2 Piece Fuel Pump Eccentric: The Complete Guide to Understanding, Installing, and Upgrading

If you own a classic Ford small-block engine, you need to know that the 2 piece fuel pump eccentric is a critical component that drives your mechanical fuel pump, and it comes with a known weakness that can leave you stranded. The two-piece design consists of an inner hub and an outer rotating ring, driven by a bent steel tab that fits into the camshaft. While this setup works fine for stock engines, the tab can shear under high-performance conditions, causing the fuel pump to stop working entirely. Upgrading to a one-piece eccentric or properly maintaining your two-piece unit is essential for reliability, especially if you're running a modified camshaft or higher RPMs. This guide covers everything you need to know about the 2 piece fuel pump eccentric, from how it works to installation tips and common failure points.

What Is a 2 Piece Fuel Pump Eccentric?

A fuel pump eccentric is a lobe-shaped component mounted on the front of the camshaft sprocket that actuates the mechanical fuel pump's rocker arm. As the camshaft rotates, the eccentric's high lobe pushes the pump arm, creating the pumping action that delivers fuel from the tank to the carburetor.

The two-piece design features an inner hub that bolts directly to the camshaft sprocket and an outer ring that rotates around the inner hub. This outer ring is what the fuel pump arm actually rides against. The key difference between a one-piece and two-piece eccentric is how they are driven. The two-piece unit uses a bent steel tab that fits into a hole in the camshaft, while the one-piece design uses a hardened steel dowel pin.

Ford originally used the one-piece eccentric on early small-block engines (up to around 1968) and then switched to the two-piece design on later models. The two-piece design was considered an improvement because the rotating outer ring reduces friction between the eccentric and the fuel pump arm, potentially extending the life of both components.

How the 2 Piece Eccentric Works

The two-piece eccentric is not a single solid unit. The outer ring is free to rotate around the inner hub, which is bolted to the camshaft sprocket. This allows the outer ring to spin independently, reducing wear on the fuel pump arm. However, the movement between the two pieces is not continuous—they only rotate relative to each other when the fuel pump arm is exerting pressure on the outer ring.

The inner hub has a bent sheet metal tab that extends from the back. This tab fits into a hole in the camshaft, providing the drive mechanism. When the camshaft turns, the tab pushes the eccentric around, which in turn moves the fuel pump arm.

One common misconception is that the two pieces are locked together and don't move. In reality, the outer ring has an inner lip that is captured by the inner hub, allowing it to rotate freely. When you install the eccentric, the outer ring should have some clearance to spin, but it should not be loose enough to wobble.

The Critical Weakness: Bent Steel Tab Failure

The biggest problem with the 2 piece fuel pump eccentric is the bent steel tab that drives it. This tab is made from stamped sheet metal, and it is the weakest link in the entire fuel pump drive system. Under normal driving conditions, it may last the life of the engine. But if you have a performance camshaft, higher spring pressures, or any camshaft harmonics, that tab can shear off.

When the tab breaks, the eccentric stops turning, and your mechanical fuel pump immediately stops delivering fuel. The engine will sputter and die, and you'll be stuck on the side of the road. This is a catastrophic failure that requires removing the timing cover and replacing the eccentric to fix.

The failure is more common on engines with:

  • High-lift camshafts that create more vibration
  • Non-OEM lobe centerlines that increase harmonic stress
  • Higher RPM operation (above 5,500 RPM)
  • Engines with significant overlap in the cam profile

One user reported that their two-piece eccentric failed even though it was torqued to Ford's specification with medium-strength thread locker. The eccentric spun on the bolt, and the car stopped dead. The replacement bolt was an ARP unit, which provided better clamping force.

1 Piece vs 2 Piece Eccentric: Which Is Better?

The debate between one-piece and two-piece eccentrics has been going on for years. Here's a clear breakdown of the differences:

One-Piece Eccentric:

  • Made from a single piece of hardened steel
  • Driven by a hardened steel dowel pin that goes into the camshaft
  • More durable and less prone to failure
  • Can be used on early Ford small-blocks (pre-1968) and later engines with a longer dowel pin
  • Requires a longer dowel pin for proper installation

Two-Piece Eccentric:

  • Has an inner hub and an outer rotating ring
  • Driven by a bent steel tab that fits into the camshaft
  • The tab can shear under high-performance conditions
  • The outer ring reduces friction on the fuel pump arm
  • Commonly found on later Ford small-blocks

For most performance applications, the one-piece eccentric is the better choice. It eliminates the failure-prone tab and uses a hardened steel pin that is much more reliable. The only downside is that the one-piece design does not have the rotating outer ring, so the fuel pump arm rides directly on the eccentric surface, which can cause more wear over time. However, with proper lubrication, this is rarely an issue.

How to Install a 2 Piece Fuel Pump Eccentric

Installing a two-piece fuel pump eccentric requires careful attention to alignment and clearance. Here's a step-by-step guide based on the COMP Cams installation manual and other resources.

Step 1: Prepare the Engine
Loosen all rocker arms so the camshaft can rotate freely. Clean all mounting surfaces thoroughly. The life of your timing chain and sprockets depends on having clean components.

Step 2: Install the Dowel Pin
If your engine originally used the two-piece eccentric with a short dowel pin, remove the old pin. Press the new dowel pin into place, making sure it is fully seated in the camshaft. Some older engines have a spacer on the camshaft—if yours does, remove and discard it. The new camshaft sprocket has the spacer machined in.

