Regrinding the crankshaft is an important part of the engine rebuilding process. It was almost an art in the ‘40s, but it’s pretty routine now. However, there are still plenty of myths and misunderstandings that are left over from the past that we still have to deal with today.
One of the most common myths is the belief that .010”/.010” shafts are less prone to fail than the ones that are ground to .020” or .030” undersize. Although this idea was once based in fact, it’s no longer true. We’ll take a look at the difference between the past and present, but let’s talk about what causes a crank to fail before we go there.
The crankshaft is the backbone of the engine. It not only converts the reciprocating motion into useable rotating motion, but it also connects everything together while it drives the transmission along with all the accessories.
In the process of doing its job, the crank is subjected to two primary forces that are constantly trying to break it – torsional and bending stresses. Torsional stresses are the sum total of all the twisting forces that are applied to the crank. They vary depending on bore and stroke, maximum engine speed, torque, firing order, engine type, compression ratio, rod length, rotating and reciprocating weight and cylinder pressure along with a few more, but the two most important factors are the reciprocating inertia and the torque spike created by the combustion process.
At low rpms, the major cause of torsional vibration is the sudden rise in cylinder pressure created when each cylinder fires, but by the time the engine speed goes up over 4,000 rpm, the inertia forces become a significant contributing factor to the torsional loads on the crank.
In fact, according to a recent SAE paper, the torsional stresses due to inertia grow at the square of the rotational speed as the rpms go up, becoming almost equal to the torsional stresses created by combustion pressure at a given point, depending on the engine.
Because of these twisting forces, the crank acts like a torsion bar that twists one way and then the other way as the pistons come to a sudden stop at top and bottom dead center while the crank tries to rotate at a steady speed. Then the crank gets twisted the other way when the piston suddenly accelerates in the opposite direction right after it slowed down to try to match the speed of the pistons that just stopped and turned around.
And so it goes, twice every revolution, at both top and bottom dead center for each cylinder, first winding it up one way and then unwinding it the other way at the rate of 100 times a second when the engine is turning 3,000 rpm.
While all of this is going on due to the reciprocating inertia, each cylinder is firing on every other revolution and subjecting the crank to another significant torsional force, so the crank twists one way and then back the other way to try to absorb the difference in the speeds between the individual rod pins that are accelerating quickly while the rest of the shaft, including the balancer and flywheel, are trying to rotate at a more constant speed.
At low rpms, this is the primary torsional force until the engine reaches higher rpms. It’s also worth noting that the sudden extreme pressure spikes that are created by repeated detonation can actually twist the shaft beyond its design limits and may cause it to fail if the bearings don’t fail first.
Although torsional failures aren’t very common in gas engines, they do occur in diesel engines. This happens because diesels have higher peak cylinder pressures that create greater variations in torque when each cylinder fires.
It sometimes helps to think of the crank as a torsion bar. You can actually feel it twist back and forth by locking the crank up in the block and then trying to rotate the front of the shaft with a BIG cheater bar on a socket wrench. The deflection you see is the torsional movement of the shaft that’s constantly taking place in the engine. If you look carefully, you may notice that this twisting is all taking place in the mains, not in the rod journals or crank webs, and you might also see that there is progressively more movement in each journal from the front to the back of the shaft.
This illustrates an important point: torsional force is additive. In this experiment, there was more deflection at the front of the shaft than at the rear because we applied the force to the front of the shaft. When the engine is running, all of the torque is added to the back of the shaft, because that’s where the load is, so the crank will usually break behind the rear main journal if the failure is due to torsional stresses.
Bending stresses are the second force applied to the crank. They are predominantly the result of the increased cylinder pressure due to the combustion process. Inertia has little or no effect on the bending forces applied to the crank. The crank bends at each rod pin when that cylinder fires, because the load is applied in the middle of the mains that support the rod pin on both sides. When the shaft bends, it’s the crank web that is actually deflecting, so most of the stress is concentrated on the fillets on both sides of the rod journal. That’s why the crank usually breaks in the fillet radius if it’s overloaded and bent too far too often.
Crankshaft deflection is affected by several factors in addition to cylinder pressure. These include the structure of the block and crank along with the strength of the bearings, the bearing clearance and the condition of the oil film. Problems in any or all of these areas can increase the amount of bending stress and dramatically shorten the life of the crank.
Since bending and twisting are the two primary forces that are trying to break the crank, the real question is, how does regrinding the shaft undersize affect its strength in these areas? And the answer is…Not by very much, if it’s done correctly.
One crankshaft manufacturer tested some shafts that were ground down to .010˝, .020˝, .030” or even .040” undersize and found that there was less than a 4% reduction in the fatigue life of the undersize shafts. Most crankshafts are engineered to be about four times stronger than required, based on mathematical stress and load calculations, so a 4% reduction in fatigue life is virtually meaningless.
