The Great Rod Side Clearance Imbroglio: Clearing Up the Confusion

In the formative days of race-engine building when men worked in dimly-lit blacksmith shops, horsepower advancements came in great waves. Airflow and combustion advancements provided power in massive leaps and bounds. Today, major improvements are scarce and more often appear as evolutionary steps in single-digit increments. This search for advancement has led to digging into areas previously unexplored, with special attention paid to reducing friction as one path to “free” horsepower.

In a recent Spinal Tap 11,000-rpm LS story, we outlined the engine built by Ben Strader of EFI University with huge valvetrain design and execution assistance from Billy Godbold at Comp Cams. One photo of the Spinal Tap engine revealed a pair of narrow connecting rods positioned on a rod journal. The photo revealed a decidedly wide rod side-clearance of 0.360-inch (0.120 at all three places on the journal). A reader quickly commented that this would not work and that the engine would suffer from a lack of oil pressure resulting from excessive rod side-clearance.

This reader was merely parroting the widely held belief that rod side-clearance determines the oil flow exiting the connecting rods, and that excessive side clearance cannot be tolerated. As we will see, this is an engine building myth that has, unfortunately, managed to sustain itself into the 21st Century.

This photo is what initiated this story – EFI University’s Spinal Tap crankshaft and connecting rod package. Note there is a ton of lateral clearance between the rods and the crank. The Spinal Tap engine uses a piston-guided connecting rod that is intended to reduce friction. This technique is used in thousands of competition engines around the world.

For those who demand we cut right to the chase, the gap between a pair of connecting rods does not control the amount of oil leakage past the connecting rods. You would be safe carving that into stone above your work bench. The forum experts are already firing up their poison keypads and that’s okay. It’s also not enough to flatly state this position without reinforcing this statement with a few facts.

We’ll also save you the trouble of informing us that Bill Jenkins, Smokey Yunick, and John Lingenfelter have all authored respected engine books that state that rod side-clearance determines oil flow off the connecting rod journals.

This is a direct quote from the Jenkins book (The Chevrolet Racing Engine) : “We also check side clearance in the normal manner. For drag racing or circle racing we use 0.015-0.020-inch…opening the clearance beyond this tolerance leads to excessive oiling to the cylinder walls (making oil control difficult for the rings) and increases the engine oil pump volume and/or volume requirements.”

All I know is, that in my world, rod side clearance has nothing to do with how much oil comes out of the rod bearings. – Jon Kaase

The Jenkins book was published in 1976 and Smokey Yunick’s book, Power Secrets, followed in 1983. We are not writing this to make these highly-respected engine builders – who have both passed away – look wrong. The more important point is that ideas and perspectives change as our understanding evolves, and this is one of those areas. The information in those books was what they believed at the time. This story will expand on what is really happening inside your engine.

Piston-guided rods employ a very tight span between the pin bosses, which only allows minimal lateral movement of the piston. This allows the engine builder to use a narrower connecting rod that is lighter and virtually eliminates friction between the rods. This is a JE piston built specifically for piston-guided rods.

Let’s start by examining the oil-flow path through an engine. The oil pump moves a given amount of oil based on its capacity and the speed at which it is operated. Pumps don’t make pressure. Instead, they move a given volume of fluid with pressure created by restrictions in the lubrication path. These restrictions reduce the volume. The best way to understand this is with the classic garden hose flow example. When the tap is opened, water escapes the end of the hose at a given flow rate. This rate can be expressed as gallons per hour (gph). If you restrict the hose outlet with your thumb, the pressure in the hose increases while the flow volume decreases exiting the hose.

The same situation is true inside an engine. The oil pump pushes a given amount of oil at a given speed and the volume is restricted by the route the oil must follow. The three major points of exit or leakage are the main bearings, the rod bearings, and the area around the lifters. Of these three, the lifters offer the largest area and that’s where much of the oil can escape. There are other leak paths as well including the oil that is directed to the valvetrain, the area around the distributor on older engines, and performance modifications such as piston oilers that direct oil to the bottom side of the piston and perhaps spring oilers. This is often termed “internal leakage.”

