The crank and rods handle all the stresses of converting up and down motion to reciprocal motion. And when you are building a performance engine, those stresses are much higher and consequences of a failure are much greater. Therefore, selecting the right parts for your build is critical to the success of your engine and happiness of the racer who chooses you to build it for him or her.
On the outside, connecting rods and crankshafts may look the same. If you measure it and it fits the dimensions, then you might assume it will work for your application. It’s either a 350 crank or it isn’t. However, it’s the things you can’t see that may be the most difficult to understand that play just as an important role in the parts you choose. Most engine builders are not set up with the equipment to perform metallurgical testing and therefore can’t see behind the physical dimensions without some type of destructive testing, and that can be quite expensive.
A lot of times it boils down to the materials you use and how well they match the application. Whether it’s for street/strip or all out race motor, going overboard with a crank or rod combination that is over-engineered for the application can be a waste of money, whereas parts that aren’t up to the task will waste money, time and possibly your reputation.
When the engine is at full tilt, the rods and pistons are trying to shove the crank right out the bottom of the engine, so strength is a prime characteristic of a crankshaft. Concurrently, it is the pulling forces on the connecting rod that do it the most harm as the rod is being stretched or elongated with the weight of the piston changing directions and crank pulling the rod down.
There are currently three types of manufacturing techniques used to make crankshafts and connecting rods – casting, forging and billet machining. The design and materials of the components that you use depend mostly on the engine and vehicle’s intended purpose.
Rotating assembly kits that are matched and balanced and ready to go are a good choice for some engine builders. You don’t have to do anything except bolt it in. For others, however, it’s more satisfying to put it all together and balance it yourself.
One manufacturer we spoke with said that rotating assemblies are the fastest growing segment for his business. One reason complete assemblies are gaining popularity is that not all machine shops or engine builder have the necessary balancing equipment or time to do it all. It can be far easier to let your supplier balance everything for you and often times it is less expensive than sending it out to some other shop that does have the equipment.
If you are building mild street engines then you probably don’t need the equipment with all the bells and whistles, but you can assemble a kit and still come out ahead of the game and make your customers happy. It won’t take as much time or overhead to assemble from a kit but your profit will probably be less than if you did it all. On the other hand, as we’ve discussed in these pages many times, there are many good and profitable reasons to own a balancer.
Many experts say that stock, cast cranks are fine for mild performance applications. But with the plethora of aftermarket performance parts it doesn’t take much these days to go past the mild threshold. The pounding, twisting forces created by those trick aftermarket heads or supercharger will wreak havoc on the cast crank until it fails. Most stock cast iron crankshafts have a tensile strength of 80,000-100,000 psi, depending on how much carbon content is in it. A typical cast crank made of 1053 has about 100,000 psi, which experts say is good enough for builds up to 450 horsepower.
If you’re building a big horsepower engine you may want to consider upgrading to a forged or billet crank made of 4340 alloy steel. The 4340 billet steel crank has the highest fatigue strength. The biggest difference in the various steel crank materials is the grain structure, heat-treating process and the mixture of elements. Cranks made of 4130 or 4340 for example, have higher amounts of chrome and nickel, which makes them stronger.
For higher horsepower street/strip or all out racing engines, a forged or billet crank is a must. Some budget forged cranks are made of a little lower grade alloy steel and, consequently, has a lower tensile strength rating than 4340 but better than cast iron. But the majority of forged and billet performance cranks are 4340 steel or another high grade alloy steel. The steel that a crank is made of is much like the ingredients in a cake. There may be a lot of chocolate cakes that look the same but the ingredients are slightly different. Manufacturers tend to have their own recipes for the material even though it is 4340 alloy.
Cranks made of 4130 alloy have a tensile strength rating of 120,000 to 125,000 psi. Cranks made of 4340 and similar alloys may have a tensile strength of 140,000 to 145,000 psi or higher, and a fatigue strength rating of 160,000 to 165,000 psi or more depending on the heat treatment and the quality of the alloy. The ingredients in the cake that boost the strength are chromium, nickel and molybdenum.