Since torsional failures are almost nonexistent on gas engines, removing .040” from our ‘torsion bar’ is not a concern. And, because the bending force is applied primarily to the web, not the rod pin, and we haven’t changed it in any way, it also stands to reason that we haven’t affected the bending strength of the shaft in any significant way. The validity of test data can be confirmed by looking at some real world examples:
It’s not uncommon to see race engines making 500 or 600 horsepower using reground OEM crankshafts that were designed for about half as much horsepower.
Caterpillar regrinds the cranks for its 3208 mid-range diesels to .050” undersize for its rebuilt engines and has had no problems with them living in trucks and tractors.
High performance stroker motors that use factory cranks that have been offset ground are pretty common today. Depending on the application, every one of these engines has had either the rods or the mains ground undersize to accommodate a different block, stroke or rod. Notice that we’re talking about taking .100 “ or .200” or even more off a journal to make a ‘mountain motor’ that’s abused without mercy. Here are some examples of what’s being done:
In order to build a 383 Chevy, the mains on a 400” shaft are turned down to 2.4493”so the crank will fit in a 350 block. That means we grind them down 0.200” and expect the engine to make 500 hp and live!
In order to build a low deck 470 Chrysler, the mains on the 440” shaft are turned down 0.125” so the crank will fit a 400 block and the rods are offset ground down to 2.200” instead of the original 2.375”. That means that the rod pins are now 0.175” smaller than they were originally.
In order to make a 495 Olds out of a 455, the rod journals are offset ground down to 2.250” instead of the standard 2.50”, so they’re .250” smaller.
In order to make a 514 big block Ford, the rod journals are offset ground down to 2.20” which means that they are 0.280” smaller than the original diameter of the rod pin.
If all of these cranks can be ground down by .050” to .280” and used in a diesel truck or a high-performance engine, it only makes sense that shaft that’s been ground .020 “, .030” or even .040” undersize will live just fine in Aunt Emma’s car. It sure would seem so, but some of our customers still aren’t convinced.
Why not? My guess is that we’re still dealing with a myth from the ‘50s and ‘60s that lingers on. Like most old wives tales, this one has some basis in fact because there was a time, way back in the ‘30s and ‘40s, when a .010 “/.010” shaft was actually better than one that had been ground down any further. There were a couple of reasons:
The rod and main bearings in most early engines were poured babbitt so when the shaft was ground undersize, the thickness of the babbitt was increased accordingly. The thicker the babbitt got, the less load it could carry, so it was more prone to distort, pound out and fail prematurely. Since it was even thicker and softer when the shaft was ground down to .020” or .030” undersize, it was even more prone to fail, so the repair shops didn’t want to buy a shaft that was ground beyond .010 “/.010”;
Up until the ‘50s, there was no way to salvage a shaft that had been ground beyond .020” or one that had burned down a journal, so everyone wanted a .010˝/.010˝ shaft because it could be reground one more time.
That’s how it all began and that’s why we continue to live with the myth of the .010/.010 crank even though everything has changed now – we don’t use babbitt any more and the bearing material is always the same thickness no matter what undersize the bearing is. The thickness of the steel backing varies instead of the bearing material, so the risk of a bearing failure is no greater with a .040” under bearing than it is with a .010” under bearing. We can repair a down journal today with the crank welders that were invented in the ‘50s, so most cores are reusable even if they have a bad journal or if they’re already ground down to .030” or .040” undersize. Most rebuilders will accept a core with a .030 “/.030” shaft without a chargeback because they can save it and reuse it again.
If the size of the journal is not the key to shaft strength and possible breakage, what makes the difference between a good and a bad reground crank? It’s all about the fillet radius. The size and condition of the fillet will literally make or break the shaft, because it’s the transition point of two intersecting planes on the shaft. Without a radius, the shaft would crack and break at this junction every time, no matter what size it was.
Think about how you cut a piece of glass. Once you scratch it, it will crack and break right on that line every time because all of the stress will concentrate at the scratch that’s the weakest point on the pane. The weakest link in the chain always breaks first, because the stress will always find the weakest point. Without the fillet radius, the corners of the journal would be the ‘scratch in the pane’ or the weakest link in the chain.
I once read that adding a fillet with a 3/32˝ radius is the equivalent of adding a one-inch steel strap diagonally across the intersection of the journal and the crank web. Whether that’s exactly true or not, the point is clear: it’s the fillet radius that triangulates the junction of the journal and web and gives it its strength, just like the triangular brace that goes from the sill to the wall when a carpenter frames a house.
There are two kinds of radiuses; there’s a ground radius, sometimes called a “cut radius,” and a “deep rolled radius” that’s sometimes called an “undercut radius.” Most engines have a deep rolled radius today rather than the tangentially cut radius that was common 20 years ago because it’s easier to manufacture and it nearly doubles the fatigue strength of the crank, especially if it’s made out of cast iron.
