2005 ML270 CDI Injector Clamp Threading gone

badchiney

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We removed the injector to replace the copper washer. After cleaning the black, sticky substance and replacing the washer and injector, the clamp bolt will not lock in the cylinder because there is no threading there.

I tried getting a tap long enough to reach the threading in the cylinder head but have had ZERO success.

Can anyone report on a working solution? Thanks for the advice
 

M80

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Be careful of going too deep, I've read here that it's possible to puncture through the bottom of the hole.
 
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badchiney

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must i tap 8mm or 10,the la tap the same depth as the factory bolt
 

Alex Crow

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yes you can tap a larger size, no more than 8mm though. ideally though you would use a thread insert or two to restore the original thread size and depth.
 

ELGRINGO

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Hi it is possible to restore original thread through a competent machine shop, or drill out to 8mm retap and use 8mm allen key bolt after out clamp.
 
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Alex Crow

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a local machine shop that only does automotive work does exactly this - 8mm thread and very solid allen headed bolt. they tell me that it works well with no problems, but i have my doubts. i suspect that in time the copper washer will get bashed flat (thermal expansion anyone?) and the injector will leak again - there must be a reason why a stretch bolt was usen in the first place. have also heard of clamps breaking in two from this mod.
 

Number_Cruncher

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>>there must be a reason why a stretch bolt was usen in the first place.

It's there to control the load on the injector, and should help controlling the crush load on the copper washer, thus reducing leaks (I know, I know!, they do leak!)

As the bolt is stressed into its plastic region, you always get a clamp load of the yield stress of the bolt multiplied by the stressed area of the bolt, even if there are some errors due to measuring errors in the +90 +90 stages. This load drops off a bit after a thermal cycle, but is way more accurate than using a non stretch bolt and some spurious torque tightening spec.

I'm beginning to take the view that the weak point on these injectors is the soft female thread in the cylinder head. I would be tempted to install a decent hard thread insert in the head every time I took one of these injectors out if I wanted to avoid comebacks, and I would not be tempted at all away from the original stretch bolt - it's definitely there for a good reason!

Despite the fact that these injectors do leak, the situation is far better than on diesel engines of old, which despite having much lower specific powers had frequent leakage problems from their conventionally designed injector sealing/clamping arrangements. The upshot of a leaking heat shield washer on AEC engines, for example, was virtually always a ruined injector nozzle.
 

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......I'm beginning to take the view that the weak point on these injectors is the soft female thread in the cylinder head....

not really, despite what you might read here thread damage is not that common. it is invariably caused because some numpty has tried to cure a failed seal by over tightening the bolt - obviously once the bolt is tightened correctly there is no extra clamping load to be gained by further tightening, only a damaged bolt or thread.

and the stretch bolt, yes i agree with your assumption that it is to control injector-to-washer-to-cylinder head clamping pressure, this is my view and good to have you confirm it george.
 

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I'm with you NC, its a poor design, to have so much depending on a small stretch bolt is poor design/engineering. These threads will wear/get pulled and so will fail at some time. Yes lots of problems are caused by "numties" but its still a poor design.
 

dieselman

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This load drops off a bit after a thermal cycle, but is way more accurate than using a non stretch bolt and some spurious torque tightening spec.
I completely agree. When the aluminium head heats it will expand at a greater rate than the steel injector so the clamping force will reduce significantly on a stiff bolt.

Given the clamp type and bolt position the whole thing seems a poor design to me, the leverage of the injector is too great, especially when the bolt interfaces into aluminium.
 

Number_Cruncher

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>>not really....

It's actually a bit more insidious than you might think AP. If you consider how load is transferred from bolt to "nut", it turns out that the upper most few threads take the vast majority of the load.

Even a standard nut yields in these highly loaded threads when assembled.

The same effect will happen in this application, where the upper most threads will certainly yield every time the joint is tightened. Despite MB's rather ham fisted attempt to provide a long engagement, the load always concentrates on just a few threads.

The long thread engagement is a bit of a clue that MB themselves considered this a bit marginal when they introduced it - I certainly wouldn't risk engineering a bolted joint like that!

This yielding provides a relaxation mechanism, and the amount of relaxation you can tolerate before the joint begins to leak is not large - the compliance of the stretch bolt is, again, an attempt to minimise the change in load, however, the relaxation of the bolt is nowhere near as benign as the load limiting you get while tightening. Rather than following a shallow plastic characteristic of load versus displacement, relaxation follows the steeper elastic gradient.

My saying that any torque to yield bolt is controlling load is really not much of an assumption - the most stark place to see this is to consider the different factors for different tightening methods which are specified in a decent high performance bolted joint design standard - VDI-2230 being a very apt example, as it's both a very useful standard, and one that's commonly used in Germany.

As an aside, when these injectors are well repaired with a good hard thread insert, how often do they leak again afterwards?
 

exeng

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I have done loads of these you should not just tap it 8m/m as the hole size is greater than the tapping size for m8 i use an insert that allow you to use the original set up but you have a hardened insert with a far greater pull out strengh than original and i have had no problems as i also recut the seat
 

Number_Cruncher

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I have done loads of these you should not just tap it 8m/m as the hole size is greater than the tapping size for m8 i use an insert that allow you to use the original set up but you have a hardened insert with a far greater pull out strengh than original and i have had no problems as i also recut the seat

That sounds like a very good repair to me.
 

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As an aside, when these injectors are well repaired with a good hard thread insert, how often do they leak again afterwards?

no idea, i have not had to fit an insert as yet, although i know many have been done. i can say though that with a seat recut for the injector and head, along with new bolt and washer the seal is good for years - never had to do one twice. and there are plenty of cdi engines out there that seemingly have never had injector sealing problems.

regarding the issue of thread loading not being spread evenly along the length of the thread, this is a good point and has been made before, and an insert must be a much better solution with harder threads.
 

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There is one reason for repeated failures that I have come across.
Often people come in to the dealership, and ask for the copper washer...
Personally I, and all my colleagues say 'and of course the bolt?'

All too often we get the answer 'NO, just the washer'

I always used to try and explain that the 2 parts MUST be replaced together, but all to often the customer looked at me, and in some cases actually accused me of trying to rip him off.

So now?
I try and sell the bolt with the washer, but will not push it...
....I now realise that they will be back soon, and probably will need more than a washer and bolt.....
 

Alex Crow

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and they do not even cost much!

these same people probably have no idea of the tightening procedure either...
 
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badchiney

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Missing and fuel dumping problem

Thanks for all the great advice. I've been offline for a while but the problem was solved by threading to 10mm.

However…
Since retreading and putting in one more copper washer (couldn't remove the existing washer), the vehicle is smoking (dumping fuel) and the cylinder is not firing. Could the copper washer -- which is not a benz washer -- be blocking the fuel from entering the piston? Could it be raising the injector and preventing fuel from reaching the piston in time? Any ideas?
 

stumo

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So.... you guys were saying...
 

dieselman

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Thanks for all the great advice. I've been offline for a while but the problem was solved by threading to 10mm.

However…
Since retreading and putting in one more copper washer (couldn't remove the existing washer), the vehicle is smoking (dumping fuel) and the cylinder is not firing. Could the copper washer -- which is not a benz washer -- be blocking the fuel from entering the piston? Could it be raising the injector and preventing fuel from reaching the piston in time? Any ideas?
By adding the second washer you have raised the injector out of the air vortex, so the fuel no longer homogenates fully.
Strip it all out, fit a proper insert and one washer only.
 

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