617 turbo engine crank question/ rebuild

[P.E.Haiges]

Omegabenz,

Sounds like old wives tales. I've had Diesel cranks reground and they worked fine, however they were not cranks that were nitrided. Why would regrinding affect oil pressure as long as the correct oversize bearings are used.

A 617 crankshaft is nitrided to make the surface super hard. I don't know how deep the nitriding goes but it is only a surface hardening. So if you grind away the hard surface, it should be renitrided. This may be the crux of the problem as I don't know how many shops are set up to renitride crankshafts. You might need a shop that works on aircraft cranks because some of them are nitrided.

P E H

PS: I don't know how this reply got placed before your question.

[omegabenz]

Hello,

I bought a car from a girl 1982 300d turbo diesel, and I think she ran it out of oil, and the engine siezed up at idle in an intersection.

My uncle took the engine apart, #5 rod split radially meaning it adheared to the crank and the rod stopped on the block somewhere.

Some of my friends say turning a diesel crank will give no oil pressure when you are done, or it will break because of the stresses caused by nitride hardening the crank from the factory.

Anyone have experience with this?

[mpnye]

I've done it MANY times . . .

Have the crank ground and have them send it out for hardening and MICRO-POLISHED . . .

If the crank is REALLY BEAT . . . it can be HARD-CHROMED, RE-GROUND and MICRO-POLISHED back to standard and HARD AS HELL ! ! !

[samiam4]

omegabenz,

The question is *why* waste your money grinding the crank? You should be able to get a perfect used one out of a core engine for $100 or so. Ok, $100 with a little elbow grease.

Never NEVER hard chrome a crankshaft. Look it up in every engineering text book. While it does wear better than a regrind, it destroys the fatigue life of the crankshaft. That's why nitrating is prefered. You improve the fatigue life with nitrating. I cannot qoute all the numbers because my books are at work- but you can easily go from a million mile crankshaft to 10,000 mile crankshaft easily if you are not careful. Nitrating is an expensive specialized service, which MB would have never done if it was not nessary(non-turbos are not nitrated).

Problem #2 is machine shops use the radius grinding stones they have... if they don't put the correct radius- you'll increase the stress concentration and break the crank(at least the cranks I've seen break have been from this).



Michael

[mpnye]

Originally posted by samiam4
omegabenz,

The question is *why* waste your money grinding the crank? You should be able to get a perfect used one out of a core engine for $100 or so. Ok, $100 with a little elbow grease.

Never NEVER hard chrome a crankshaft. Look it up in every engineering text book. While it does wear better than a regrind, it destroys the fatigue life of the crankshaft. That's why nitrating is prefered. You improve the fatigue life with nitrating. I cannot qoute all the numbers because my books are at work- but you can easily go from a million mile crankshaft to 10,000 mile crankshaft easily if you are not careful. Nitrating is an expensive specialized service, which MB would have never done if it was not nessary(non-turbos are not nitrated).

Problem #2 is machine shops use the radius grinding stones they have... if they don't put the correct radius- you'll increase the stress concentration and break the crank(at least the cranks I've seen break have been from this).



Michael

(a) Try to find an MB crank for 100 bucks . . .

(b) My old 77 300D which I sold 10 years ago with a HARD CHROMED crank and a BAE TURBO is still goin' like hell with at least 475K miles on the engine I built 15 years ago . . . It runs back and forth between San Diego and Seattle almost on a regular basis . . .

[samiam4]

Oh,

And to be statistically signifcant- you need to rebuild at least 12 cars=0) At least that was the only thing i remember from that class!


Michael

[mpnye]

Originally posted by samiam4
mpnye,


I have zero ideas of what was done to your car. But, as a rule of thumb following mercedes service instructions gives great results.

Did you send the crank out to a aircraft service facility???? They don't do hard-chroming for the reasons I've stated. At one time in the past, they did use a process called "chromite" which uses chrome, but does not have the negative effects of hard-chroming. Doesn't have the positive effects of nitrating...

Not sure what you had with an aftermarket turbo and no oil cooling pistons like the factory?? Seen those setups on 240's but they don't tend to last. Did you put a turbo IP and ALDA setup on the car?


