Bewildering Crankshaft Discoloration...pictures included - PeachParts Mercedes-Benz Forum

WANT '71 280SEL · #1 07-05-2005, 08:31 PM

I have here two pictures of my crankshaft during my recent oil pan replacement. Notice that the discoloration is only on the crank and not on the caps. Do you guys have any idea as to why this might be? For the past couple years my car has had the oil cooler bypassed, could this be a result? The PO had it done, not me. Right now I'm trying to find the $100 for a used cooler and hoses. In the mean time however, this has to do.

Thanks
David

t walgamuth · #2 07-05-2005, 08:38 PM

like heat discoloration. i dont really know if that is normal or not. i am guessing not. by all means reconnect the oil cooler. it is one of the many keys to longivity.

WANT '71 280SEL · #3 07-05-2005, 08:46 PM

That's what my uncle thought but he said the wierdest thing about it was that it was limited to the crank and not at all on the caps. Why wouldn't it also be on the caps? Is there some reason that the main bearing would have heated up more than the rod bearings?

Thanks
David

boneheaddoctor · #4 07-05-2005, 09:09 PM

Unless they welded up that journal and reground it in the past? thats very odd, it takes a LOT of heat to get like that.

WANT '71 280SEL · #5 07-05-2005, 09:20 PM

I have the PO's service records and nothing indicates any major engine work. Just the usual monovalve every 3 years and pesky electronic things.

Thanks
David

J. R. B. · #6 07-05-2005, 11:39 PM

You should pull a rod cap off and examine the bearing shell and the bearing journal of the crank. Look for streaks and gouges where foreign material could have gone through the system. Next take a micrometer and "mike" the crank journal. Make sure you "mike" it twice 90 degrees apart so you can check for out of round. Go and no-go tolerances are listed in the engine manual.

WANT '71 280SEL · #7 07-06-2005, 12:21 AM

Well now I have the pan back on and it would take some major persuasion to get me to redo that job. That was a nightmare!

Thanks
David

jbaj007 · #8 07-06-2005, 01:31 AM

I'm guessing that discoloration is not from engine heat, but rather from manufacturing process heat.

boneheaddoctor · #9 07-06-2005, 08:15 AM

Quote:

Originally Posted by jbaj007

I'm guessing that discoloration is not from engine heat, but rather from manufacturing process heat.

His crank shows one journal tha thas been heated up quite hot AFTER it was made....heat reating and such take place over the entire crank at once equally....having worked with a LOT of metal this was a localised heating...and could not have happened in the engine. Becasue the bearing shells would have turned to goo at the temps that did that. But memory escapres me of what temps were required to discolor the crank like that.

J. R. B. · #10 07-06-2005, 10:29 AM

Quote:

Originally Posted by boneheaddoctor

His crank shows one journal tha thas been heated up quite hot AFTER it was made....heat reating and such take place over the entire crank at once equally....having worked with a LOT of metal this was a localised heating...and could not have happened in the engine. Becasue the bearing shells would have turned to goo at the temps that did that. But memory escapres me of what temps were required to discolor the crank like that.

Bonehead here are a few color temps from The Machinerys Handbook. This book is the bible for machinists. I'm only taking a few of the important ones as there are probably over 30 of them. The temps I'm listing are in Farenheit.
Dark yellow--480
Light purple--530
Dark purple--550
Dark blue--570
Red heat visible in the dark--752
Red heat visible in daylight--975
Dark red--1292
Cherry red--1652
Bright cherry red--1832
Orange yellow--2192
White welding heat--2552
Brilliant white--2732
Dazzling white--2912
One must realize though that this is inacurate as one person will percieve colors differently than the next. Also the temps I have given are from an older edition of the book. By looking at his pictures I would say that the crank was rather hot in localized areas that is why I suggested pulling a rod cap off and miking the journal.

J. R. B. · #11 07-06-2005, 10:38 AM

Quote:

Originally Posted by WANT '71 280SEL

Well now I have the pan back on and it would take some major persuasion to get me to redo that job. That was a nightmare!

Thanks
David

The second time you do a job is always faster than the first time.

dkveuro · #12 07-06-2005, 11:24 AM

Mercedes used to heat treat the cranks by gas flame...they now do it with induction.
The 'blueing you see here is the result of factory hardening ...the bearings would be shot by now if the had reached the temperature needed to make the 'blue'.

Mecedes has a fische about this on the 6 cylinder turbo 300SDL engine to explain which components are interchangable.
The connecting rods where hardened too.

.

boneheaddoctor · #13 07-06-2005, 11:26 AM

Quote:

Originally Posted by dkveuro

Mercedes used to heat treat the cranks by gas flame...they now do it with induction.
The 'blueing you see here is the result of factory hardening ...the bearings would be shot by now if the had reached the temperature needed to make the 'blue'.

Mecedes has a fische about this on the 6 cylinder turbo 300SDL engine to explain which components are interchangable.
The connecting rods where hardened too.

.

I agree the berings would have been ruined.....but I think there is something missing from previous owners paper trail to explain this. Now what would have been truely bizarre was if the PO was his parents who bought it new. But I don't think that is the case.

dkveuro · #14 07-06-2005, 11:43 AM

The reason I'm up to speed on the 300SDL motor is that I have fully overhauled one last January..new liners/pistons, used crank/used rod set......and it returned last week for it's oil service and hood pad.

During the build, ( which was due to the oil pump eating a piece of metal at 70 mph on I-35..)..I had a lot of different input from other sources about the construction and fitting of the early and late crankshafts and the use of re-sized conrods......I got hold of the factory fische, and it explained in detail the engine numbers and crank id's to allow using later made components.

Mercedes changed the width of the journals and the rod, to increase the strength of the crank......they changed the hardening process too, to electric induction from flame hardening with information on the Rockwell of the metal, so information was required to allow rebuilds with compatable components along with increase in head deck thickness..

They also changed the rifle drilling and a few other discrete parts...

All in all, it told me that next time I get a blown 1987 and up 300SDL, find another engine and swap it out......but do not rebuild the siezed motor.

.

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