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Old 02-26-2003, 07:59 AM
Registered User
Join Date: Apr 2002
Location: Charlotte, NC
Posts: 25
Question Crank pulley and balancer -- How did it come apart?

I have a 1991 Mercedes 420SEL and I'm stumped about how the crank pulley and harmonic dampener can come loose (last week --the pulley moved forward and was flopping around, cutting into the fan blades -- dampener totally loose). Isn't there a big 27mm bolt and other smaller bolts holding all this in place?? Am I missing something???

I'm an aviation mechanic, and I can do basic work on my car. I just had the situation repaired, but I don't understand how it all came apart.... any explanation would be greatly appreciated...

Bob Irving
Charlotte, NC
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Old 02-26-2003, 08:52 AM
Fimum Fit
Posts: n/a
There is a rubber filler between the two metal parts.

This is what does the damping to control harmonic vibrations. As the assemblies get older, the rubber ages and cracks up from heat and eventually the two metal pieces cease to be bonded together -- it can happen more quickly if an oil seal or antifreeze leak allows fluids onto the rubber. This is an especially big problem on classic SAAB 900s, whose harmonic balancer sits at the firewall end of the engine, just above the place where the headers meet the catalytic convertor, a very hot spot indeed.

Addendum: some late model M-B motors (the V6s?) got some defective balancers, but I think that yours is far too old to be involved in any recall or extended warranty.

I worry about some of the late model BMW cars which achieve incredible smoothness with what they call a "dual mass flywheel" which potentially puts the same problem at the transmission end of the crankshaft, but I haven't heard of any big panics yet.

Last edited by Fimum Fit; 02-26-2003 at 09:42 AM.
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Old 02-26-2003, 05:37 PM
Posts: n/a
To add to Finnum's excellent reply, yes there is a 27MM wrench size bolt that threads into the crankshaft snout. That bolt is supposed to be torqued to around 200 ft/lbs. It is not uncommon for these bolts to get under torqed and come loose. Is the bolt still in place?

Good luck,
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Old 02-26-2003, 11:43 PM
Larry Delor's Avatar
What, Me Worry?
Join Date: Jun 1999
Location: Sarasota, Fl.
Posts: 3,028
I had that bold snap in two on two seperat occasions on my 300D. So far, no one has been able to explain why that happened. I bought a rebuilt engine, and gave up on replacing the bolt every two weeks. (there were also 6 smaller bolts that held on the pulley, and two pins to line up the dampener to the shaft)
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Old 02-27-2003, 12:40 AM
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Join Date: Jan 2003
Location: Sydney, Australia
Posts: 627
Prior to our '90 300TE we owned an '80 300TD (OM617 diesel). Prior to our purchase it had some engine work that obviously involved the crankshaft pulley and harmonic balancer. The mechanic obviously neglected to fit the correct washers under the six smaller bolts.

Initially we had an engine vibration that I could not find. Then one night the vibration suddenly got worse followed by the sound of something hitting the fan (the s**t hit the fan so to speak). Investigation with a flashlight revealed the pulley sitting at a funny angle. Three bolts were missing and the remainder either sheared off or tore through the pulley hub. It was a tow-it-home job and a bit of unplanned work for me.

Fimum Fit mentions the dual mass flywheel on BMWs. Mercedes manual transmission cars with the M104 and the later M103 (I think) also got dual mass flywheels.
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Old 02-27-2003, 08:42 PM
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Location: Carol Stream, Il, USA
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When I replaced my crankshaft pulley (from the late model V6 with defictive rubber), I put some blue locktite on the bolt to help prevent it from coming loose.

If the 27mm bolt came off easy, then it wasn't put on tight enough to begin with.
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Old 02-28-2003, 02:39 AM
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Join Date: Jan 2001
Location: oregon
Posts: 2,013
takes a 27 mm socket , is easier to work on with fan off and raditor out . just take bolt out if theres no damage on threads clean it up put on a bit of blue lock tight and torque it to specs my 500se specs were 250 lbs you will need to lock the flywheel to apply that much torque.............
William Rogers.......
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