MB Service gone wrong - spark plug snapped!

[Number_Cruncher]

There's no problem at all with a heli-coil repair - if done properly.

In the area of engineering where I work, it isn't usual practice to thread directly into soft metals like Aluminium alloy, and wire thread inserts are thus routinely fitted to safety critical equipment from new.

The presence of fluctuating loads, whether from gas pressure or other mechanical sources isn't a problem.

Sometimes, the wire thread will unscrew with the spark plug at service time. If you obtain some spare wire thread inserts, they are very quick and easy to renew in-situ. It's only the initial installation, where the Aluminium alloy needs to have a new thread cut into it that is at all critical.

THE vital thing to get right when installing thread repairs of any kind is to be absolutely sure the new hole and new thread is absolutely square to the seating face. If not, the plug may not seal properly, and will impose a bending load on the plug body every time the cylinder fires.

 

[LTD]

Sometimes, the wire thread will unscrew with the spark plug at service time. If you obtain some spare wire thread inserts, they are very quick and easy to renew in-situ. It's only the initial installation, where the Aluminium alloy needs to have a new thread cut into it that is at all critical.

THE vital thing to get right when installing thread repairs of any kind is to be absolutely sure the new hole and new thread is absolutely square to the seating face. If not, the plug may not seal properly, and will impose a bending load on the plug body every time the cylinder fires.

Precisely the sort of reason to reject any repair - it might well come back to haunt you.

 

[Number_Cruncher]

Precisely the sort of reason to reject any repair - it might well come back to haunt you.

I think that's a good point.

On the other hand, the OP might consider accepting a repair, and using those negative technical points as bargaining items in return for a hefty reduction in his bill for the service, or for some other offer the garage might make.

 

[iscaboy]

There's no problem at all with a heli-coil repair - if done properly.

That simply is not true.

Alloy head Ford 4.6 and 5. engines in the US are absolutely *infamous* for blowing out helicoiled spark plugs.

This wasn't a problem in days gone by when the standard spark plug thread was 3/4" deep, and when heads were simpler shapes and so had simpler patterns of thermal expansion, but that is no longer the case.

If helicoil was as good as you say, it would be certified for spark plugs, and it ain't.

Actually, having just done some checking, Time-sert is approved by all sorts of agencies, not just the FAA, but also as it happens Mercedes-Benz.

http://www.timesert.com/html/faq.html

 

[LTD]

I think that's a good point.

On the other hand, the OP might consider accepting a repair, and using those negative technical points as bargaining items in return for a hefty reduction in his bill for the service, or for some other offer the garage might make.

Small term gain, long term pain.

New head - end of. Takes away any doubt, debate and discussion.

They didn't fit a repaired head in the factory.

 

[219]

Accidents can indeed happen , I once broke a spark plug whilst trying to remove it - this was on an older car I had just bought and I don't know how long that plug had been in there ; the other 5 plugs came out without difficulty but this one was stubborn , tried it with the engine hot as well as cold , gave it 24 hours with Plus Gas soaking in , eventually used a big breaker bar on the socket and the inevitable happened ! In my case the hexagon part sheared off from the threaded part along with part of the ceramic internals . Lucky I had a spare engine with another head !

If the vehicle in question is still under any warranty I would expect nothing less than a new head , otherwise a competent repair .

If , due to the error , you have lost confidence in the workshop , you would be entitled to have the vehicle repaired elsewhere and send them the bill . I don't feel you have reached this position , but if the car is not 'right' when you get it back then this would be another option .

Since they have admitted responsibility and are fixing the car at their cost I would wait and see what happens - only if you are dissatisfied take the car elsewhere for independent examination .

 

[LTD]

Accidents can indeed happen , I once broke a spark plug whilst trying to remove it

The problem was caused when they were FITTING the new plug.

 

[Number_Cruncher]

That simply is not true.

If helicoil was as good as you say, it would be certified for spark plugs, and it ain't.

Actually, having just done some checking, Time-sert is approved by all sorts of agencies, not just the FAA, but also as it happens Mercedes-Benz.

LAS Aerospace Ltd

 

[cyberflex]

That simply is not true.

