I’m weighing my options on my 425 build. I know the crank needs to but cut due to a spun bearing. I keep reading about forged cranks being nitrited but I can’t get a definitive answer so I’m looking for opinions. It’s should be a mild build 450hp aluminum heads. 99.9% street and will only see 6000 rpm if my foot gets stuck.

I have an opportunity to buy a good used crank but the only way to get it is to buy the whole engine and I think the guy is asking too much.

is nitriding critical? Were these nitrided from the factory?

 

Nitriding doesn’t go very deep anyway. Don’t. Lose any sleep over it.
Any of the N big block cranks are more than adequate for a 450hp build.

 

If it was initially carbonitrided and it needs to be cut just have it re-nitrided. A good machine shop can handle this.

 

Am I wrong, or aren’t 425 crankshafts forged (not cast) from the factory? If so, I’d sincerely doubt that you could have any concerns unless you’re turning NASCAR/NHRA typical rpm’s. And then, there would be all sorts of additional concerns.. Please educate/explain if I’m wrong, I don’t generally get easily offended, and LOVE to LEARN!!😂

 

Yes the 330, and 400/425 cranks were forged. What I’m wondering if if I HAVE to have it re-nitrited. I feel that the extra step will bring the price up too high. I got a quote for about $380 to mag/ cut the crank. I can get another 425 for $450, the cylinders are scored but the crank is good. I’d like to keep a forged crank. If I in fact do HAVE to get it re nitrited I might as well buy the engine.

 

You guys keep saying that but there are VERY VERY few shops in the country that can re nitrite a crankshaft. I certainly do not know of any within several hundred miles of my area.

 

Nitriding is one of several "proprietary" surface treatments. The main purpose of these processes was to surface harden and reduce friction. Surface finish is the important part. Your crank wasn't nitrided from Oldsmobile. Proper clearances and good oil is what you need. Don't worry about 6,000 RPM's. You'd need a good cam to make power at that RPM.

 

If a crank was initially carbonitrided its not required but is strongly suggested to reapply the heat treating post-machining to maintain the intended surface HRC for integrity/performance.

Consult the machinist and do your own homework.

Polishing should not affect the nitirde (cased) surface layer, it doesn't remove enough material, (again consult the machinist).

The crank is the foundation of the engine. Let the machine shop holding the warranty handle this discussion.

 

I would trust Cutlassefi's post. Who made you wonder about your crank being nitrided ?

Case hardening can be various depths. In a big factory where I worked, "Carbo-nitride" was used to treat "low alloy and carbon" steel for a hard outer case and tougher core then the original steel. There was a (case hardening depth) .003-.005", .005-..011", and .020-.030".process line for each. Various alloy and carbon content parts were primarily run. Typically the thicker the "case" the more distortion and size change. The .020-.030" parts were finish ground if size was important, such as axle bearing journals. .
I would consider surface hardening flat tappet cam lobes, but I wouldn't do it to the cam journals. In fact, some cam grinders offer nitriding as a service.

I would have a reputable heat treat specialist check your hardness, but i think you're over thinking this. The reason I mention this is because a Wilson-Rockwell tester in competent hands will tell you what you need to know quickly. I'll bet they won't use the HRC scale because the diamond penetrator would likely pierce through a thin .003-.011" "case" from the initial "load". More likely they will start using the HRB settings with a 1/16" ball.

 

I’m going by what I read online about cutting forged steel crankshafts. A lot of people are saying once you cut a steel crank (which removes the nitride coating) that you have to have it renitrided but I could never find a good answer as to why. If the crank will be fine with just a cut and polish for street driving than I’m fine with that.

 

#1. No one has established your crankshaft was ever nitrided from the factory or nitrided after.

#2. You are on a reputable Oldsmobile site that can provide you with the best information available about your Oldsmobile 425 crankshaft.

#3. In the "Now time" of AI, you can read all the past myths, old wives tales and just plain wrong information.. A point of clarification for you.....Crankshaft journals are all ground on a specialized, dedicated crankshaft grinder. "Cut" is a slang term for machining the crankshaft journals to a precise size.. The grinding wheel grit size helps to determine surface finish.

 

How badly damaged is the crankshaft; does it "ring" clearly? In other words is it worth sending out for regrind and will a 0.020" undersize provide a good finish or does it need to go to 0.030" or 0.040"? Has the crankshaft grinder looked at it yet and if so what has the crankshaft shop told you about it? I personally would be satisfied with a 0.010" or 0.020" undersize but not anything further. The '65 to '67 400/425 crankshafts were not factory nitrated and don't need it. That other 425 may be the better choice in the long run.

 

Is this strictly your personal choice or is there another reason?

 

I just want to debunk this other old wives theory. I have had crankshafts that rang like the best church bell you could find but had so many cracks in them that it was pitiful. I have also had crankshafts that sounded like you were hitting a piece of wood and yet when magnafluxed they had absolutely no cracks.

 

Keep it simple. Don't worry about nitriding. I'd rather spend the funds on coated bearings than nitriding the crank. You're not going to stress the crank at this engine level.

Turning a crank is very common and as long as the grinder is skilled then it works just fine. No need to kill yourself finding a pristine OEM crank. Note that it's difficult to get the radiuses as nice as factory on a grinder, so double check the rod side clearance and get narrow rod bearings if you can. Most high performance rod bearings are narrow anyway.

 

Sounds good. I don’t mind cutting the crank as much as it takes. People take .100” off to make strokers so I’m ok with .030”. I’ll skip the nitriding. Thanks for the info guys!

 

Make sure you can find the correct size bearings before you cut to a particular size.

 

Thats not true. The factory ground crankshafts with a crankshaft grinder originally. Those radii are ground on there for good reasons. It was to eliminate a stress area and for strength. The radius can be "touched up" by having the correct radius dressed on the grinding wheel and allowing it to dwell on the radius.

Let me ask you this, does your favorite machinist "plunge grind" or "traverse grind" a journal to size ?

 

cutlassefi Availability of good quality bearings and allows for further regrind if another oops. For strictly a street cruiser I have run a 0.030" undersize crank in an "E" block with no issues.

 

BillK To clarify my point about ringing. We are talking about an Oldsmobile forged crank here and if it doesn't ring well it most probably won't pass mag so I would not spend my money on it, I would replace it. Chances are it will ring well and mag just fine; takes a lot to kill one of these.

 

Ok gotcha. FYI, the thickness of the bearing shells themselves are adjusted for size. In other words, a .020 or .030 under bearing will typically have a thicker metal shell behind it. So support and quality are seldom an issue.