Bearing clearance is one of the fundamental aspects of engine building and one that continues to spark debate and divisive opinions—and as with everything from nitrous systems to eating chocolate, there's a balance to strike between just right and regret.
In an engine, the critical bearing clearances we're talking about are for the mains and connecting rods; and the clearance is the amount of space between the shaft and bearing surface that's filled with the vital, lubricating cushion of oil, known as the hydrodynamic wedge.
And it ain't much of a cushion. Even if the installed gap between the bearing and shaft is only 0.0015-inch, the oil is displaced with the loaded bearings. The oil wedge lifts the shaft when it starts rotating to keep it moving with minimal friction, but that oil film can be as thin as 0.00002-inch.
It doesn't take much in the way of excess heat, inaccurate machining, or other factors to affect that critical oil wedge. Bearing clearances play the most important role in maintaining the optimal balance of that oil wedge, ensuring the engine's performance and longevity.
In this story, we're going to look at the basics of bearing tolerances and the theories that can help you decide whether to build an engine with tighter or looser clearances. But no matter what you decide, it's guaranteed that someone else will have his or her own opinions on the topic. At least you'll be armed with facts to help make a more informed decision.
The Rule of Thumb
Generally speaking, bearing clearance should be 0.001-inch for every inch of journal diameter. Let's take the traditional 350 small-block as an example. The factory specs include 2.450-inch main bearing journals and 2.100-inch rod journals, which means the target bearing clearance should be about 0.0024-inch (0.0025 is the common spec) on main bearings and 0.0021-inch on the rod bearings. Again, in general terms, it's better to have the clearance slightly larger to ensure optimal performance and oil temperature, particularly if the engine is built for higher performance.
The idea that a slightly larger bearing clearance is better for overall engine performance, however, is another generality and is at the core of the age-old tolerance debate. Some builders will force a tighter bearing clearance with oversized bearings to achieve a horsepower advantage, while others will maintain the factory's target spec and accept a slightly larger clearance to foster engine durability.
Loose vs. Tight
The advantages of a "loose" bearing clearance—one that is on the high side of the recommended specification range or even higher—is lower friction, particularly at start-up, which translates into lower oil temperature. Ultimately, that can mean greater engine longevity. The flip side of that is excessively large clearances will push excess oil out of the sides of the bearings, which can enhance power-robbing windage and lower oil pressure.
A "tight" bearing clearance, with a smaller hydrodynamic wedge between the journals and bearing surfaces has advantages, too. It can provide more even and uniform loads on the bearings, as well as more uniform oil pressure on them. Additionally, less oil is required to maintain the hydrodynamic wedge, which translates into a reduced load on the oil pump that can result in more horsepower.
That all sounds great, right? But there's a downside to tighter bearing clearances: heat. A thinner film of oil between the bearing and journal is going to heat up faster and rise to a higher temperature than an engine with a looser clearance. It's less of a concern in a dedicated racing engine, but for an engine that's driven daily, it can increase wear and make it easier to spin a bearing, which will totally ruin your day.
As a rule of thumb, main bearing clearance should be targeted between 0.0025- and 0.0020-inch for most street and street/strip engines.
The Safety Factor
Having a looser bearing clearance is also a safety factor for the less-than-perfect manufacturing standards of stock-type engines and their machining and surface finishes. In other words, in a generally stock engine rebuilt to stock specifications, the journals and/or the backs of the rods or mains may not be perfectly round. Extra bearing clearance helps ensure everything keeps rotating as intended without excessive friction and heat that will ultimately break down the oil film.
The opposite is true, too. A more precisely machined engine block and rotating components enable the builder to run tighter tolerances and leverage the inherent performance advantages because the components are more closely matched in roundness, and that promotes the more even oil support on the bearing surface, as discussed above. Again, there's a fine line to walk with how much tighter to go, but more precise machining affords that leeway.
Don't Skimp on Machining
When considering building an engine with tighter bearing tolerances, the main and rod bores must be as round as possible—no more than 0.001-inch variation on a stock-type rebuild and 0.0005-inch on a performance engine. Main bore alignment is also very important, with perhaps no more than 0.0005-inch misalignment between the adjacent bores and only 0.001-inch overall, when using conventional tri-metal bearings. That overall tolerance should be no more than 0.002-inch when using aluminum bi-metal (lead-free) bearings.
Such precise machining not only ensures optimal bearing clearance, it accommodates minute yet important manufacturing variances with the bearings themselves. In short, if you're going to run tighter clearances, spend the extra time and money to make sure the block, main caps, and rods are as true as possible.
Keep it Looser With Power-Adders
Nitrous oxide, supercharging, and turbocharging exert tremendous loads on an engine that results in high cylinder pressures and, consequently, much higher temperatures. They also typically come with tremendous and immediate increases in engine speed, all of which can be a challenge for oil control.