Alloy head Ford 4.6 and 5. engines in the US are absolutely *infamous* for blowing out helicoiled spark plugs.

This wasn't a problem in days gone by when the standard spark plug thread was 3/4" deep, and when heads were simpler shapes and so had simpler patterns of thermal expansion, but that is no longer the case.

If helicoil was as good as you say, it would be certified for spark plugs, and it ain't.

Actually, having just done some checking, Time-sert is approved by all sorts of agencies, not just the FAA, but also as it happens Mercedes-Benz.

++ TIME-SERT Frequently Asked Questions ++

This method certainly looks like the one that works very well and is the way forward, but given the circumstances when this type of thing occurs, who really knows what will go on behind closed doors in order to correct the problem, at least with an 'indy', you get to speak to a tech, can't see that happening very easily at a dealership, especially, given the present set of circumstances.
I do feel sorry for the poor guy who was allocated the job, the damage may/may not have been 'initialised' during a previous plug change, after learning a fair bit from you guys this evening, I think if I were the OP, I would ask for a replacement part.
If the request is declined, accept a repair with some form of written warranty. What else, realistically, can be achieved without the expense/time of legal action, other than increasing stress levels/blood pressure over a motor??

Hope you get it sorted soon mate.

 

[Number_Cruncher]

Just to be clear, I have nothing against a time-sert type of repair - my point is simply that heli-coil is also a good repair.

 

[iscaboy]

LAS Aerospace Ltd

helicoil SERRATED inserts....

chalk and cheese compared to standard helicoil.

Their website is also blatantly lying, time-sert is FAA approved, actually used them on a twin-wasp, which then had the repair flight approved.

 

[grober]

At the moment its all speculation until the original poster finds out exactly what the MB dealer intends to do. I would urge the poster to go down to the dealer in person- its all too easy to fob someone off on the phone. If the service manager is unavailable, ask to see the workshop foreman, - if he's not available ask to talk to the technician scheduled to carry out the repair- make it clear that you want to know exactly what is going to happen to your mother's car and this would be best achieved by talking to someone directly involved in the repair. Its quite possible they are intending to remove the head and use a time cert repair. If they intend to use another method you are entitled to ask if its THE Mercedes approved repair for this type of problem. I feel its quite likely they would resist the suggestion to replace the head completely since this would considerably increase the cost of a repair. [ this doesn't mean you are within your rights to ask for this but expediency might dictate a compromise rather than a long drawn out dispute which might cost you money] Now having settled how the repair is going to be done you have to ask the big question [all the while asking them to bear in mind that your mother's car was in excellent condition when it entered their workshop ] what form of guarantee/warranty are they going to offer on the repair? If they have faith in the quality of their repair there should be no problem with their reply. It should be possible to conduct all this discussion in an amicable and transparent fashion. Any deviation or obfuscation should be viewed with suspicion and clarification sought. -------- Finally with everyone happy go back home and later in the day accurately summarise your conversation in writing and send it in a recorded delivery letter to the service manager asking for their comments if any.

This last bit is optional depending on how you are treated by the dealer [ this is why its always best to deal with people face to face as you get a better impression of who you are dealing with] but you can always use the excuse you are acting for a third party--- your Mother- and you wish to avoid any misunderstandings in the future.

 

[Bellow]

Doesn't a Time Cert introduce a thermal impediment to the plug giving up its heat to the cylinder head thus creating the possibility of pre-ignition?
I accept that they are approved by the FAA, but do aero piston engines make the same specific hp/litre as do road engines?
And is there sufficient material to guarantee no weakening between the newly cut 'hole' for the Time Cert and the water jacket and also the valve seats?

 

[Number_Cruncher]

Usually, the highest thermal resistance in the conduction path is within the spark plug ceramic itself, so changes to the conduction path outside the plug don't tend to make a massive amount of difference.

By introducing an extra interface, you do introduce an extra contact resistance - but the most direct thermal conduction path is actually through the sealing surface - taper or metal gasket - and so, the extra thermal resistance of the thread insert can be in the least important parallel path.

As an aside, one interesting advantage of wire thread inserts is that their flexibility allows a much better and more even contact to be made between them and the other parts of the joint, and this effect has both mechanical and thermal benefits.