Michael

(a) Why in the hell would I send a crank to an AA certified shop to have my machine work done ? ? ?

(b) The company that I use GUARANTEES a 15-N 82 ROCKWELL INDEX which is IDEAL for such applications ! ! !

(c) I had my pump shop build a pump with a FAT lower mid-range curve to keep from torching the pre-chambers, which they supplied me with shims to DIAL-IN the ENRICHMENT via the ANEROID COMPENSATOR . . .

(d) It's run on a STEADY diet of TORCO 15w40 SYNTHETIC DIESEL OIL which is changed RELIGIOUSLY every 25,000 Miles and a FILTER ONLY change every 6250 miles . . .

ALL that with a 3000 RPM STALL CONVERTER . . . FAAAAAAST ! ! !

[samiam4]

mpnye,


I have zero ideas of what was done to your car. But, as a rule of thumb following mercedes service instructions gives great results.

Did you send the crank out to a aircraft service facility???? They don't do hard-chroming for the reasons I've stated. At one time in the past, they did use a process called "chromite" which uses chrome, but does not have the negative effects of hard-chroming. Doesn't have the positive effects of nitrating...

Not sure what you had with an aftermarket turbo and no oil cooling pistons like the factory?? Seen those setups on 240's but they don't tend to last. Did you put a turbo IP and ALDA setup on the car?


Michael

[samiam4]

Ref Juvinall and Marshek “fundamentals of machine component design”


Snip “shot peening is more effective with high strength steels… particularly beneficial in steels with tensile strengths above 200 ksi”
“Chrome and nickel plating, even though good for the surface in providing corrosion protection, can substantially reduce the endurance limit of the steel parts. This is well known as “hydrogen embitterment.” This damage can be minimized by taking special care in using low plating current densities and baking the parts at 600-900 F after plating.”

Note this minimization of effects does not gain any benefits over an endurance limit of a non-plated part. So, if they do it properly- your still loosing 50% of the endurance strength.

Endurance strength= the highest value the part can sustain for a reversing load 10+8 cycles, or infinite life.

Thermal and chemical surface hardening treatments:

“The purpose of thermal and chemical surface-hardening treatments is usually to provide surfaces with increased resistance to wear: however, they also serve to increase the fatigue strength, and for this reason are considered here.”
“The strictly thermal processes of flame and induction hardening of steel parts containing sufficient carbon produce surface residual compressive stresses as well as surface hardening. Nitriding is adding nitrogen to the surface layer, together with appropriate heat treatment.”

Endurance limit comparison (via ASME Handbook of metals)
Without notch 90 ksi nitrided vs 45 ksi Not Nitride
With notch 87 nitrided vs 25 ksi Not Nitrided

So, If you grind a metal shaft perfectly, it would could only take 29% of the alternating force as the nitrided part for an unlimited life. If your hard-chrome plating is properly baked, it would also be a 29%. If you shot-peen the area before plating this would be somewhat helpful, but it is a industry standard to keep the plating 1/8 inch away from the tangency of the fillet. And the stress peak is in the fillet(k=2+).

I don’t think it’s a good rule of thumb to decrease the fatigue strength by 2/3, period.

Michael

[mpnye]

Originally posted by samiam4
Ref Juvinall and Marshek “fundamentals of machine component design”


Snip “shot peening is more effective with high strength steels… particularly beneficial in steels with tensile strengths above 200 ksi”
“Chrome and nickel plating, even though good for the surface in providing corrosion protection, can substantially reduce the endurance limit of the steel parts. This is well known as “hydrogen embitterment.” This damage can be minimized by taking special care in using low plating current densities and baking the parts at 600-900 F after plating.”

Note this minimization of effects does not gain any benefits over an endurance limit of a non-plated part. So, if they do it properly- your still loosing 50% of the endurance strength.

Endurance strength= the highest value the part can sustain for a reversing load 10+8 cycles, or infinite life.

Thermal and chemical surface hardening treatments:

“The purpose of thermal and chemical surface-hardening treatments is usually to provide surfaces with increased resistance to wear: however, they also serve to increase the fatigue strength, and for this reason are considered here.”
“The strictly thermal processes of flame and induction hardening of steel parts containing sufficient carbon produce surface residual compressive stresses as well as surface hardening. Nitriding is adding nitrogen to the surface layer, together with appropriate heat treatment.”

