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deeps
Original Poster
4,216 posts
111 months
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Following on from my other thread, here's some pics of the cracked piston. My local mechanic was surprised at how short and stubby it is. Contrary to what WSM Mercedes originally thought, the piston is actually cracked right across the face (which is 9mm thick) down the sides and across the underneath of the face, including the tops of the holes for the gudgeon pin. The only part that doesn't show any visible crack is the underneath of the pin holes. Any engineers about? Your opinions would be welcome  Is it likely to be a one off event caused by a slight fault from manufacture? Or what other reason could have caused it? Thanks... 
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deeps
Original Poster
4,216 posts
111 months
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SLacKer
2,047 posts
77 months
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Have no specialist knowledge on the engineering side of this but that crack is something else.
Has there been any damage to the cylinder bore? Did you have to remove the engine in the end?
Maybe you should ask this question on benzworld.org which tends to have more technical expertise on it.
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ilovevolvo
1,630 posts
94 months
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That's shocking your lucky it did not fall apart
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P.Nott
145 posts
59 months
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That looks to me like a manufacturing fault rather than a combustion related issue. Do all the other pistons on that bank look ok and have a similar colour to the damaged one?
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deeps
Original Poster
4,216 posts
111 months
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Thanks for replies, they didn't remove the engine but had to remove a cross member to lower engine I was told. I didn't see the other pistons, but was told they were checked and looked fine. I think it's a design fault or manufacturing fault too. I could perhaps understand it if the car had increased boost and a remap, but it's always been standard with no mods.
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SFO
2,608 posts
53 months
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send pics to AMG and see what they have to say
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Egbert Nobacon
2,783 posts
113 months
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Just a thought.
The handbook for my E63 states "AMG vehicles should only use petrol with a minimum octane number of 98 otherwise the engine could be damaged. 95 RON can be used as a temporary measure if no other available but you must avoid driving at full throttle"
I don't know whether this applies to the SL55 but if not adhered to prior to your tenure it could have started or exploited a weakness.
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TISPKJ
1,252 posts
77 months
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As mentioned above I would think AMG would be very keen to look at this, pistons dont just crack like that without some underlying reason, could well be a bad casting.
They may even offer some kind of compensation towards the rebuild, even if only the parts has to be worth a shot.
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959
121 posts
62 months
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This is plainly a symptom of abuse during the running in period before the rings have expanded fully.
Excess play will have slammed the piston to the bore causing it to crack.
I would be considering legal action to recover the cost of repair from the imbecile first owner who did this, on the grounds of 'criminal damage'.
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Johnboy Mac
2,666 posts
48 months
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SFO said: send pics to AMG and see what they have to say +1.
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Johnboy Mac
2,666 posts
48 months
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959 said: This is plainly a symptom of abuse during the running in period before the rings have expanded fully.
Excess play will have slammed the piston to the bore causing it to crack. Would this problem not have shown up sooner as bore damage and thus excess oil use and maybe even lower compression on that particular cylinder?
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cptsideways
9,895 posts
122 months
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That looks like a fault in the hot forging process, I doubt they are cast pistons probably hypereuretic, which is a half cast/half forged process. Looks like the moulding process stopped & restarted with an insufficient mix of the alloy before it was moulded.
I'm sure AMG would like to see it.
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P.Nott
145 posts
59 months
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959 said: This is plainly a symptom of abuse during the running in period before the rings have expanded fully.
Excess play will have slammed the piston to the bore causing it to crack. Sorry, but I think that is nonsense!
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Johnboy Mac
2,666 posts
48 months
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P.Nott said: 959 said: This is plainly a symptom of abuse during the running in period before the rings have expanded fully.
Excess play will have slammed the piston to the bore causing it to crack. Sorry, but I think that is nonsense! Must agree, as it doesn't make sense to me.
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959
121 posts
62 months
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The lining of the bores will be a much harder material than the piston (nickasil or similar) which would only score if it went beyond its maximum operating temperature.
Also the piston will not impact the same point as it rattles around.
I don't believe AMG wouldn't pick up on a bad casting thru x-rays etc given their reputation.
This engine was on the limiter as soon as the showroom doors were opened until the clown noticed a drop in power and got rid.
I'd be interested to know if there is a running in period for this engine and what AMG recommends.
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1point7bar
1,091 posts
18 months
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Looks like the cylinder has overcompressed.
Have you driven through water?
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SLacKer
2,047 posts
77 months
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959 said: The lining of the bores will be a much harder material than the piston (nickasil or similar) which would only score if it went beyond its maximum operating temperature.
Also the piston will not impact the same point as it rattles around.
I don't believe AMG wouldn't pick up on a bad casting thru x-rays etc given their reputation.
This engine was on the limiter as soon as the showroom doors were opened until the clown noticed a drop in power and got rid.
I'd be interested to know if there is a running in period for this engine and what AMG recommends. Do I sense a dislike of Jeremy Clarkson? 
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Pentoman
4,137 posts
133 months
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Water is a possibility... Although what breaks first in that situation, the piston or the rod?
This is also a very early car, one of the first with this engine, so that could be relevant.
A machine shop may be able to tell you more.
Out of interest what symptoms did it show when it was running with the cracked piston?
Could be worth a write to Top Gear for a laugh???
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1point7bar
1,091 posts
18 months
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Pentoman said: Water is a possibility... Although what breaks first in that situation, the piston or the rod?
This is also a very early car, one of the first with this engine, so that could be relevant.
A machine shop may be able to tell you more.
Out of interest what symptoms did it show when it was running with the cracked piston?
Could be worth a write to Top Gear for a laugh??? Can I make a distinction between hydraulic lock [lots of water and a bent rod at least) and overcompression (a bit of water and cracked piston crowns).
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