I haven't run into any trouble with respect to clearance from the water jacket and the valve seats using a thread repair on spark plug holes - but, I can't claim to have repaired every type of engine in this way.



I have been using wire thread inserts to repair engines for over 25 years, and I've been designing safety critical parts (some of which are used on civil and military aircraft) which use wire thread inserts for 15.

Oddly, the only problems I've had in that time have been with solid inserts - where the repair method I had written was being sloppily followed in one of the maintenance depots which carried out the repair meant the inserts were being installed off-square. This caused the bolts in those repaired joints to fail in bending fatigue, and it was this experience which made me caution about the alignment of the hole.

If a solid insert is used, I would want to be sure that the hole and the sealing surface are square to within less than 0.5 degrees - especially so if the plugs use a taper seat - for this, the head must be taken off, and the head drilled on a machine rather than by hand.

 

[swift1]

Update from the SM:

rang this morning (9.30am), again said he was busy and will ring back, he rang back 30mins later, had a long 20min chat. So heres what has happened:

They have taken the head off and sent it to a specialist to have the plug removed. Apparently, they were able to wind out the plug leaving the thread wholly and completely intact. They have screwed in and out several times a spark plug and has gone in and out without any problems.

A new head gasket has just come in from germany this morning, and they will be rebuilding the engine today and should be ready by cob today or monday.

I mentioned are they using any thread repair such as helicoil or time-sert, he said they wouldn't touch that, they would just replace the head at their expense, he mentioned they cost £1500!

The head gasket will have a 2 year warranty and they will also write a letter explaining what has happened and what they have done to repair matters along with a warranty should anything happen to this specific spark plug in cylinder 2 for the next 2 years (I think). I did say that the next spark plug change won't happen for 4 years and what if a problem arose then, he said he will speak to his manager and see what they could do on this.

I mentioned to him that my mother is doing the school run with kids in the back and I want complete assurance that this plug is safe. He assured me it was.

I asked how had this happened even if the porcelain had snapped that would still leave the steel shell to unwind, he said that the metal had sheared off and thats why they had to take the head off.

I asked if the person working on the car was an apprentice and did he make a newbie error. He said no he is an experienced mechanic, mercedes-benz trained and with several years of experience. It is the same person working on the engine dismount/rebuild!

The SM was reluctant to blame the tech and instead tried to convince me it was the spark plugs fault. He said the parts arrive on a lorry and sometimes they get abused, pushed and shoved. He said if a spark plug drops on the floor they would not use it as it may have caused damage to it. He said they spark plug unbeknown to the tech may have already been damaged, had a crack or something and so when it went to tighten it, it sheared off. He said spark plugs 1 and 3 went in fine.

So hopefully all will be well. It will be interesting to see if they provide any goodwill on the bill - heres hoping

 

[martin_a]

Apart from simply not admiting human error it seems they are doing everything right so far. Deffo push for that four year warranty, especialyl as they are being so specific about it applying to plug No 2.

m.

 

[Bellow]

Everyone happy there aren't any bits of spark plug in the cylinder in need of retrieval?

 

[Number_Cruncher]

It's an interesting twist to the tale.

Perhaps the mechanic, instead of being a bit poor, has actually done exactly the right thing? He's encountered a problem - which could have been the fault of the plug - he's stopped, and instead of struggling in the confined space and perhaps causing damage, he's taken the head off to sort it out properly.

Getting every last trace of broken ceramic out of the cylinder won't be an easy task - especially if any is wedged between the piston and the bore. I would be tempted to make up a small nozzle and attach it to a strong vacuum cleaner, and give the area a good going over while turning the crank to move the piston down the bore.

It might be worth going and looking, and getting some digital pictures of the site, to be absolutely sure there really is no damage to either the threads, or the piston and cylinder.

 

[andy_k]

Everyone happy there aren't any bits of spark plug in the cylinder in need of retrieval?

it shouldn't happen if the base of the plug was intact.

I had a plug shatter once when I was fitting it and had to take the head off to get the core out - I just vacuumed the top of the piston and around the rings figuring that anything left would be small enough to get blown out the exhaust valves the first time the engine started

 

[nigel cross]

Sounds like a good outcome for everyone.