Determining The Optimum Oil Clearance For Your Engine

Discussing oil clearance and what type of oil to run in your engine always elicits strong opinions from people who may or may not be the best judge of your particular setup. Many keyboard mechanics go on internet forums to offer their not-so-sound oil expertise to anyone who will listen. But you can throw a dart and have a better chance of being right.

So when in doubt, ask an oil expert like Len Groom. He is the technical marketing manager for AMSOIL, responsible for engine oil products made and sold to racing and performance markets in North America. He heads the team that creates motor oils from concept to shelf and has given his fair share of advice to engine enthusiasts over the years.

The engine builder probably has the most power in the decision over which oil to use for your specific application. If your engine builder says ‘use this oil,’ most people will do it and not ask questions. But outside of that direct expert advice, people are often left searching,” — Len Groom, AMSOIL

What’s The Clearance, Clarence?

All engines need to make space for the oil to flow to vital internal components. You also need good oil pressure to provide lubrication, cooling, and hydraulics for lifters, variable valve timing, and more.

For years, according to Groom, engine builders relied on a basic rule of thumb of about .001-inch of bearing clearance for every inch of journal diameter. But the caveat here is that this rule only applies to stock engines. The rule changes a bit for a performance application, depending on how mild or wild it is. While rules are generally meant to be broken, it is better to err on the safe side. A performance engine that spends a lot of its time at high RPM may need some extra gap (about .0005˝) between the rods and mains to account for crankshaft flex and rod elongation. As the piston reaches TDC in performance applications, the connecting becomes more elliptical than round.

For years, engine builders have relied on a basic rule of thumb of about .001-inch of bearing clearance for every inch of journal diameter. However, the rule changes a bit for a performance engine application, depending on how mild or wild it is, and the RPM range in which it operates.

“You’re always going to be safe if you start at one-thousandth per inch of journal diameter,” says Groom. “It will generally get you into the ballpark of not hurting your engine with this formula. It may not be the fastest engine, but it will keep you from destroying it, and then you can go from there,” he explains.

While some naysayers may call this “Old School” thinking, it allows enough safety margin and protects your expensive investment. Groom says that as the metals and cylinder block construction have evolved over the years, oil clearance has gotten tighter as we move toward steel cranks and aluminum blocks. There are also different cap designs, bolt designs, and improved machining/manufacturing techniques that have contributed to this ‘tightening-up’ philosophy.”

You’re playing that game with metal expansion and contraction – aluminum vs. steel – which moves more at a given temperature. Not all components are going to expand at the same speed or the same temperatures with the same amount of movement.

The Society of Automotive Engineers (SAE) uses certain temperature thresholds to determine oil’s viscosity. The first number refers to its cold-cranking ability (not weight), and the second number is based on the oil’s viscosity at operating temperature (100ºC/212ºF).

Go With The Flow

Many enthusiasts misunderstand the terminology of multi-viscosity oil. The first number followed by a “W” (e.g., 10W) doesn’t mean “weight,” it stands for “winter” and the viscosity it represents at freezing temperatures. The number after the “W” means how resistant it is to flow at hot temperatures. It essentially measures friction. For example, a low viscosity oil, such as a 0W-20, flows faster than a high-viscosity oil like 20W-50. It is like the difference between water and honey. Water will always flow faster, but you need resistance to build pressure. There is a fine line between flow and resistance (i.e., pressure).

Internal clearances increase as temperatures rise, and the viscosity of the oil is reduced. This effect reduces resistance as the demand for oil increases.

A key factor to any lubrication system is the engine’s thirst for oil. The highest specific rate of demand (gallons per engine revolution) is when the engine is running at a hot idle. The oil pump must be appropriately sized to provide adequate flow at hot idle, and the clearances inside the engine provide the resistance, which results in oil pressure. In addition, engine resistance is affected by temperature. Internal oil clearance increases as temperatures rise, and the viscosity of the oil is reduced. This effect reduces resistance and the demand for oil increases.

Choosing the appropriate viscosity grade of motor oil is extremely important, whether for a grocery getter or an all-out horsepower monster. Automotive engines are typically tolerant of viscosity changes up to a point. But go too light or heavy on the viscosity grade scale, and the engine will wear faster, reduce performance, and could suffer internal damage.

