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Ford 351 Cleveland Engines: Crankshafts

by George Reid

(Taken from Ford 351 Cleveland Engines: How to Build for Max Performance)

Where other Ford V-8 engine families had both cast and steel crankshafts, the 351C was produced in cast only. Based on years of experience with these engines, it is clear to me Ford never produced a mass-production steel crankshaft for the 351C, 400, or 351M aside for perhaps racing and rare factory experimental XE pieces. Hank The Crank began producing steel Cleveland cranks in 1974 for drag racers such as Jack Roush. The aftermarket has followed suit in the years since with a great selection of cast, steel, and steel-billet Cleveland cranks.

Cleveland crankshaft identification is easy and based on codes stamped into the first counterweight. Codes 4M, 4MA, or 4MAB indicates a 351C or 351M crankshaft with a 1.750-inch throw or 3.500-inch stroke. A 351C crankshaft has a 2.750-inch main journal, which makes it a different crank than you find in the 351M, which has the 400’s larger 3.000-inch main journal.

The 335-series factory nodular-iron crankshaft is fully capable of use in high-performance applications up to 7,500 rpm and hasn’t exhibited failure issues on a grand scale. I’ve seen hundreds of 351C engines raced over the past 40 years without consequence. The same can be said of the Cleveland’s connecting rods, which are very durable pieces with beefy large ends and 3/8-inch rod bolts.

There are three factory nodular iron crankshafts identifiable by markings, stroke (throw), and main journal dimensions. The 351C crank is marked with an identification number of 4M or 4MA. Where you must be attentive is main journal size because it’s easy to unknowingly pick up a 351M crank, which has the 400’s larger 3.000-inch main journals along with the 351C’s 3.000-inch stroke and rod journals. The 351C has smaller 2.750-inch main journals, which means you would need the journals turned down by a competent machinist.

Cleveland crankshafts all look the same at a glance; however, they are quite different. There are three basic types: The 4M with a 3.500-inch stroke and either a 2.750-inch main journal (351C) or 3.000-inch journal (351M). The 400 crank has a 3.000-inch main journal and a 5M stamp. On the left here is a 351M with a 3.000-inch main journal. In the middle is a 351C with 2.750-inch main journal (both with a 3.500-inch stroke and “4M” markings). The 351M also had a “1K” mark later in production. On the right is a 400 with a 4.000-inch stroke and 5M marking. (Photo Courtesy Tim Meyer, TMeyer, Inc.)

There’s a lot of scuttlebutt about 351C, 400, and 351M crankshaft identification numbers, but basic identification has never changed in all these years. When it comes to mainstream 351C, 400, and 351M crankshafts, basic identification numbers of 4M (351C, 351M) and 5M (400) and 1K (351M only) hold true as excellent means of identification. Though you see 4MA, 4MAB, 5MA, 5MAB, the additional “A” or “B” is nothing more than an engineering revision to the casting. It does not mean Brinell tested. Any crank that has been Brinell tested for hardness gets the Brinell test mark in the first counterweight, which is little more than a small 1/4-inch dent.

Ford’s 400 bottom end was designed with durability in mind with its 3.000-inch mains, 2.311-inch rod journals, and 4.000-inch stroke. This engine was born to make torque for full-size cars and trucks; hence the heavy-duty “5M” crank and D1AE 6.580-inch (center to center) connecting rods. Magazine tech writer and book author Richard Holdener proved you can throw nearly 600 hp at this bottom end and it stays together. (Photo Courtesy Richard Holdener)

We’ve been long taught to believe we must have a steel crank to do high-performance work. However, the 335 nodular-iron crankshaft, regardless of displacement, is a durable crank thanks to main journal sizing and the good consistency of nodular iron. Based on four decades of experience with this engine and the many people I’ve seen build Clevelands, this is a crankshaft with a great reputation for durability, especially if you inspect and prepare it properly.

