPs with new ones (and an anti seize compound on the GP threads).
Comments?

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Last edited by Crazy_Nate; 09-29-2009 at 05:56 PM.
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  #2  
Old 09-29-2009, 03:14 PM
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I'm sure cleaning the threads wouldn't hurt. But the problem is in the carbon build-up below the threads. So use the reamer and use anti seize on the newly installed GP's. trust me on this. It was a very expensive mistake I made. If you read my past posts regarding broken GP in my '98 E300, you'll see. I was so disgusted with the outcome, I stopped posting. But in the end, the car turned out to be garbage with a destoryed engine. Had to sell the car for parts. Then found myself a decent 1999 in good condition. I loved the car so much, I bought that one. But I'll never make a GP error again!
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  #3  
Old 09-29-2009, 04:12 PM
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That sounds like good procedure..
but I would add due to seeing on other threads " DO NOT use the engine to try to blow out any carbon droppings " ... because you do not know which part of the stroke your piston is on and might draw them into the combustion chamber, or precombustion chamber.. which might clog some of those tiny fuel spray calibrated radially drilled holes pointing your fuel in the proper direction...
Also, do not use compressed air.... either grease or vacuum to remove loose carbon.
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Old 09-29-2009, 04:49 PM
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Ok, after doing a little research "15-0705 Function of Preglow System", the proper size for the OM603 and OM602 is M12 x 1.25.

Quote:
The pencil-type glow plugs consist essentially of a housing with a M12 x 1.25 external thread and a heating element press-fitted into the housing.
In the link (repair tap example) I posted earlier, the full length tap for thru and blind holes is about $50.

I haven't been able to find the size for the OM606 (I have the W124 CDs - basically the 94 or 95 E300D). There simply isn't the same information as with the OM603, just how to test the preglow system and the R&R of the glow plugs.

Are they the same size as the OM603? It looked like different parts on fastlane.
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  #5  
Old 09-29-2009, 04:59 PM
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The threads on a 617 are 12x1.25.
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  #6  
Old 09-29-2009, 05:02 PM
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Quote:
Originally Posted by toomany MBZ View Post
The threads on a 617 are 12x1.25.
I see a pattern...

Thanks
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  #7  
Old 09-29-2009, 05:03 PM
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Quote:
Originally Posted by Crazy_Nate View Post
Ok, after doing a little research "15-0705 Function of Preglow System", the proper size for the OM603 and OM602 is M12 x 1.25.



In the link (repair tap example) I posted earlier, the full length tap for thru and blind holes is about $50.

I haven't been able to find the size for the OM606 (I have the W124 CDs - basically the 94 or 95 E300D). There simply isn't the same information as with the OM603, just how to test the preglow system and the R&R of the glow plugs.

Are they the same size as the OM603? It looked like different parts on fastlane.
For the above in blue I would go to your favorite internet parts site and compare the pictures.
So far as I know all the Pencil type Glow Plugs have M12 x 1.25 threads.
The difference is in the length of the Glow Plug; this can be seen by way of the pictures at the parts site.
The other physical difference on the long Glow Plugs has to do with how the wires attach to the end.
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  #8  
Old 09-29-2009, 05:11 PM
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I doubt carbon in the threads is causing them to get stuck. My guess would be corrosion due to dissimilar metals. Steel GP into an alum cyl head is a recipe for thread corrosion.

I've never cleaned the threads during a GP change, not on my VW's or MB's. I don't feel it is necessary.
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  #9  
Old 09-29-2009, 05:41 PM
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Ok, speaking hypothetically again, the use of an anti-seize grease (ie. zinc - for the aluminum / steel combination) would help reduce corrosion. Is this "stuck" glowplug problem solely a corrosion issue resulting from a lack of prevention?

Cleaning the threads is certainly not surefire way for prevention - but, if you're careful, all it can do is bring the threads closer to their original condition. Does anybody know if the engines came from the factory with anything on the GP threads? The service manual doesn't mention anything.