It’s important to emphasize that this story is not intended as a condemnation of production-rod side clearances or a recommendation to increase this spec. Current clearances are intended to prevent the connecting rods from either being too tight and causing problems as well as to prevent them banging around between the crank cheeks. However, if the situation presents itself where one or two rod journals fall outside the stock side clearance maximum, this is not necessarily a situation that requires investing in new rods.

As an example, if on a small-block Chevy the side clearance on a pair of rods measures 0.023-inch and the stock Chevrolet spec is 0.008- to 0.013-inch, the additional clearance is not something to stress over. While the side clearance is out of spec, no damage will occur as a result unless this is a high-RPM engine where excessive lateral movement could cause problems.

These three books from three very famous and well-respected race engine builders all state that rod side clearance determines oil flow. Back in the day that was “common knowledge,” however today, we know that information wasn’t entirely correct.

We ran across a statement on a respected engine forum from a gentleman who claimed that excessive rod side-clearance was the sole cause of the engine’s high oil consumption. He tightened the clearance with different rods and deglazed the cylinder walls. When the rebuilt engine stopped using oil, he pronounced that the solution was the tighter side clearance. What is odd is that no one caught the fact that deglazing the cylinder walls was more likely the reason the oil usage problem disappeared.

To get a professional engine builder’s perspective on this, we called Jon Kaase (pronounced Kah-zee) at Jon Kaase Racing Engines (JKRE). Kaase is perhaps best known currently for his Boss Nine aftermarket version of the original Ford Boss 429 heads, and his Boss Nine engines are capable of 1,000 naturally aspirated horsepower. He’s also a seven-time Engine Masters champion, and builds engines for IHRA Pro Stock competition. Suffice it to say, he knows his way around a race engine.

When we offered the opinion that rod side-clearance determines oil flow, Kaase was quick to say, “If that’s true, then the whole starting field of a NASCAR race is doing it wrong!” His point is that current Pro Stock, NHRA Competition Eliminator, and NASCAR engine building efforts (among many others) are aimed at reducing friction by using what are called piston-guided rods. This approach minimizes rod side movement with tight clearance between the small end of the connecting rod and the piston pin bosses.

This allows the engine builder to use narrower connecting rods and bearings. This is all aimed at reducing rotating weight and using wide clearances to minimize friction between the connecting rods. No one would offer a guess as to what all this is worth – likely the horsepower numbers for a small-block would be in the low single-digits. Yet despite this minimal return, this is normal procedure for high-RPM competition engines where every advantage must be maximized.

This is a copy of test results published in an old Callies crankshaft catalog. Note how bearing clearance has a major effect on load capacity, oil temperature, and oil flow. Increasing clearance from 0.0015 to 0.003-inch nearly quadruples the amount of oil flow exiting the bearing.

This fits right in with the now-common approach of reducing rod journal size to slow bearing speed. The most popular small rod bearing journal diameter now is 1.850-inch, down from the Honda 1.88-inch bearing. This also means the bearing clearance will be quite tight at perhaps 0.0015-inch or less and used with extremely light viscosity oil such as 0w20 or even 0 weight oil. We have reproduced three graphs from an old Callies Crankshaft catalog that shows how tighter bearing clearances increase load carrying capacity yet reduce flow. This flow is also affected by oil temperature, with tighter clearances causing a higher oil temperature. This is one reason why thin oil is necessary. In the ‘60s and ‘70s, engine builders did the opposite, using wider oil clearances and heavy 20w50 oil.

Side clearance means nothing to oil flow. – Tom Lieb, SCAT Crankshafts

Strader cautioned that piston-guided rods are definitely a race-only application as the pistons require both a custom small-end spec and also demand a specific skirt shape to compensate for the additional loading. He also said that regarding rod and bearing width, it’s critical to maintain a given width to prevent side loading from “corkscrewing” the piston down the cylinder which could edge-load the bearings. This is the kind of effort that is the reason race engines are so expensive and difficult to build.

In terms of oil flow, this is a chart created from a Driven Racing Oil test looking at oil flow versus viscosity at 250 degrees oil temperature. It’s no surprise that when using thinner 0w20, the oil flow increases roughly by a half gallon per minute at most of the RPM points compared to the more viscous 5w20 oil. While the difference appears significant on this graph, it represents a change of less than 6-percent of total flow.

