Clearing the Air on Bearing Clearances
Engine connecting rod and main bearing clearances have tightened in recent years for many racing applications, but there are caveats to this trend that engine builders need to acknowledge before bending traditional guidelines, especially when adapting those practices to street engines.
"There are specific classes that really go towards the tighter oil clearance and thinner oil," says John Himley, engine builder at CNC Motorsports. "NASCAR and obviously Pro Stock, those guys nit-pick every tiny bit of horsepower they can out of that engine. That's where you see the tighter oil clearances being used." As with any high-end racing innovation, there's always the temptation to leverage that technology into sportsman classes and even to street vehicles.
"If it's too tight, everybody knows. Too loose, and you know," quotes Himley.
The time-honored formula for determining bearing clearance is .001-inch for every inch of crankshaft main journal or rod journal diameter.
"Then if you want to go loose, add .0005," suggests Lake Speed, Jr., certified lubrication specialist at Driven Racing Oil. "And for tight, take away .0005. This is on the gross. So, for a 2.5-inch main bearing, standard is .0025, loose is .003 and tight is .002."
LOOSE IS FAST?
Mahle Aftermarket, which manufacturers Clevitte 77 bearings, offers a slightly different starting point with .0007 to .001 per inch of shaft diameter, and adds an option of an extra .0005 for performance engines.
"Then, you can work down from there as experience dictates," says Bill McKnight, lead trainer at Mahle Aftermarket. "It boils down to weight and grade of oil. If you're going to run a 0 or 5W oil, you don't need the extra .0005."
Running a little on the loose side has often been a preferred option for engine builders. Fifty years ago, "loose is fast" was a common belief. At the very least, this tactic extended the lives of racing engines—although we're just now starting to really understand why. More on that thought later. But, first, why are tighter clearances becoming more of the norm in race engines?
"In most cases it’s easier to do pushups on your palms and not on your fingertips," says Speed.
That analogy refers to the hydrodynamic wedge that moves the shaft off center and keeps it from contacting the bearing surface. A low-viscosity or light oil will be squeezed over a greater area of the bearing surface when there is a tighter clearance.
"Today's oils have more load-carrying capacity than older oils," explains Speed. "You're not concentrating the load in a smaller area; you're spreading it out and actually reducing the amount of load per square inch. That combination actually frees up more horsepower than a looser clearance and heavy oil in the same engine. A lighter viscosity oil frees up drag on the pump and there's less drag on the ring pack—all without compromising the bearings."
CONSIDER THE CRANKSHAFT
Tight clearances will get an engine builder in trouble if they don’t consider two very critical factors: crankshaft stiffness and crankshaft journal surface finish.
"If you have distortion in the crank, you’ll knock out the bearings. The flex will take up all the clearance you have," says Speed. "Tight clearances only work when you have strong crankshafts."
"Flexing leads to rubbing on the edge of the bearing," warns McKnight. "We call it edge loading."
Consider the most extreme engine environment in racing—Top Fuel. With so much boost and cylinder pressure, crankshafts barely survive more than a dozen quarter-mile runs of less than four seconds each after being subjected to so much flex. Nitro engines run very generous bearing clearances and use high-viscosity oil in the range of 70W.
"You have to have the clearance to accommodate that much motion," reminds Speed.
And that dynamic is one possible reason early hotrod and speed pioneers discovered that "loose is fast." When racers started heavily modifying stock motors with more compression, boost and aggressive camshafts—and then running those motors at higher rpm than what they were originally designed—the old cast-iron crankshafts moved around too much. Thicker oils and more clearance helped solve some of those issues until stronger, forged-steel and billet crankshafts became standard equipment in race engines. And when those modern crankshafts were developed, they also featured ultra-smooth journal finishes.
"If you're running tight clearances, you have to run thinner oil," cautions Speed. "And you can only do that if the crank surfaces are smooth enough. The rougher the finish, the more clearance you need."
Other factors that need to be considered when setting bearing clearances include the engine-operating environment, cylinder, block material, and oil system.
"If you've got a circle track engine running 40- or 50-lap features, you have to take into account the oil temperature," says Himley. "Some guys get up to 260 and 270 degrees."
NASCAR engines are designed to run extremely hot so that crew chiefs can tape off more of the grille and gain an aerodynamic advantage on the track. EngineLabs has seen a build book for an engine that ran six years ago, and the main clearance was set at .0022 on a 2.248-inch shaft, while the rods were set at .0019 on a 1.850-inch shaft. This team used a formula of just under .001-inch for every inch of shaft diameter on the main and just a hair over .001 for every inch of rod journal diameter. Rather old school, wouldn't you say? A lot can change in six years; however, especially with the addition of EFI and roller camshafts, the exact clearances running today can be different — and likely tighter.
Aluminium blocks expand more than iron blocks, so many engine builders will tighten up the main bearing clearance when building the engine at room temperature, knowing that when the engine attains operating temperature, the clearance will have expanded. That's one of the reasons why engines in V8 Supercars, IndyCar, WEC, Formula 1 and other high-end disciplines are pre-heated before startup.
"Most of the LS engines that are hot today like a little bit tighter clearance," affirms Himley. "On any aluminum block, we'll run a little tighter clearance due to the expansion."
WHAT'S BEST FOR YOU?
Dry-sump engines respond better to tightening up the clearances than wet-sump, simply due to the consistent oil control in the pan.
Measuring the crankshaft and journal bearings requires precision equipment and the knowledge of how to evaluate the readings. It would take a very lengthy story to cover that topic, but these skills are necessary if engine builders are considering tighter clearances.
"If you’re going to go tighter and run thinner oil, then all of that stuff has to be spot on," cautions Speed.
Moving towards tighter clearances and thinner oil can free up some horsepower in the right combination. But there are risks if the engine builder doesn’t take into consideration the many factors just outlined, especially the crankshaft.
"It's easy to whip these numbers out," sums up McKnight. "But you have to remember, how good is the shaft? Is it round? Is it straight? Is there any taper? If so and you’ve got tight clearances, then you'll have issues."