Apologies for my curiosity...it's getting the best of me, haha
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Old 09-29-2009, 05:47 PM
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Quote:
Originally Posted by Crazy_Nate View Post
Ok, speaking hypothetically again, the use of an anti-seize grease (ie. zinc - for the aluminum / steel combination) would help reduce corrosion. Is this "stuck" glowplug problem solely a corrosion issue resulting from a lack of prevention?

Cleaning the threads is certainly not surefire way for prevention - but, if you're careful, all it can do is bring the threads closer to their original condition. Does anybody know if the engines came from the factory with anything on the GP threads? The service manual doesn't mention anything.

Apologies for my curiosity...it's getting the best of me, haha
No you do not want to put anything on the threads. The threads are an electric interface. It is the ground for the current flowing through each GP.

There is no stuck GP problem. The problem is folks trying to remove the GP's on a cold engine. Want them all to come out nice and easy? Go for a good hard drive for 20 minutes, get the engine gootundhot. Come back and immediately start pulling the GP's out. They will all come right out like butter. Here the dissimilar metals are working in your favor since the alum head expands more (and faster) than the steel GP's.

Yes, you'll need to wear heavy cotton gloves to avoid burning your hands while working on the hot engine.
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  #11  
Old 09-29-2009, 06:06 PM
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Ok, list appended (in first post). I fully understand the differences in coefficients of thermal expansion (for volumetric aluminum is roughly 3x steel - at room temperature, linear is about a factor of two).

With that said, do you install on a cool / cold engine? The torque for installing (and if you're cleaning the threads - tolerances on the tap) will have varying effect depending on temperature. For instance, if you apply a certain torque on a hot engine...and it contracts while cooling, the effective torque would be more. If I had a choice, I would definitely ream / clean / install on a cool engine.
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  #12  
Old 09-29-2009, 06:59 PM
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Yes use anti-seize. The anti-seize with which I am familiar, all use metal in the paste, it will conduct electricity just fine.

Is the glow-plug problem just dissimilar metals corroding (electro-galvanic corrosion)? If so, why do my 603 glow plugs come out of cold heads without protest (with no anti-seize apparent)? The problem seems to manifest itself almost exclusively on the 606 engines, which are almost 10years newer. Possibly an alloy with more magnesium / more anodic? More zink/zinc?

Cleaning the threads can't be a bad idea, but getting particles in the pre-chamber is a risk. I would consider turning the engine over without glow-plugs after the cleaning and vacuuming in an attempt to force any stray carbon out through the hole and prevent the scenario in post #3.
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  #13  
Old 09-29-2009, 07:07 PM
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The problem isn't to do with corrosion or the threads at all.

What happens is that combustion gas gets past the taper seal, and carbon gets rammed into the cylindrical gap between the glow plug and the bore in the head.

If the glow plug hasn't leaked past this "seal", then, after initial loosening of the glow plug, you can twist them out by hand - the threads run that freely.

If, however, the seal has leaked, the glow plug will fight you every degree of turn on its way out, squealing and squeaking while it moves.

Penetrating oil can help, but, as the glow plug sits horizontally, and the carbon is well rammed in there, not much penetrating oil actually penetrates very far.

Having the head hot and expanded is about the best you can do.

When replacing the glow plugs, making sure the taper seat is in tip-top condition, and using the correct installation torque is, again, about the best you can do. Lubricants in this gap between glow plug and head are going to become carbon very quickly at those working temperatures, so, I wouldn't bother too much about using it.
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  #14  
Old 09-29-2009, 07:10 PM
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Anti-seize uses lubricants such as zink, which won't melt and become carbon.
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  #15  
Old 09-29-2009, 07:28 PM
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Number cruncher said it perfect. The problem with the 606 is the carbon buildup in the tapered part. No matter what you do, carbon will build up in there. I have never heard of such a problem with other diesels. This is clearly a characteristic of the 606. Do yourself a favor and once you clean out the carbon real good with the reamer, remove GP's every 50k miles before the carbon gets that packed again. if you have the IM off for any other reason during regular maintenance, then clean out the GP's for good measure. Not saying replace them every time. just remove, ream, clean, never-seize and re install. TRUST ME! I RUINED A PERFECTLY GOOD ENGINE by breaking off 2 GP's. Had head removed and never got it running properly again. Car eventually was sold as a parts car.

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