Connecting-Rod Selection Help From SCAT Crankshafts

It’s a brave new world — and a much more powerful one as well — as factory-stock production engines are making 700hp. But, with all this newfound power and RPM potential come new challenges. That means foundation work with the rotating assembly is essential. Not that long ago, Bill Jenkins built Pro Stock-level small-blocks using stock connecting rods. However, in the present-day, unless there are rules demanding it, there’s no reason to do that anymore.

We thought we’d take a look at SCAT Crankshaft connecting rods to see how much this somewhat-obscure component has improved engine life. We’ll limit this discussion to steel connecting rods and use the ubiquitous small-block Chevy as our time-tested traveler through this evolving story. Why? Because it’s been around for so long. But, you could easily substitute a small-block Ford, a big-block Chrysler, or even an ancient flathead Ford into these descriptions — the physics don’t change.

SCAT Crankshaft

SCAT’s H-beam rods are offered in Premium Pro Sport and Ultra-Lite designs and are often employed in stroker combinations. An Ultra-Lite 6.0-inch H-beam for a small-block Chevy can be 35 grams lighter than a typical SCAT Premium Pro Series I-beam. This reduced weight comes from the Formula 1-derived hole in the beam. It does not weaken the rod but does remove rotating mass.

The Basics

A traditional production-focused small-block Chevy connecting rod has — for decades — been constructed of 1045-grade steel. This is a strong material with what metallurgists call an ultimate tensile strength (UTS). The UTS is defined as the material’s ability to withstand a load attempting to pull it apart. The 1045 steel has a UTS rating of 82,700 psi. Most high-performance aftermarket connecting rods are forged from a 4340 chrome-alloy carbon steel. This is generally rated to around 145,000 psi, which is roughly 75-percent stronger.

SCAT Connecting Rod Weight Chart

Scat Pro Sport H-Beam 6.125-inch connecting rod shown.

Here is a weight comparison of three small-block Chevy SCAT rods: Pro I-beam, Ultra-Lite H-beam, and the new Ultra HP Pro Sport rod. The differences in weight might seem small, but at high RPM, a reduction of more than 70 grams (36 grams x 2) on a rod journal drastically reduces the working force.

Component Weight:

Premium Pro, I-beam 6.00-inch SBC rod – 605 gms
SBC Ultra-Lite w/ 3/8-inch bolts, 6.0-inch – 581 gms
Ultra-Lite 6-inch SBC H-Beam, 7/16-inch bolts – 569 gms

We must also keep in mind that strength is only one part of this discussion. The steel’s ability to be malleable is also important. Malleability can be thought of as steel’s ability to deflect (slightly) while under high load and still retain its original shape. The ability to easily machine the material is also another important consideration — beyond just strength. This is why 4340 has become the steel alloy of choice for most high-performance connecting rods.

Building It Better

When talking with SCAT owner Tom Lieb, he mentions the company approaches the construction of its forged connecting rods differently than others. A typical forged-4340 rod is generally pressed from a single, large steel plate. While this process works, Lieb discovered a better way to build a stronger rod while his company was working on a development program with the Ford Motor Company.

scat crankshaft

Most of SCAT’s rods come with hollow dowel pins to more accurately align and maintain the position of the cap on the rod. The cap should be carefully positioned when installing to prevent damage to the dowels during assembly. The rod bolts should not be used to pull the caps into position.

This enhanced process begins with a round bar of 4340-steel that is placed in an induction furnace. The round bar is pushed through a roller press that forms the steel into what looks like a weightlifter’s dumbbell with large and small ends.

scat crankshaft

All Scat rods are ready to install with finished machine work on both the big and small ends. Full-floating rods are equipped with pin-fitted bushings on the small end.

The shaped metal is then placed in a forge-die, and a press forms the grain structure around both ends of the rod. This creates a grain structure in the cap area that forms around the cap. The “older” process cuts the grain structure area “like slicing an onion,” Lieb says. The result using Lieb’s new method delivers a much stronger rod.

Two Simple Choices

There are two basic connecting-rod designs that permeate the world of steel connecting rods. The choice for all production rods has always been a simple I-beam design. This classic configuration can withstand tremendous vertical loads and can be extremely light. The alternate version is the H-beam, which has developed a significant following.

