What Is Rod Knock and What Causes It?

What Is Rod Knock? Causes, Fixes, and the Truth About Additives for Rod Knock

As an experienced auto technician who has diagnosed my share of internal noise issues, I can tell you firsthand—rod knock is one of the most dreaded sounds a driver can hear. That rhythmic metallic knocking isn’t just annoying—it’s often the death rattle of your engine. But before you panic, let me walk you through what rod knock is, what causes it, and whether there’s a way to fix rod knock—either through mechanical repair or by using specially formulated additives for rod knock.

What Is Rod Knock Sound?

Rod knock refers to a deep knocking or banging sound that comes from the lower end of the engine. The sound is usually loudest when the engine is under load or when accelerating and becomes more pronounced as engine RPM increases. It’s caused by excessive clearance between the connecting rod bearing and the crankshaft journal.

In a properly functioning engine, a pressurized oil wedge keeps the connecting rod bearings gliding smoothly around the crankshaft. But when wear sets in—or when lubrication fails—that clearance increases, causing the connecting rod to hit the crankshaft, causing a knocking sound. That’s the origin of the dreaded rod knock sound.

Differentiating rod knock from other engine sounds

Rod knock is a distinctive metallic tapping or knocking that changes with RPM. It’s different than valve train ticking, also called valve clatter or tappet noise. Rod knock has a deeper and more rhythmic tone—it almost sounds like someone tapping a wrench on a steel pipe in sync with the engine’s combustion cycles.

Excessive valve-to-rocker clearances or clogged hydraulic lifters can cause valve clatter. If you detect valve clatter or valve tappet noise, and you have adjustable valve clearances, perform a valve adjustment before adding any chemicals.

However, if you don’t have adjustable valves, additives can sometimes help unclog hydraulic lifters, thereby quieting them.

Here are some examples of valve lifter/tappet noise

You may think they sound alike. If so, the key to differentiating whether you’re dealing with rod knock or valve clatter is to use an automotive stethoscope to listen to the engine to determine where the noise is coming from.

Touch the probe of your stethoscope to the valve cover. If you’re hearing tappet noise, it will be immediately obvious the instant you touch the valve cover. Rod knock, on the other hand, is heard near the bottom end of the engine

Common Causes of Rod Knock

So what causes rod knock in the first place? Here are the top culprits:

1) Low Oil Pressure – This is one of the most common causes. If oil level or oil pressure drops, the thin film separating the rod bearing and crankshaft disappears, leading to metal-on-metal contact. Check for a low oil level and refill if needed.
2) Oil Starvation – A clogged pickup tube, oil pump failure, or extreme sludging can prevent oil from reaching the bearings.
3) Worn Bearings – Going too long between oil changes can cause bearing wear, especially in high-mileage engines.
4) Engine Overheating — Overheating can distort engine components, including the crankshaft and connecting rods, creating uneven surfaces and excessive bearing clearance, leading to rod knock.
5) Over-revving – Spinning the engine beyond its redline can stretch rod bolts or deform bearing surfaces.
6) Detonation or Pre-Ignition – Repeated knock events can pound the bearings and widen the clearances.

Any of these can lead to excessive bearing clearance, resulting in rod knock.

Can You Fix or Quiet Rod Knock Without Tearing Down the Engine?

Here’s the straight talk: fixing rod knock properly requires opening up the engine. But in some mild cases, or in high-mileage engines where cost is a concern, temporary solutions using additives for rod knock might help reduce the noise—though they won’t cure the root problem.

Additives for Rod Knock: Do They Really Work?

There are several additives for rod knock on the market. They generally fall into two categories:

1) Viscosity Index Improvers (VII)/Modifiers – These thicken the oil to help maintain the lubricating film between the rod bearing and crankshaft. Lucas Oil Stabilizer is a popular VII additive/

2) High-Metal Content Additives – Some contain zinc, moly, or other extreme-pressure additives that improve boundary lubrication.

In my experience, products like Liqui Moly Motor Oil Saver, Lucas Heavy Duty Oil Stabilizer, and Rislone Engine Treatment can help quiet rod knock temporarily—especially in engines with minor bearing wear.

But let me be clear: these additives for rod knock are not a permanent fix. They may buy you time, reduce noise, or prevent catastrophic failure if used early enough, but once the bearings are worn, no additive can replace the lost metal. In other words, no additive can restore worn bearing clearances.

There’s also a downside to using a VII improver like Lucas. First, Lucas Oil Stabilizer isn’t a lubricant. It’s just a viscosity improver. If you add a quart of Lucas Oil Stabilizer, that means you’re adding one less quart of lubricating oil. That means your engine will have less lubrication and fewer anti-wear additives, and that oil will degrade faster. So, if you add a VII, you’ll have to change your oil more often.

Second, while VII additives can thicken the oil and increase oil film strength in the bearing clearances, thicker oil doesn’t flow as well, and that has consequences by itself. Oil’s primary functions are to lubricate, provide a cushion between metal parts, and dissipate heat. VII doesn’t lubricate. And, a thicker oil viscosity creates more heat due to higher internal friction. Therefore, it flows more slowly, reducing its ability to remove heat.  In reality, adding VII to correct rod knock, while quieting the noise, actually accelerates engine wear.

If you hear knocking and want to extend engine life a bit longer before doing major work, additives can help. But they’re a band-aid, not a cure.

Mechanical Fixes to Permanently Fix Rod Knock

If you’re serious about how to fix rod knock, mechanical repair is the only long-term solution. Here are your options:

Rod Bearing Replacement – This involves dropping the oil pan, removing the connecting rods, and installing new bearings. This is risky unless the crankshaft journals are still within spec.

Crankshaft Grinding – If the journals are worn or scored, the crankshaft must be removed and machined down to the next undersize, then fitted with matching oversized bearings.

Full Rebuild or Engine Swap – If multiple rod bearings are damaged or the crankshaft is beyond saving, a complete rebuild or replacement engine may be your best bet.

To fix the noise problem properly means addressing the internal clearances and ensuring proper oil flow and pressure throughout the rotating assembly.

When to Act

If you’re hearing the early signs of rod knock, don’t wait. The longer you run the engine in this condition, the greater the damage—and the higher the repair cost. In some cases, a bearing that could have been replaced for a few hundred dollars turns into a destroyed crankshaft or seized engine, costing thousands.

Yes, I Get It

I’m telling you that no additive will fix your noise problem. But I’m not stupid. I know you’ve got this noisy vehicle and you don’t have $5,000 burning a hole in your pocket for a new/used engine. So you want to try additives. I get it. I would too. I just don’t want you to think that you’re solving the root problem. You’re just buying time. So try additives. But don’t get delusional. Start saving money for a repair or a new vehicle.

©, 2025 Rick Muscoplat