Selecting a rotating assembly is a major decision for any builder. Choosing the right crankshaft, bearings, connecting rods, and pistons forms the foundation of engine performance. Get it right and you’ll achieve long-lasting performance. Get it wrong and it will underperform, or worse, have serious issues when you turn up the power.

In this article, we’re going to look at nine top-level tips on how to select a rotating assembly for your next project. For expert advice, we turn to Nick Rinehart, General Manager, and Erik Johnson, Sales Technician, at Competition Products. Located in Oshkosh, Wisconsin, Competition Products has been a leader in the industry since 1970, and is an engine builder’s source for everything from street to full competition builds.

Tip 1: Know How You’re Going To Use The Engine 

The first step begins with the long-term vision you have for the engine. Will this be a Sunday car show cruiser, something that sees a few trips down the track, or a full-on race engine? Each of these applications will require a different rotating assembly. Having a plan and sticking to it is one of the most time-saving, budget-friendly steps you can take when selecting a rotating assembly.

Tip 2: Match The Rotating Assembly Strength To Horsepower Goals

You’ll want to determine a desired horsepower goal before you start looking at rotating assemblies. Dreaming of big horsepower is fun, but you need to be realistic with your horsepower goals when you start to look at rotating assemblies. When it comes to the rotating assembly, horsepower goals dictate materials. That often means choosing between cast, forged, or billet parts, depending on how much power you want to make.

Typically, if you’re shooting for the 500-600 horsepower range without a power-adder cast parts will be fine. When you’re looking at more than 500 horsepower and want to use a power-adder, forged parts should be on the radar. Now, if you’re really trying to make big horsepower in the 2,000+ range, and use a power-adder will be used, billet parts are going to be required.

Future-proofing your engine build with a strong rotating assembly is a good idea. You never know if a power-adder could be in the future for your project, and you don’t want to have a rotating assembly that limits what you can do.

Tip 3: Build The Rotating Assembly For The Future

The last thing you want to do is create a problem for your future self when picking a rotating assembly for a build. You should think about future-proofing the engine just a little bit by selecting a stronger rotating assembly to start with. That way, if you decide to bolt on a power-adder you have a little bit of headroom to work with.

“Let’s take the 500-to-550 horsepower typical Small Block Chevy rotating assembly build,” Johnson says. “You can do a pretty budget-friendly rotating assembly. For a build like this, I would recommend using a cast steel crank, 5140-steel rods, and a hypereutectic piston with no problems. But if there are plans down the road for more power, then by all means step up to a better kit now. This means looking at something with forged components that are designed to handle more power.”

“Where a lot of people get into trouble is they do a build like this and then decide to add a 150- or 200-shot of nitrous. Now these parts are not up for the task because the car is now 750 horsepower. It’s important to know what the true end goal of the build is before you get started.”

Tip 4: Think About Power-Adders

As Johnson explains, adding nitrous, or later installing a turbo or supercharger, can quickly step up your horsepower, in some cases by as much as 40 or 50 percent. It’s easy to overshoot what your pistons, rods, and crank were designed for, especially with stock or mildly upgraded components.

If you have even the slightest thought of adding a power-adder down the road, invest in quality rotating assembly parts now to prepare for that goal. You can never overbuild enough for durability, but you can certainly underbuild, and the consequences can be devastating.

Creating a plan and sticking to it is important when picking a rotating assembly. There are certain areas where there might be some wiggle room with how parts are selected or used, but the rotating assembly isn’t one of them.

Tip 5: Know How The Rotating Assembly Will Be Balanced

In several older engines, you’ll find that the crankshaft reciprocating weight is located outside the block on the flywheel and harmonic balancer. These are referred to as externally balanced, as opposed to internally balanced, where all the weight is located inside the block.

Common classic muscle cars with externally balanced engines include the Chevy 400 small-block, Chevy 454 big-block, and Ford small-blocks. It’s critical to select the right rotating assembly balance for block fitment, and to ensure the flywheel/flexplate and harmonic balancer are matched to avoid excessive and damaging vibration.

In that regard, balancing the entire rotating assembly is always necessary because the reciprocating mass must match the crankshaft bobweight. Rinehart emphasizes, “Balancing the rotating assembly is a must. The cranks come through with a target bobweight, but with the amount of aftermarket parts available and the different weights of these components, it has to be done.”

Tip 6: Keep The Deck Height In Mind

Deck height is the distance from the centerline of the crankshaft to the top of the engine block. It’s an important measurement to ensure an accurate engine build. Factoring in rod length and piston height, you’ll want your assembly to sit flush at top dead center. Too tall, and it can cause piston-to-head contact. Too short and performance falls off. You need to select a rotating assembly that will fit the deck height of your block to avoid any issues.

Thankfully, you have the ability to mix and match parts to create your own custom rotating assembly. This flexibility gives you and your engine builder the latitude to pick the parts that will work for your application.

Tip 7: Don’t Forget About Compression

More factors come into play when moving from street performance to a race engine, such as compression ratio, fuel octane, and RPM limits.

Compression ratio is the volume of the cylinder and combustion chamber from bottom dead center compressed to top dead center. For example, a 10:1 ratio means the air-fuel mixture is squeezed into one-tenth of its original volume. This matters because the higher the compression ratio, generally, the more power is created. But with more horsepower comes more heat, so there’s a trade-off.

Street performance builds typically run 9.0:1 to 10.5:1 compression. Piston design plays a key role: dome pistons typically increase compression, dished pistons typically lower compression. Why lower compression? Running boost effectively increases cylinder pressure by forcing more air-fuel into the chamber. By lowering your compression ratio, you’re reducing the chance of detonation, commonly called engine knock, while giving up a little power.

Tip 8: Know What Fuel The Engine Will Be Fed

Detonation occurs because fuel octane fails to properly control the combustion event. If the cylinder becomes too hot, lower-octane fuel may self-ignite in the combustion chamber. Detonation leads to serious engine damage.

 Fuel requirements and costs are another reason why planning ahead is critical. When selecting a rotating assembly and determining the compression ratio, you’ll want to keep in mind what your goals are. You don’t want to build an engine that requires an exotic fuel if it’s not required. 

Tip 9: Set Safe RPM Limits for Your Rotating Assembly

The last step in selecting a rotating assembly is determining your maximum RPM. Not only can higher engine speed make more horsepower, but it also directly affects piston speed.

Higher engine RPM means higher piston speed and greater stress on rods and pistons. This is an important factor when setting a safe RPM limit for durability and selecting connecting rods that are a part of the rotating assembly. A longer stroke will generate higher piston speed at lower RPM, while a short stroke can spin to higher RPM before reaching the same piston speed. You need to make sure your rotating assembly parts are capable of handling the RPM level you plan to spin the engine to and handle the horsepower goals you’ve set.

Conclusion

It all comes down to choosing materials that match your horsepower goals. Johnson concludes, “The main thing that needs to be considered is whether the parts are up to the task of the load being applied to them. The engine doesn’t care what brand name is on the crank or rods, only whether they can handle the power.”

Selecting a rotating assembly is an investment in the durability and performance of your build. Knowing these key tips will give you a solid foundation to make informed decisions. Plus, it will help you understand how future performance adders will impact components.