One of the more obvious questions that come up relates to which rod to pick for a specific application. The common assumption is that increased horsepower creates elevated cylinder pressures, especially in nitrous or supercharged engines, and these applications create the greatest load on the connecting rod.

While the additional cylinder pressure does create greater compressive load on the rods, this is not really a serious concern, unless talking extreme power-adder-equipped race engines. For street-performance engines, even stock rods can handle the higher cylinder pressures from mild supercharged engines in compressive load situations.

scat crankshaft

Each rod beam and cap is individually numbered to ensure the caps are never mixed.

A Loaded Equation

Of greater concern is what engineers call tensile load, which is a force that attempts to stretch or pull the rod apart. Tensile loading occurs when the piston changes direction at top dead center as the crankshaft pulls downward on the piston and rod assembly.

In a four-stroke engine, this situation occurs twice. The first time is during the intake stroke, where a certain amount of cylinder pressure is present during overlap as the intake valve opens. This situation occurs again during the exhaust stroke, as the last of the exhaust component is exiting the chamber.

It is generally accepted that during the exhaust stroke – when the pressure is the lowest – is when this change in direction offers the highest tensile or stretching force. As you can imagine, these loads are very minimal at low engine speeds but build as RPM increases. Two aspects that play a big part in this load are the weight of the piston and rod assembly, and engine RPM. Of these two variables, RPM is the dominant component.

All SCAT connecting rods are fitted with ARP bolts, but the higher-end versions are offered with two options, using either the standard 8740 fasteners or the higher tensile strength ARP 2000 bolt. Higher quality bolts provide more insurance against the dire consequences of an accidental over-rev.

To underscore this point, a well-dressed small-block Chevy using a 600-gram piston, wrist pin, and connecting rod — in a reciprocating package with a 6-inch rod at 6,000 rpm — produces a tensile load of more than 11,000 pounds at 100 times per second. This means there is 5,500 pounds of force on each connecting rod bolt that is attempting to yank the cap off the end of the rod.

The combination of the strength of the 4340-steel connecting-rod material, the design of the big end of the rod, and the strength of the rod bolts all combine to create a rod that resists this enormous tensile load at higher engine speeds. This is also why RPM is the most important area to consider. Because of the loads achieved, a doubling of the engine speed means a four-time (4x) multiplication of the load. This is also why higher engine speeds require a stronger and higher-quality rod and rod bolt.

ARP rod bolts are installed in each SCAT connecting rod because they can handle this load. Engine speed is the 300-pound gorilla in the room when it comes to questions about rod durability. For performance street engines, we’ve had excellent luck with SCAT’s Pro Stock I-beam rod. So, it really comes down to specific recommendations for applications that might need rods like the Pro Sport H-beam, the Ultra-Lite, or Scat’s latest HP rod.

Connecting Rod Material Ultimate Tensile Strength

  • 1045 steel = 81,900psi
  • 4140 steel = 95,000psi
  • 4340 steel = 145,000 – 148,000psi

Weight is obviously a consideration when choosing a connecting rod, and SCAT offers several choices. I-beam rods are slightly heavier than H-beam offerings, and SCAT takes the lightness approach a step further with the Ultra-Lite stroker — and the newest rod — the Ultra-Lite Pro Sport rod. These two H-beam units feature an interesting hole located directly above the large end of the rod. Lieb told us he got the idea from a Formula 1 V12 engine.

The hole reduces the mass of the rod in the “dead” space directly above the big end of the rod. This weight reduction does not affect the structural integrity, but it is worth a reduction of roughly 20 grams in rotating mass. Any weight reduction here contributes to improved acceleration of the engine.

Confirming bearing clearance is an important step in the build process, which demands that a quality rod-vise is clamped over the rod and cap when torquing the fasteners to prevent damage to the rod.

The Value Of Torque

As part of the interview for this story, Lieb also opened a discussion about rod-bolt torque. One of the final steps before finalizing the rod is the finish machining of the inside diameter on the big end. During this process, the rods are cleaned, and the ARP bolts are coated with Ultra-Torque lubricant and tightened to the specific-torque value specified in the instructions included with the rods.