Endurance limit comparison (via ASME Handbook of metals)
Without notch 90 ksi nitrided vs 45 ksi Not Nitride
With notch 87 nitrided vs 25 ksi Not Nitrided

So, If you grind a metal shaft perfectly, it would could only take 29% of the alternating force as the nitrided part for an unlimited life. If your hard-chrome plating is properly baked, it would also be a 29%. If you shot-peen the area before plating this would be somewhat helpful, but it is a industry standard to keep the plating 1/8 inch away from the tangency of the fillet. And the stress peak is in the fillet(k=2+).

I don’t think it’s a good rule of thumb to decrease the fatigue strength by 2/3, period.

Michael

Yer really good at the ol' CUT n' PASTE ! ! !

NASA would be IMPRESSED ! ! !

[mpnye]

But, ya know what . . . I spoke with the present owner's son this morning . . . It's STILL ALIVE . . . CHROME CRANK and ALL ! ! !

[JimSmith]

Just to add my perspective on an interesting discussion...

Chrome plating or any other layer of foriegn material to enhance the surface condition or performance in service is a common practice, and typically can achieve the intended or desired results. You have to be pretty sure about what the intended results are and make sure you are careful in the processing steps. It is a route "fraught with peril" to quote an unforgettable, and likely inadvertant mentor of my past. Coatings, including plating of Chromium or Nickel, is subject to a variety of conditions that will deposit atoms of the intended material along with unintended materials. In addition, they can cause absorbtion of gasses, like Hydrogen, which, as noted earlier, are prone to alter the mechanical properties of high strength steels (Hydrogen embrittlement weakens grain boundaries, leading to fractures at relatively low, steady state stress levels). In addition, anything sticking to the surface of the base material does nothing to improve the base material properties at the interface, and subjects the interface "bond" to the same mechanical stresses/loads that the base material sees. Often this results in failures of the coating bond, which leads to more complex failures.

Nitiriding, and shot peening for that matter, put the outside layer of the base material in compression while increasing hardness, kind of like preloading the surface by either mechanically working it (shot peening) or chemically jamming Nitrogen molecules into the grains/grain boundaries of the base material. With the surface layer in compression, the fatigue performance, or endurance limit , is improved in the application in question. Kind of like putting a fastener in a condition of preload so that the alternating loads of the application do not physically excercise the fastener.

So, on a production basis the Nitriding approach is far superior to the chrome plating, however, if all you want is a harder, more wear resistant surface because the base material is fine for the application as it is, other than hardness, carefully chrome plating should work ok.

In some cases I would question whether or not chrome plating actually helped - the condition of the original material might have been adequate to begin with and chrome plating only made things that were ok, a little better. In my past experience though, a good quality chrome plating job was so rare, I would tend to avoid them altogether.

Hope this helps, Jim

[leathermang]

"hydrogen embitterment"(sic) as mentioned by Michael is where the EX gets the new Hydrogen Fuel cell car in the divorce settlement....

I am sorta with everyone on this... I know the first time I read about HE was in relation to people chroming motorcycle frames.... certainly something to worry about in that enviornment...

As an old blacksmith I wonder if maybe the drawbacks to the chroming in this particular example ... of a fairly thick and strong piece of metal.... might have been mostly referring to the surface area which was ground off to make room for the new chrome....and the new chrome once installed....

In other words, the ductile strength of this relatively thick metal may have been determined by its initial metal type and cooling rate...

"baking the parts at 600-900 F after plating.” This constitutes basically retempering the item.. but on a crank I think a special jig would be required to keep it from warping.. unlike most items...

[samiam4]

Hey,

I'm leaving this alone now=0) My point was fatigue life not just strength is an issue. Chrome-plating addresses the wear area where nitrating(and to a lesser amount flame hardening) address fatigue *and* surface hardness.


Michael

[mpnye]

Yah . . . BUT ! ! !

The engine now has over 475, 000 MILES on the HARD-CHROMED crank . . . HELLO ! ! !