Another clutch master cylinder push rod (240d Manual) - PeachParts Mercedes-Benz Forum

Shern · #1 08-23-2017, 04:09 AM

Driving along and thud, no clutch. Pull over, have a look and see that I've snapped yet another master cylinder push rod. This also happened back in January, leaving me stranded. It's a poorly constructed part, yes.
Why this thing isn't entirely metal, I'll never know, but doesn't seem like something that should snap this often.

Think the original part was OEM, the replacement FTE. Both appear to be the same quality, however.

Any idea why this is happening?

moon161 · #2 08-23-2017, 08:32 AM

I broke the corresponding part in a 190D, a very similar assembly but a bit smaller. It was a clean fatigue break at the neck of the pivot up top. The head of pivot moved stiffly on the bushing, not bound, but it explained the fatigue break. Every time (hundreds a day?) the pedal cycles, that neck was stressed because is was hard to turn on the pivot. When replacing it, I'd be sure that the replacement offered little resistance to turning on the pivot. If you have to ream something or polish a shaft down w/ emery tape, I'd do it.

Shern · #3 08-23-2017, 12:19 PM

Right at the neck is exactly where it's been breaking. I'm not sure if there's an alignment issue or what -or why the neck isn't a solid piece of metal. Have you had to ream the push rod head any larger? Thought perhaps there was an alignment issue/I'd over-tightened the nut.

I've ordered two so I have a spare but it's a major bummer when this happens, and potentially a dangerous one.

97 SL320 · #4 08-24-2017, 07:48 PM

Quote:

Originally Posted by Shern

Right at the neck is exactly where it's been breaking. I'm not sure if there's an alignment issue or what -or why the neck isn't a solid piece of metal. Have you had to ream the push rod head any larger? Thought perhaps there was an alignment issue/I'd over-tightened the nut.

For clutches in general, there should be a sometimes adjustable rubber stopper to prevent the master cylinder from bottoming out before the pedal, make sure this still exists / is adjusted.

After the part breaks, are you able to rotate the head easily? I'm thinking the rod pivot is binding and flexing the rod to failure. With the pedal up, you should be able to wiggle the head left / right.

I'd also be tempted to unbolt the clutch slave cyl, compress it back to operating height, have someone hold it against something solid then press the pedal by hand while jiggling the rod looking for binding. ( The slave will extend while pressing the pedal, this is must occur, just make sure the slave does not fully extend and come apart.

Shern · #5 08-23-2017, 07:09 PM

Any further thoughts?

Taking her by my local indie tomorrow once i've replaced the rod... he suggested the clutch pedal itself might need some adjustment.

winmutt · #6 08-24-2017, 07:20 AM

I've driven a variety of manual Benz (123/124) over last decade and never had this issue. No suggestion but it does sound wierd. Iirc There is an adjustment, maybe you had it adjusted wrong?

Shern · #7 08-24-2017, 12:30 PM

Beyond bolting it to the pedal assembly, I'm not sure what else to adjust.
Once it's installed, it doesn't seem to be perfectly straight. The piston comes in at a slight angle which I think is what's been placing stress on the neck. How to adjust for that...

Clemson88 · #8 08-24-2017, 12:34 PM

OE replacements breaking?

Shern · #9 08-24-2017, 12:36 PM

I don't think the first replacement was MB, but the one I'm installing today sure is.

moon161 · #10 08-24-2017, 12:58 PM

If the pin that the head slides onto is not perpendicular to the shaft, that might be the problem. You might see this assembly flexing or feel binding or load in the assembly as you move it through it's travel. If it's sheet metal, I'd bend it with some big pliers. If it's a casting, I'd be nervous.

Shern · #11 08-24-2017, 02:18 PM

I think this is the case. It could be possible that the pedal arm itself has bent perhaps through a few hard clutches.

Quote:

Originally Posted by moon161

If the pin that the head slides onto is not perpendicular to the shaft, that might be the problem. You might see this assembly flexing or feel binding or load in the assembly as you move it through it's travel. If it's sheet metal, I'd bend it with some big pliers. If it's a casting, I'd be nervous.

winmutt · #12 08-24-2017, 05:50 PM

Sorry I thought there was an eccentric bolt at the top for pedal height adjustment.

Shern · #13 08-24-2017, 09:17 PM

Think I figured it out... the simplicity or stupidity -haven't decided yet- is astounding. Replaced the part and couldn't remember which end the eccentric went into. Compared wear marks on the other two pistons and installed it to match. Didn't seem right though, so I installed it the other way round and immediately noticed the pitch looked much straighter. Drove over to my indie who happened to be working on a 240D manual with the panel off, climbed under to take a look and indeed confirmed my instincts were correct.

The original part (likely replacement) when I bought the car was installed incorrectly. It broke, and my first replacement (being unfamiliar with the car) was installed to mirror it's predecessor.

Well, to anyone searching the archives, make sure the eccentric goes into the side with the indent.