We also thought it would be a good idea to get a perspective from a crankshaft and connecting rod manufacturer, so we spoke to Tom Lieb, owner of Scat Enterprises. Lieb knows quite a bit about the subject with over 50 years of experience. As soon as we broached the subject that connecting rod side clearance established oil flow, he laughed and replied – “Yeah, all those guys are off base. Side clearance means nothing (when it comes to oil flow).” Instead, Lieb supported the position that the restriction to oil leakage through the mains and rods is the bearing clearance.

He also made a great point that journal circumference is another variable in this discussion. There is a huge difference in circumference when comparing a big-block 429/460 Ford rod journal at 2.50 inches versus the 1.850-inch journal used on many small-block race engines. The circumference for a 2.50-inch bearing is a massive 7.85 inches compared to the smaller journal’s 5.81-inch measurement. That’s a difference of 26-percent, with the larger circumference offering more area for the oil to escape.

Lifter bore clearances plays a big part in reducing total oil flow through the engine and, yet, is rarely discussed. This clearance is not something normally verified, but using this special split ball dial bore gauge from DiaTest USA we can measure the inside diameter of a small- or big-block Chevy 0.842-inch lifter bore. A worn or bell-mouthed lifter bore can dump gallons of oil onto the crankshaft. A clearance of 0.001- to 0.0015 is considered optimal both for performance and to minimize oil leakage.

Lieb also pointed to oil pressure as another variable that contributes to the amount of leakage past the rods. His point was that if you are concerned with the volume of oil coming off the rods and mains, that pressure and where you read it has a significant impact on flow. With a set restriction, adding pressure pushes more oil past the bearings.

Lieb also pointed out that oil pressure is not the same at the front of the engine as it is at the back, near the bellhousing where most enthusiasts tap into the circuit. There could be a substantial drop in pressure to the front of the engine, which is why it might be a good idea to know what the pressure is at the front as well as the back of the engine. Loose bearing clearances will increase oil flow while reducing pressure.

During our research for this story, we noticed that nobody that we could find ever bothered to do the math calculations to determine the true dimensions we’re talking about here.

Math is not my favorite subject – it often is downright frustrating for me – so I called my longtime friend, and professional engineer, Don Young to help me. We won’t go through step by step on how the numbers crunch out, but if you really want to know leave a comment below and I’ll forward the numbers. What we did was first determine the area of the 0.002-inch of bearing clearance around a 2.200-inch small-block Chevy rod journal and the bearing. This edge orifice came out to 0.007 square-inch for one side of the rod. We doubled that to account for the two bearing edges feeding into the common area between the rods, creating 0.014-inch of total oil flow area.

This is what happens when oil flow drops off due to too-tight bearing clearances or a lack of oil, so erring on the side of creating sufficient oil flow is a good idea.

Next, we measured the outer diameter of a small-block rod at 2.600-inch and then determined the area of that circumference using an extremely tight 0.005-inch side clearance. This computed to an area of 0.040 square inch. So it should be obvious that even with a very tight rod side clearance of 0.005-inch that the area between the rods is slightly less than three-times as large as the area the oil must squeeze around the bearings.

We next tried 0.003-inch bearing clearance since that might have been a clearance early racers would run with 20w50 oil. This change delivered a 0.011 square-inch on one side of the rod. Both sides combined would equal only 0.022 square-inch of area, which is still roughly half the area of a tight 0.005-inch rod side clearance of area. If we increase the rod side clearance to acceptable numbers, the area between the rods obviously increases in area but the rod bearing clearance does not. The numbers don’t lie.

So to wrap all this up, there’s nothing wrong with using factory rod side clearances. They’ve been working for decades. However, rod side clearance does not have an effect on the flow of oil exiting the bearings. The viscosity of oil, the bearing clearance, and the temperature of the oil, along with lifter bore clearance all are far more important players with respect to the amount of oil slinging around inside the crankcase.

It’s that simple.

This story has focused on steel connecting rods, but aluminum rods do require much more side clearance than steel. Manley specs 0.025 to 0.050-inch for side clearance on its connecting rods. The larger spec accounts for aluminum’s greater rate of expansion, which his roughly twice that of 4340 steel.

Article Sources

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.
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