The two big factors here are heat and crankshaft flex. The heat, of course, comes from the higher temperatures, while the fast run-up in engine speed can cause flexing of the crankshaft, both of which can lead to bearing clearance problems. It's not necessary to dial in excessively loose clearances when running a power-adder, but don't tighten them up. Aim for the high side of the normal range, and add 0.0005-inch if the measurement comes out on the lower side of the range.
What About Oil Viscosity?
Regardless of their weight, the formulations of motor oils today deliver greater load-carrying capacity than a couple of decades ago. That means they're able to spread out that load with less pressure per square inch. That means lower friction and heat, allowing a lighter-weight oil (thinner viscosity) to accomplish the same as a previously higher-viscosity oil in the same engine.
The consequently lower drag on the oil pump and piston ring pack that comes with a lighter oil frees up a few horsepower, too, and allows for tighter bearing clearances than would have been previously recommended.
Now, does that mean you can just pour 0W30 oil into your vintage 350? Not necessarily. Although modern oils offer greater load-carrying capacity, an original, unrestored engine may need the thicker oil for other reasons. But if the engine has been rebuilt with more precise machining, then yes, experiment with thinner oils to see what works the best—just be aware that you'll probably see lower oil pressure as a result.
Bearing Clearance vs. Oil Pressure
Oil pressure is the result of two things: oil flow volume from the pump and oil flow resistance in the engine. Higher pressure comes from greater resistance, while lower pressure is the result of less resistance.
When it comes to bearing clearances and their effect on oil pressure, a looser clearance will reduce flow resistance and lower the oil pressure. Conversely, tighter clearances increase resistance to increase oil pressure.
Oil viscosity contributes to resistance, with thinner oils reducing it and thicker oils increasing it. That's why it's more important and advantageous for performance engines with tighter bearing clearances to use thinner oils and vice versa for looser bearing tolerances and thicker oils.
Keep in mind, too, that a large increase in pressure does not equate with a large increase in oil flow. Pressure increases exponentially with flow, so a large increase in pressure will only net a comparatively small increase in oil flow.
Oil Flow Importance
Regardless of the clearance, oil flow through the bearings is critical for carrying away the heat generated by friction. When oil flow isn't sufficient enough to carry away heat, the oil will overheat and the thin oil film will break down quickly, allowing metal-to-metal contact and quite possibly catastrophic engine damage or seizure shortly thereafter.
For even mildly built, street-performance engines, that means a high-volume oil pump is a smart choice, especially when running tighter bearing clearances and thinner motor oil. It will help move the oil through the engine to maintain optimal bearing temperatures.
On engines with looser clearances, a strong oil pump is also essential to ensure the bearings aren't starved for lubrication, as more oil to them is required than a tighter-clearance engine.
Racing Engine Myths
Don't be swayed by tales of extreme clearances—tight or loose—for racing engines. If you're building an engine primarily for the street, with occasional or even regular blasts down the quarter-mile, stick to the rules of thumb described in the tips above. Racing engines come in all forms and are built for a variety of special operating conditions, and the urban legends you've heard thirdhand from the cousin of a guy who used to sweep the floors in a NASCAR shop don't mean much when it comes to building an engine for your vintage Chevelle.
In fact, whether it's a NASCAR engine, a Pro Mod drag racing engine, or even a Late Model circle track engine, each has its own parameters; and each is built typically with specialized bearings, which are used with very specific oils that are often blended with antiwear additives such as ZDDP (zinc dialkyldithiophosphate).
Then there's the whole question of whether the engine is naturally aspirated or fed with a power-adder. A NASCAR engine may run super-thin oil and have tighter tolerances, while a Top Fuel engine runs very loose clearances and very heavy oil to help keep the monster alive.
Bottom line: The variables for racing engines and how they're built are endless, so don't get bogged down by what you've heard on cruise night or in the pits at the track. Unless you're building a Top Fuel engine or jumping into NASCAR, ignore those tall tales and stick to the basics.
Mix and Match for Optimal Clearances
Generally speaking, if you want to go a little loose, subtract 0.0005-inch and if you want to tighten up the clearance, add 0.0005-inch. The twist to that is bearings aren't typically sold in such increments. They're offered in standard size, undersized by 0.001-inch or oversized by 0.001-inch. You go with the undersized bearing to loosen the clearance and the oversized bearing to tighten it.
Start with the standard bearings and if you find a clearance adjustment is required, move up or down, as necessary. And because 0.001-inch in difference can be more than desired, you can mix bearing sets to achieve the 0.0005-inch desired adjustment. Simply mix one of the shell halves of a standard bearing with a shell half from an over- or undersized bearing. Yes, it requires the purchase of two sets of bearing sets, but that's the price to pay for clearance optimization.
One more thing: When mixing the bearings, make sure all shell halves are aligned. That means install all the standard halves on the block side and all the undersized halves on the cap side or vice versa. It doesn't matter which side they go on, only that the same sizes are on the same sides of the components. CHP
Photography by Barry Kluczyk