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Mercedes w123 240D cracked piston. What caused the piston to crack?

5K views 6 replies 5 participants last post by  teodor 
#1 ·
I wrote a 2 threads about the low compression on my w123 240d and when I pulled the head out it turned out that piston #1 is cracked. I will post a picture and if any one has a clue what could have caused the crack please tell me. It has not been overheated (head gasket was in very good condition). The piston hasn't crashed in the valves because the valves are fine. The only thing wrong is the crankshaft balancer which was mounted on the opposite side (180deg out). Can the wrong mounted balancer cause this?
And also the middle piston ring was snapped in 2 pieces but I might have snapped it when I took the piston out
or maybe it was like that I don't know.
 

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#2 · (Edited)
I'm wondering if that balancer was off 180*, it may have allowed the engine to get into a resonating frequency (which all engines have, but if that harmonic balancer is out of wack, it can move where those frequencies are). Most engines are designed so the frequencies are above the max rpm, and below idle speed. There are usually many sets of frequencies that are all multiples of each other. A resonating frequency is the same thing that happens when an opera singer shatters a glass, or when the wind collapsed the Tacoma Narrows Bridge. I know on a slow speed diesel I worked on, it said "Do not operate engine between the ranges of 36-39rpms", which was just between Dead Slow and Slow ahead. When you accelerated through it you felt the engine shake a little (a 3 story tall engine shaking is kind of scary). I know they had some cracked liners from the same thing, so maybe it's what happened you your 240?

PS: for sh*ts and gigs, you can see a picture of us pulling a liner on that engine in my profile under "7RLB90"
 
#6 ·
I would check the pump timing for sure. It could be the culprit if it is advanced too far. Could be the previous owner was trying to boost the engine power and advancing the timing was just the way to do it. Except that he went too far.

Advancing ignition timing on gasoline engine cars to increase performance has been a favorite trick of many mechanics for decades. However,if you go too far you cause engine damaging detonation. The same may hold true for a diesel engine.
 
#7 ·
Yeah IT seems that the crown crack can be cause by too much advanced timing and the cracked rings lands (which in my case was just the middle ring broken) can be caused by pre-ignition which could occure if the timing is too much advanced. So I should use the 1 drop per second metod to check it? Can I use the injector pipes insted of that thing that I'm supposed to use?
 
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