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The hardening of components is an important step in manufacturing mechanical components. The principle of hardening is to induce residual
compressive stresses in the material, which increase the critical load at which cracks will occur. Different techniques are used in the process. These include shot-peening, work hardening, ion-nitriding or ion-implantation, laser and induction hardening and deep rolling.

If extreme durability is the goal, you might talk to your machinist about nitriding. This process case hardens the crank to a depth of .020" or .030", without affecting the characteristics of the base metal. Surface hardness doubles from 30 or 35 Rockwell C to 60. This technology was primarily limited to military and commercial aircraft until the 1960s when Chevrolet specified nitrided crankshafts for the 327 Corvette. "Tufftrided" crankshafts were also installed in extreme-duty Chevrolet truck engines. The largest users of such crankshafts today are Mercedes-Benz, Perkins Diesel and Honda.

Nitriding iron can be done in several methods. The old process where the metal is soaked in a "Salt Bath Ferritic Nitrocarburizing" solution for a required period of time to increase fatigue strengthed is one method. The level of strengthening is directly related to the amount and in what form nitrogen occurs. The cost of hardening a crankshaft using this process is about $200, where most of the cost is concentrated on disposing of the solution once the hardening process has been completed.

The grey, sometimes almost black nitrided cranks encountered today were heated in an oven to 950° F in the presence of gaseous nitrogen.

Light grey cranks were subjected to ion nitriding, a process that uses microwaves for heat generation. The cost is similar to the salt bath method.
Anyone remember that IH used to advertise the "Tocco Hardened" crankshaft?
HB

I'm no expert by any means, am not a metallurgist nor a machinist, but I can float you a theory or two. I would think that it is a question of both stress and balance.

I don't like the idea of cutting down a journal at all; but rather, would opt for the idea of first building it back up and then cutting it back to the original size if replacing the complete crank is just not an option.

The reason I say this is because in the whole of the engine, the rod has the most load placed upon it. Both at the wrist pin as well as at that journal.

The journal is where the power flow is converted into rotary motion. At both extreme ends of the rod's travel, for any given speed, the velocity of this moving piece of iron is at it's absolute greatest and then it is required to immediately & completely stop and then abruptly reverse directions; twice, once at each end of the travel. So, even at a conservative idle speed of say, 600 Rpm, we are changing this direction what, 20 times per second?

Now, let’s drill a couple of angled holes in the center of this journal for the pressure lube and we’ve taken away material at the very point of the most stress.

So then, if we were to cut down the diameter of the journal, the flank of that journal is now heavier and the whole of the throw is out of balance with itself. Even this small amount of weight, when compounded by speed, is magnified tremendously.

Add to this the fact that the overall load on the crank is divided fairly evenly between all cylinders; cutting one journal down in size compared to the others is opening the door for a failure, IMHO. Cutting all the journals down would simply lower the overall reliability of the complete unit.

Under normal conditions, that crank is in a state of constant flex, both lengthwise and sideways and is the reason it is constructed from a relatively soft material, witnessed by the fact that the journal can easily be damaged merely by the act of bumping it with a rod bolt during assembly.

Also, this ‘flexing’ is why the stronger engines have a main bearing placed between each rod throw in an attempt to control this unwanted movement, which is constantly going on.

It is no wonder then, that an engine shakes when it is missing; or if run with a bad or missing harmonic balancer, it will set up vibrations, which are literally trying to break the crankshaft in two. When an engine is running without a balancer, you can actually see the end of the crank “wobble” at about 1200 Rpm.

There is a lot of stress going on with the crankshaft even under the best of conditions, as there is a lot of heavy iron flying about. This is why I think a line bore is so very important at the time of an overhaul.

Just my take on it, anyway,

Allan