Step 3: Align the Engine
Rotate the crankshaft until the number one piston is at top dead center. The crankshaft key should be pointing straight up. Rotate the camshaft until the dowel pin is pointing straight down. This ensures proper timing alignment.

Step 4: Assemble the Timing Set
Select the appropriate keyway and timing mark on the crankshaft sprocket. Loop the chain over the camshaft sprocket, align the timing marks, and loop the chain around the crankshaft sprocket. Lift the assembly into place and make sure the sprockets are fully seated.

Step 5: Install the Eccentric
Install the eccentric onto the camshaft sprocket, making sure the dowel pins fully engage the proper holes in the eccentric. Install the proper bolt and washer (supplied) and thread it all the way in, but leave it slightly loose.

Step 6: Check Clearance
Check for clearance between the washer and the dowel pin. If the washer touches the pin, remove the pin and grind one end until the pin no longer protrudes through the eccentric, then re-install the dowel pin. This step is critical to prevent binding.

Step 7: Torque to Spec
Tighten the bolt to the manufacturer's torque specification. Use thread locking compound to prevent the bolt from loosening over time.

How to Convert from 2 Piece to 1 Piece Eccentric

If you want to eliminate the failure-prone tab on your two-piece eccentric, you can convert to a one-piece design. This is a common upgrade for performance engines.

To convert, you need:

  • A one-piece eccentric (available from Ford, COMP Cams, or aftermarket suppliers)
  • A longer dowel pin (approximately 3/8" longer than the stock pin)
  • The proper bolt and washer

The one-piece eccentric uses a hardened steel pin that goes through the camshaft sprocket and into the eccentric. The pin is longer than the stock dowel, so it extends through the sprocket and provides a solid drive for the eccentric.

Some users have reported success by making a custom "T-shaped" pin that fits into the eccentric and indexes into the camshaft sprocket. This requires a lathe to machine the pin, but it provides a reliable solution.

Symptoms of a Failing Fuel Pump Eccentric

A failing eccentric will show symptoms that are similar to a failing fuel pump. Here's what to watch for:

  • Inconsistent fuel pressure that varies with engine speed
  • Engine stalling under load or at high RPM
  • Hard starting after the engine has been running
  • Unusual noises from the fuel pump area (clicking or knocking)
  • Complete loss of fuel delivery (if the tab shears)

If you experience any of these symptoms, it's important to diagnose the issue quickly. A failed eccentric can leave you stranded, and driving with a partially failed unit can cause the fuel pump arm to wear unevenly, leading to additional damage.

How to Check the Eccentric for Wear

To inspect the eccentric, you'll need to remove the timing cover. This is a significant job, so it's usually done when you're already replacing the timing chain or doing other front-end work.

When you have the eccentric exposed, check for:

  • Cracks or deformation in the bent steel tab (if two-piece)
  • Wear on the outer surface where the fuel pump arm rides
  • Loose fit between the inner and outer pieces
  • Damage to the dowel pin hole in the camshaft sprocket

If the eccentric shows any signs of wear, replace it immediately. The cost of a new eccentric is minimal compared to the inconvenience of a breakdown.

Tips for Avoiding Eccentric Failure

Here are practical tips to keep your 2 piece fuel pump eccentric working reliably:

1. Use thread locking compound on the eccentric bolt. The bolt can vibrate loose over time, especially on performance engines. Use medium-strength thread locker (like Loctite 242) to keep it secure.

2. Torque the bolt to the correct specification. Over-tightening can strip the threads in the camshaft, while under-tightening can allow the eccentric to spin. Consult your engine's service manual for the correct torque value.

3. Inspect the tab during timing chain replacement. If you're replacing the timing chain, take a close look at the eccentric's drive tab. If it shows any signs of stress or deformation, replace the eccentric.

4. Consider upgrading to a one-piece eccentric. If you're building a high-performance engine or plan to rev past 5,500 RPM, the one-piece design is significantly more reliable.

5. Use a high-quality bolt. The stock bolt is often marginal. Using an ARP or other high-strength bolt can provide better clamping force and reduce the risk of the eccentric spinning on the camshaft.

Common Questions About 2 Piece Fuel Pump Eccentrics

Does the outer ring of a two-piece eccentric rotate?
Yes, the outer ring is designed to rotate around the inner hub. It reduces friction between the eccentric and the fuel pump arm. However, the rotation is not continuous—it only happens when the fuel pump arm applies pressure.

Can I use a two-piece eccentric on a high-performance engine?
You can, but it's risky. The bent steel tab is prone to shearing under high-RPM conditions and with aggressive camshaft profiles. Most engine builders recommend upgrading to a one-piece eccentric for performance applications.

How do I lubricate the two-piece eccentric?
The eccentric is lubricated by oil splash from the timing chain and the front of the engine. No special lubrication is required. The oil that gets between the two pieces helps reduce friction between the inner hub and outer ring.

Will a two-piece eccentric work with any fuel pump?
Yes, the fuel pump doesn't care whether you have a one-piece or two-piece eccentric. The pump arm rides on the outer surface of the eccentric, and the shape is the same regardless of the design. The only difference is the drive mechanism.

What is the torque spec for the eccentric bolt?
The torque spec varies by manufacturer, but it's typically in the range of 30-45 ft-lbs for small-block Ford engines. Always refer to the specific instructions that come with your timing set and eccentric.

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