Heat treatments in general can raise tensile strength of an alloy steel from as low as 55,000 psi to nearly 300,000 psi in some cases. As far as crankshafts are concerned, 165,000 psi is about the strongest crank available from billet steel alloy.
Crankshafts should have the ability to stretch and bend and then return back to their original shape without any distortion, which is called elongation. Higher end cranks are made of materials that allow elongation and greater fatigue strength so they can withstand the constant pounding forces of combustion without failing.
A stock cast crank is often required for a particular racing class or division. While you can certainly rebuild a stock cast crank, you never know what you’ve got. Cranks have a life-cycle and when they aren’t abused they can last for a long time before any fatigue issues start to set in, but it’s not always easy to tell if one has be been abused. So it’s always a good idea to perform a magnetic particle inspection before going forward with any crank rebuild or grind.
Besides heat-treating cranks to provide additional strength and durability, journal surfaces are either hard chromed, nitrided or induction hardened. Some heat-treatments can double the surface hardness and increase fatigue life by up to 25 percent. Many of today’s high end cranks are nitrided, which involves injecting nitrogen into the surface of the crank to a depth of about .010? at a controlled heat. Air is vacuumed out and replaced with a chemical that penetrates the surface of the part. One of the drawbacks of nitriding is that the part has to be re-treated if you do any machine work to the crank.
When building engines with more horsepower than a mild street build it is recommended that you upgrade the connecting rods to a stronger material such as a forged or aluminum rod. But it really depends on what you’re building because there are a lot of materials and designs to choose from.
Connecting rods typically don’t bend or break under compression but are pulled apart at high rpm and tend to break during the exhaust stroke. Big torque monster engines pound down on the connecting rods while higher revving engines pull them apart. Because of this, performance rods have more compression strength and stiffness for the higher loads, and the higher tensile strength of the upgraded rods help keep them from a catastrophic failure at high rpms.
But experts usually say lighter is better, and in the case of rods it certainly helps. In certain applications such as drag racing, it is especially important in these types of builds that a light rod is used. In these builds, an aluminum or even titanium rod may be used. The lighter the rods, the faster the acceleration, but aluminum can stretch over time and needs rebuilt or replaced more often than steel rods, according to experts.
A discolored con rod is the result of excessive heat, and experts say the cause is almost always a spun rod bearing. Rod bearings spin due to a breakdown of the oil film between the crank journal and rod bearing. While there is debate as to why this happens, when discoloration is visible some experts say the rod has been damaged enough to ruin the heat treatment in the steel. This can have unpredictable effects on the strength of the rod. It is impossible to be sure if the rod can be used reliably once this has happened. The best course of action is to replace the rod.
Some manufacturers are starting to use 300M alloy. Some experts say that there is not that much difference between 4340 and 300M, however, 300M has a higher level of silicon and a little more carbon and molybdenum for added strength. This little bit extra strength can mean a lighter component as some connecting rod makers say it is good for a 10 to 20 percent weight reduction because they are able to thin out the cross-section areas of the rod.
A big topic of debate in recent years is whether foreign made products are of a lower quality than U.S. made components. While it is inaccurate to label ALL foreign-made product as inferior, one supplier says his company is constantly testing the forgings to make sure the steel meets specifications and often audits the facilities they deal with overseas.
Certainly there are foreign-made components that don’t meet quality standards, which is why you need to know what you are getting. There have been a few rogue suppliers who have burned end users and it has left a mark for all overseas manufacturers to overcome. However, many of the quality products are sourced from around the world and not only from China but Japan, Brazil, England as well as the good old U.S. of A. Unfortunately, as with almost any engine part, you will always have to be on your guard to watch out for knock-off brand name components. Talking to your suppliers and learning how to identify the authentic product will help you avoid this situation.
The good news for engine builders is that the performance aftermarket wants to give you what you want, and even offer several price points and options such as fully balanced rotating assemblies to meet your needs. Engine builders push their suppliers and manufactures push back with more new and improved products for you to sell to your customers.
For a complete list of performance crankshaft and connecting rod suppliers, visit our online High Performance Buyers Guide.