I have to admit, although I am telling you how to treat a Cleveland’s factory internals, it makes more sense these days to pump up the displacement and get more durable components in the process with a good aftermarket stroker package. But let’s just say you want to keep your Cleveland all factory inside, you have a great foundation to work with. The 351C crank is an extremely durable piece that can take up to 7,500 rpm. Main thing is a detailed inspection and good prep work going in. A crankshaft’s primary job is to convert heat energy into rotary motion and power. Because there are many torsional stresses on a crankshaft, a lot of thought needs to be given to crankshaft selection. Although the hot ticket always seems to be steel crankshafts, they’re rarely necessary for a street project. Cast is good for anything up to 450 hp. If you’re going to push it beyond that, you’re going to want steel. It really depends on how you will treat an engine at 450 hp. If you’re going to treat it to nitrous or a blower, you want steel to reduce the chance of breakage. A cast crank does well with steady, consistent applications of power. It is the shock nature of nitrous and pressurized induction that can break an iron crank. I believe you can road race an iron crank depending on how fast you want to go and how much violent twist there will be.

The thing to remember about crankshafts is that they twist and flex as combustion pulses hammer each journal and you put a load on them, which oscillates material such as rubber back and forth. Then, you load the crank with a wide-open throttle. Aside from the obvious stress, there are other loads such as oil pump and distributor, which also contribute to twist.

Twist and oscillation affect timing and power output. As pistons and rods rise to compression/ignition stroke, oscillation becomes more intense, acting on not only the crankshaft, but also connecting rods and pistons. It all moves violently with changes in power application. The crank’s torsional action rebounds against the piston and rod as they ascend on the compression stroke. There’s also the harmonic balancer, which acts as a shock absorber for crankshaft twist. As the crank rebounds, it works on the balancer, which softens rebound shock and reduces the risk of crankshaft breakage.

Regardless of how you look at this dynamic, cyclic issues affect crankshaft life. How do you recondition a crankshaft to make the most of its durability? I get nervous about turning a crankshaft beyond .010-inch undersize; however, crankshafts are stronger than you think. Automakers engineer tremendous strength into even the most modest cast-iron crankshafts. Crankshafts do break from time to time, but rarely due to material failure. This means you can machine your Cleveland’s crank beyond .010-inch undersize without concern for failure, especially if you’re building a street engine. You can comfortably turn your journals to .020-inch undersize and still have durability. Some machinists are good with .030- and .040-inch undersize, which is discouraged because it tends to negatively affect hardening.

Over the years, I’ve studied crankshaft failure and talked with those who have also studied it and concluded most failures occur in the journal’s fillet radius where heat and stress seem to occur most. If you study most crankshaft failures, look at where they fail. The fillet radius where crank journal meets the counterweight is the most common failure spot between journal and the rest of your crank. Rarely do you see a mid-journal failure or a break at a counterweight. This is why you must pay close attention to the fillet area with proper machining and finishing technique, plus the bearing’s smooth and comfortable relationship with the fillet. Radiuses of both bearing and fillet must be identical for perfect mating and without stress issues. When you grind and finish a crankshaft, it is so easy to overlook the fillet radius. You spend so much time focused on journal surfaces, oil hole chamfering, and balance you forget to examine fillet radius and the bearing’s important relationship with it. To add insult to injury, you sometimes install rod bearings backward, which is failure before you even get started.

A good rule to follow is to machine the crank journals to the bearings, and swap bearings around to get optimum clearances. In other words, swap bearings around to where you get the best clearances throughout. And when machining the crank, micropolish the journals to get the best oil flow and wedge. Main and rod journals get extremely hot, especially under a load at high RPM where temperatures can be as high as 350 to 400 degrees F. Oil begins to break down at 260 degrees F. Synthetic begins to break down at 300 degrees F. Oil can tolerate extremely high temperatures for a short time, which is why steady volume across the bearing and journal is important. You want enough of an oil wedge at the journals to keep moving parts apart, yet enough flow to carry heat away from the journals. This is why the middle ground is suggested when it comes to bearing clearances.

Whether you’re doing a mock-up or final assembly, journals and bearings should be generously lubed with engine assembly lube. Some builders use engine oil, but assembly lube has staying power.

If in the process of machining and mock-up you find excessive crankshaft endplay, King Engine Bearing has two affordable options: MaxFlange and ProFlange thrust bearings. MaxFlange is a process used on all King engine bearings. It reduces crankshaft endplay by supplying a flange on the high side of the tolerance to compensate for excessive crankshaft thrust wear.

Excessive thrust clearance is more common with manual transmission Clevelands due to clutch activity, which leans on the main thrust heavily each time the clutch is disengaged. ProFlange is a King line of bearing sets with oversize flanges, which allows the crank’s thrust to be ground to .010-, .020-, or .030-inch undersize. Both approaches are designed to save crankshafts from rework or replacement.

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