Retorqueing 6.9 Head Bolts

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Styria

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Some of you may re-call my reference to abl567's 6.3 engine that developed a profuse oil leak at the rear of the left hand head after the engine was first re-assembled. It was some time after thereafter that the engine was actually started and the car driven to any extent.

I have just recently completed the valve grind and de-coke job on 'Gleaming Beauty'. After some two thousand miles,and the engine being trouble free, I am beginning to think that maybe, just maybe, I should consider re-tightening the head bolts. There'd be a bit of time involved to carry out this work but, then on the other hand, should I err on the side of caution ? Thanks to abl567's help in procuring the head gaskets from Tom Hansen in the States, I have in fact used the correct 'Reitzle' brand gaskets, with the claim being advocated by Benz that the heads do not need to be retorqued.

The exhaust system, or rather the sound of it, has a somewhat, shall we say, uncharacteristic sound burble to it. It may be the muffler system, for all I know, with the 350 first muffler, and an after market stainless steel unit, as being the end muffler (I just HAD to use it as it was so shiny), thus giving me THAT sound - but the exhaust pressure seems VERY MUTED, and I am thinking "why".

Car sounds good, with a raspy exhaust on acceleration, and I have done absolutely no tuning nor checking of the engine tune after all this work save for a clean-up of plugs and a slight muxture adjustment with the help of abl567's exhaust gas analyzer. Other than that, have not touched the car. Any thoughts or advice on this subject matter ? Regards Styria
 

abl567

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I'd retorque the heads, unless you used all new hardware, head bolts etc, MBs recomendations do not count. Peace of mind.
I was surprised on the M100 rally how much quieter my 6.3 was compared to others with genuine factory exhaust systems. The factory system is much less restrictive that I thought it would be, almost a straight through design.
Fitting the muffler you supplied, a balance pipe and returning my exhaust to close to factory made a big difference to the breathing in my car without making it to noisy.
 

John S

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Did the 6.9 use the stretch bolts, or the standard type? If you used stretch bolts you dont need to retorque.
 

s class

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Both of my 6.9's had head bolts that were finger-tight when I last checked. I would check them. It only takes an hour or so.
 

WGB

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Did you use genuine M-B head gaskets.

I thought the deletion of the torqueing was due to headgasket design and was really only to save work which would have probably been paid for M-B.

I would feel happier with a re-torque if it was mine.

Bill
 

abl567

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I recall a thread on the M100 site about the heat soak issues that all M100 engines are subject to.
The 6.9 tends to loosen the rear head bolts, those closest to the firewall, due to thermal expansion.
I guess due to the covered valley of the engine. The 6.3 has an open valley so the heat can cause vapour lock in some MFI cars but does not soak into the heads as much.
 
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Styria

Styria

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Hi guys, thanks for your input. As indicated, I did use genuine MB head gaskets and, as far as I could tell, they really weren't all that different to steel gaskets used and supplied by the ROVER company for their V8 engines.

I do tend to think that I should really bite the bullet and retighten those head bolts - just for peace of mind. Should I start with a warm/hot head, or let the engine cool down overnight before touching those bolts ?

I know of an Engineering shop (they were the ones to cause the problems on abl567's 6.3 engine) who tighten the heads an extra 5-7 lbs./sq.in. when cold and during the course of re-assembly and, on that principle, they "HAVE NEVER HAD TO RE-TIGHTEN HEAD BOLTS". I am curious about that philosophy ! Regards Styria
 

s class

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Overtightening when cold to prevent the need to retorque sounds like highly dubious, bark yard type technique. Its likely to distort the heads.

I would stick to the standard MB torque spec.

The Aus 1980 workshop manual says :

For a 100.985 engine :

Cylinder head bolts : 90Nm on a cold engine and 90Nm on a warm engine (2).

The (2) refers to a footnote which I quote below : (I have also included footnote (1) though you are past that step)

Slide1.jpg

I know how you are feeling though - its tempting to leave well alone what isn't broken - so good luck with whatever you choose to do.
 
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Styria

Styria

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Hi S-Class, you know, thanks for that. Obviously I have read this previously (over the years from time to time) but when you really analyze it, it does contradict MB claims that "there is no need to retorque cylinder head bolts due to the design of the head gaskets". It does make you wonder where you stand, doesn't it ?(..and BTW, I do appreciate you posting these specific details).

So, I think what I'll do is as follows: Run the engine for about ten minutes, remove rocker covers, loosen one head bolt at a time in order of tightening sequence, leave it at that and not worry any longer. Sounds alright ? Regards Styria
 

abl567

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So, I think what I'll do is as follows: Run the engine for about ten minutes, remove rocker covers, loosen one head bolt at a time in order of tightening sequence, leave it at that and not worry any longer. Sounds alright ? Regards Styria


Please retorque them after you loosen them:D
 
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Styria

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I think it's quite staggering that eleven years have passed since I carried out this valve grind and de-coke ! I am not quite so sure if I have my time frame correct, but looking at the dates, that has got to be it. I seem to recall another thread which covered this job and which had been carried out with the help of Mercules. I don't recall ever retightening the heads. Assuming all is as described in this thread, I can confirm that

1. I have not experienced a single water leak from whence the job was completed, and
2. Not a single drop of oil leaking from the engine block or heads.

Whilst I have used Gleaming Beauty very little for a number of years now, I can confirm that it still starts first turn of the key each and every time. I have proved this by continuous starting and stopping the engine as part of my Suspension Testing procedures. GB is now, and has been for a little while, in need of replacement of the Pressure Cells. I had to purchase five units from the UK, and I am just hanging out for some time to carry out this work. Regards Styria
 

sean sherry

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A very interesting Post with sensible replies..... Reminds me of the issues of cylinder head Oil leaks under warranty on our W204 Class in 2010. Engine out twice and run without the Transmission to locate the leak. Was from the rear head Bolts and Mercedes Factory cure to the Dealer was to use a special Sealer. At 75000Ks starting to leak again. If it gets worse I will try the Stop Leak from "Liquid Intelligence" According to my local Guy their Products do actually work.
 

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