Big-End vs Small-End Connecting Rod Weight Explained (With Clear Examples)

When you first start learning how to balance connecting rods, you’ll quickly encounter two important measurements: big-end weight and small-end weight. These aren’t random values — they determine how forces are distributed through the crankshaft, bearings, and piston assemblies.

Most beginners only look at total rod weight, but proper balancing requires isolating each end. This article breaks down exactly what each measurement represents, why the split matters, and how to measure both accurately at home.

If you want the full step-by-step workflow, check out the Complete Connecting Rod Balancing Guide.

What Is Big-End Weight?

The big end of the rod is the crankshaft side — the portion that rotates around the crank journal. Its weight directly affects:

• Rotational balance

• Main bearing load

• High-RPM stability

• Crankshaft counterweight effectiveness

Because it rotates 360° with every crankshaft revolution, even a 1-gram difference between rods can introduce measurable vibration at high RPM.

Where material can be safely removed

Only remove material from:

• The exterior of the connecting-rod bolt bores, evenly

Avoid:

• The beam

• The bearing tang areas

• The big-end bore surface

Removing material from structural zones can weaken the rod.

What Is Small-End Weight?

The small end is the wrist-pin/piston side — the reciprocating mass that moves up and down with the piston. Small-end weight affects:

• Vertical reciprocating forces

• Vibration at TDC and BDC

• High-RPM harmonics

Imbalance here may not cause the same rotating vibration as the big end, but it can create harshness and contribute to uneven loading.

Where material can be safely removed

Only remove material from:

• The exterior of the wrist-pin bore, evenly around the outer surface

Avoid:

• The wrist-pin bore surface

• The wrist-pin bushing

• The beam

These are structural and cannot be safely thinned.

Why Total Weight Alone Doesn’t Tell the Full Story

Two rods can have the same total weight but be completely different internally.

Example:

• Rod A → 382 g big end / 155 g small end

• Rod B → 379 g big end / 158 g small end

Total: Both 537 g
Balance: Entirely different forces on the crank and piston

This is why proper rod balancing always separates big-end and small-end measurements.

How to Measure Big-End and Small-End Weights Accurately

To isolate one side of the rod, you need to suspend the opposite end at a consistent height — that’s exactly what a balancing fixture is for.

A metal fixture typically costs $300+, but DIY builders can achieve identical results using a digital scale and an accurate support jig. If you want an accessible option used by many hobbyist engine builders, check out the 3D-Printed Connecting Rod Balancing Jig.

What you need

• A digital scale (0.1 g or 0.01 g resolution)

• A rod balancing jig

• A flat surface

Steps

1. Support the small end → place the big end on the scale → record weight

2. Support the big end → place the small end on the scale → record weight

3. Measure total weight for consistency

4. Repeat for every rod

This gives you a consistent profile for each rod in the set.

Ideal Target Tolerances

General guidelines for rod balance across different build types:

Most factory rod sets are nowhere near these numbers, meaning there’s significant room for improvement.

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Where to Remove Material (Safe Zones Only)

Big End

✔ The exterior of the connecting-rod bolt bores, evenly
❌ Never grind the beam
❌ Never grind the tang areas
❌ Never touch the big-end bore

Small End

✔ The exterior of the wrist-pin bore, evenly
❌ Never touch the bore surface
❌ Never touch the bushing
❌ Never thin the beam

Even removal is crucial — you want to preserve the rod’s geometric balance.

Final Thoughts

Understanding big-end vs small-end weight is the foundation of proper connecting rod balancing. With a good fixture and a consistent workflow, you can achieve machine-shop-level results at home.

If you want the simplest and most accurate way to measure rod end weights on a digital scale, take a look at the 3D-Printed Rod Balancer Jig — designed specifically for DIY engine builders who want professional results without the $300 price tag.