The ground radius is created by dressing a radius on the edge of the crank wheel so it grinds a corresponding radius on the shaft. It’s important to diamond dress the wheel with the special fixture that’s set up on the grinder so the abrasive is actually cut, and not chipped off like it is when the wheel is rounded off by hand with a carbide stick to create the radius. Using the diamond dresser also ensures that the angle of the radius is correct and uniform all the way around on both sides of the wheel. Most car cranks have a radius somewhere between .060˝ and .080˝, but some engines require a more generous radius, so be sure to follow the factory specifications for a given application.
Cranks with a cut radius can be welded as long as the radius is maintained and there is uniform penetration up on the cheek of the crank. If there are any gaps or flaws in the radius after it’s been welded and ground, it must be completely redone or the crank should be junked. Always double check the size of the fillet radius to make sure there is plenty of room for the bearing when it’s installed on the journal, too, because the fillet will edge load the bearing and cause an immediate failure if it’s too wide.
Be sure that the fillet surface is polished along with the journal because leaving any noticeable grinding marks in the fillet radius will create miniature ‘stress risers’ that can eventually contribute to crank failure.
The second type of a radius is called an undercut or a “deep rolled radius.” It’s created by machining a groove in the shaft on the edge of the journal and then burnishing it by rotating the shaft between rollers that are pressed tightly against the shaft by a couple of big hydraulic cylinders. This compresses the metal into the groove and creates a smooth, dense, uniform grain structure that provides the strength that’s needed in this highly stressed area.
Think of it like the comparison between a forged and a cast piston. The deep rolled radius has a tight grain structure more like the one on the forged piston, while the grain of the cut radius is more like the one found in cast piston. Or, to put it another way, the tight, uniform grain structure of the rolled radius is what you would see in a piece of oak compared to a piece of particle board.
The rolled radius is also preferable because the surface finish is always very smooth and free of the miniature stress risers that are created in the cut radius by the abrasive grain of the grinding wheel. However, there are limits to the strength of a rolled radius so there are times that a big, fat, cut radius is the only way to make the crank strong enough, but the incredible strength of the rolled radius was best demonstrated by the Buick V6 turbo motors that we used to build. It seems amazing that the split pin crank could even make it to the grocery store, but some of the Stage II motors we built in my younger days made 650 hp and ran in the 10s in the quarter mile with a turbo crank that was identical to the stock crank except for the addition of the rolled radius on both the rods and the mains! That’s a real testimonial to the strength of this crankshaft and the rolled radius.
By the way, you can weld a crank that has a rolled radius, but the technique may seem backwards to some people. When welding journals that have a rolled radius, start from both sides and work to the center as usual, but be sure to fill up the radius on both sides when you’re welding the journal because if you don’t you will leave sharp edges that create stress risers hidden under the weld that hangs over the edge of the journal. Welding them up won’t really affect the strength of the fillet radius one way or the other, but it will eliminate the breakage that may occur otherwise. Make sure that you grind the journal wide enough to prevent edge loading on the bearing after welding up the rolled radius or the engine will probably fail on the test drive.
Now that we’ve talked about the importance of the crank radius, let’s talk about what causes the crank to break at the rod pin. Most broken cranks in gas engines are caused by bending fatigue. Think of the shaft like it’s a piece of mechanic’s wire. If you bend it back and forth long enough, it gets hot, turns soft, loses its strength and then breaks.
The same is true for the crankshaft. The greater the deflection and the more often we bend it at the rod journal, the sooner it breaks. And, if there are any nicks, scratches, notches, flaws or imperfections in the fillet radius, the failure will happen much sooner. You can prove it to yourself by putting a nick in a length of mechanic’s wire before bending it back and forth several times. It doesn’t take long for the wire to break at the notch. A crank is no different, so an imperfection of any kind in the fillet radius will cause the crank to break right at the flaw.
You can carry this experiment one step further. Notch another piece of wire and try to get it to bend somewhere other than at the notch. You can’t do it because the wire will always flex at the point of least resistance, and that’s right at the notch. The same thing happens to the crankshaft, too. The same kind of stress that attacks the weak link in the chain will find the weak point in the crank and attack it until it breaks, too. You can’t hide a flaw from the bending stress that’s applied to the crank, so it will break wherever the stress risers are, especially if they’re in the fillet radius.
All this leads to an interesting conclusion. The strength of a reground shaft doesn’t depend on how far we grind it down, but it depends completely on the quality of the fillet radius we leave behind. Whether the radius is “cut” or ‘rolled’, it must be sized right and uniform all the way around the shaft and it must be smooth, free of any nicks, notches, grooves or any other imperfections. If the radius is right, the reground crank will live in most any reasonable application. There are thousands of reground cranks in every possible undersize in service today that prove it.
So, let’s put the myth of the .010˝/.010” shaft to bed and move on into the 21st century. It’s time to recognize that the strength of every reground shaft is the same no matter what undersize it is as long as the fillet radius is machined right.
Our job is to make sure that the radius is done right.