This torque value will approach the bolt’s proper stretch dimension. But, there is a range to the tensile strength of each bolt which affects its stretch value. Lieb says that if only relying on the stretch value, it’s possible the two bolts will not be tightened to the same torque value.

“This can create a situation where the rod is now subjected to asymmetrical torque values, which can cause the housing bore to shift slightly,” says Lieb. Because the housing bore was initially machined using a symmetrical-torque value for both bolts on the cap, Lieb feels that symmetry across both fasteners is more important to the integrity of the connecting rod. Of course, SCAT does insist the installer use ARP Ultra-Torque lube to ensure the consistency of the actual torque number. This does require the torque value applied to the rod bolts to be accurate.

In the case of a clicker-style torque wrench, if the wrench has not been calibrated — or it is not always returned to a relaxed position after use — it is very possible the wrench could under-torque the rod bolt. This could also cause the fastener to fail. If you’ve invested thousands of dollars in a performance engine, calibrating the wrench should be considered part of the cost of building the engine.

Stock Rebuild Versus SCAT's Stock Replacement

We decided to see if rebuilding a stock small-block Chevy connecting rod was less expensive than a stock replacement SCAT rod. Keep in mind, even this entry-level SCAT rod is a 4140 mild-carbon-steel forging that enjoys a higher tensile-strength rating than a standard small-block Chevy rod using 1045 steel.

Stock Rod Machining Procedures

Remove old pistons           $50

Re-size rods                        $80

Magnaflux                           $50

New ARP bolts                   $65

Total                                   $245

SCAT part number: 3-ICR5700P

$232.00 at Summit Racing (5.70-inch small-block Chevy rod with ARP Wave Lok bolts)

Need More Room?

Another important consideration for a connecting rod is its ability to clear other components like the camshaft. This is especially true when used in a stroker combination. SCAT offers a complete line of both I-beam and H-beam rods intended for stroker applications.

In the early days of popular stroker combinations, some engine builders’ answer to the clearance issues between the rod and the camshaft was to create additional clearance by grinding the connecting rod bolt head. It should be obvious by now, this is not an intelligent solution.

scat crankshaft

Each set of SCAT rods includes a torque recommendation that should be closely followed. SCAT also suggests establishing this value in a two-step process using ARP’s Ultra-Torque lube on the threads and also underneath the bolt head. The best rod bolt torque results will come from coating the threads and the under-head radius of the bolt before the final torque is achieved.

A much better alternative employed by SCAT is to trim both the transition area between the big end of the rod and the beam and use capscrews (through bolts) instead of studs and nuts. Also, SCAT uses a slightly shorter bolt than usual to create more clearance. This falls under the area of a simple solution to a common problem.

It’s never been easier to build more horsepower than in today’s performance engines. But, it also means all the components will be stressed that much more with the additional torque and RPM. A quality connecting rod may not be as romantic as a new supercharger, but it’s a smart move to place a quality set of rods on the essential side of the parts list for your next engine.

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
Read My Articles

Horsepower delivered to your inbox.

Build your own custom newsletter with the content you love from EngineLabs, directly to your inbox, absolutely FREE!

Free WordPress Themes
EngineLabs NEWSLETTER - SIGN UP FREE!

We will safeguard your e-mail and only send content you request.

EngineLabs

We'll send you raw engine tech articles, news, features, and videos every week from EngineLabs.

EngineLabs

EngineLabs NEWSLETTER - SIGN UP FREE!

We will safeguard your e-mail and only send content you request.

EngineLabs

Thank you for your subscription.

Subscribe to more FREE Online Magazines!

We think you might like...



Late Model LS Vehicles

Drag Racing

Performance Driving

EngineLabs

Thank you for your subscription.

Subscribe to more FREE Online Magazines!

We think you might like...

  • Late Model LS Vehicles
  • Drag Racing
  • Performance Driving

EngineLabs

Thank you for your subscription.

Thank you for your subscription.

EngineLabs

Thank you for your subscription.

Thank you for your subscription.

Loading