If you’re looking to go to a different viscosity grade than what is recommended by the manufacturer, keep in mind that some engines may not be able to tolerate a significant change. Small engines with tiny valvetrains utilize tighter oil clearance and therefore may experience a hydraulic stacking effect when heavier oil is used. It’s like dropping a marble in honey at 50° F vs. 100° F. Over time, this can result in fatigued valvetrain parts or even broken valves. But even similar viscosity oils can react differently at different temperatures, depending on the base oil used. Premium motor oils use better base oils and flow better than similarly based conventional oils.

Machining practices have greatly improved over the past decade, and engine clearances have tightened up as a result. But if the machining falls short, the oil clearance needs to be opened up to compensate.

If you look at a hydraulic lifter, be it roller or flat tappet, those are oil pressure dependent. You don’t want to artificially increase your oil temperature or oil pressure, because the thicker the oil, the higher the oil pressure will be at a given RPM. And that has a direct effect on hydraulic lifters.

Groom continues: “When you set a hydraulic lifter, you always set it a quarter turn to a half turn past first contact, which means you move the plunger spring down slightly, so the motor doesn’t rattle. Now, that plunger is activated with oil pressure. But oil pressure that is too high will push the valve off its seat. And now you’ve got a miss. Or, you get overlap, and you run into other issues. At higher RPM, the power will fall off because the valves are not shutting and sealing the cylinder. You’ve already pushed it past that point where you’ve unseated the valve.”

Modern engines with variable valve timing (VVT) systems use oil as a hydraulic fluid, therefore it must not be too heavy or it won’t advance or retard the cam timing effectively.

The most important decisions you make about your engine should occur before turning any wrenches. It’s best to have a conversation with yourself, your engine builder, or parts supplier before starting your build. Groom says you must decide early on what you’re going to do with the application. “Then we can start making some decisions, but you have to be very real with yourself as far as what you’re going to do with it. Do I want to run it from stoplight to stoplight? There is a certain way we’re going to build it if you’re just looking to burn tires in a burnout contest. Do I want to take it out to track days, plus drive it to work? That’s another type of engine…”

While that conversation upfront is essential, everyone initially wants the most horsepower they can get for the least amount of money – and it has to last forever. But the reality is, something has to give.

“Building an engine is like building an oil – there’s always going to be compromises based on what you want to do with it. That’s how we construct oil at AMSOIL, too. We have to look at the application and where it is going. That’s why a generic oil that will fit all your needs doesn’t exist anymore. You used to grab a 40-weight to use in everything; you can’t do that anymore because of the differences in engines today, the way they’re put together, and their likes and dislikes.

Today’s oils are designed for specific applications, whether it is intended for a high-lift hot rod (which requires more zinc), or for long drain intervals (which requires more detergents). Companies like AMSOIL build oils like engine builders – for specific uses – which is why you see so many types of oils on the market.

You are buying a particular product today designed for an application, Groom comments. “That’s how we formulate and why you see all these different tiers of products at AMSOIL. We have a Z-Rod product that’s high zinc; we’ve got a racing line, that’s also high zinc, but it has high film strength designed for bearing protection. Then we’ve got our long-drain-interval oil for your passenger cars and light trucks. And small engine oil, that’s a completely different ballgame. It’s got a ton of detergent in it.”

Any way you slice it, viscosity directly relates to the amount of oil film between the bearing and the crank, or the bearing and the rod journal, Groom concludes. “When you take oil pressure out of the mix, the space will be thicker or thinner based on the viscosity that you’re choosing. Oil pressure is going to help bolster that. But at the end of the day, you still have to look at your oil clearance. The general rule of thumb is the thicker the oil, the bigger the oil clearance. And then you go from there.”

Article Sources

About the author

Brendan Baker

Brendan Baker is an automotive writer who lives in Akron, Ohio with his wife and two dogs. He started racing and building cars at a young age, building his first "racecar" at age 12 (a quarter-midget), which he put on pole position in its first race.
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