Balancing your Nashville NA engine is one of the most effective ways to ensure smooth power delivery, reduce vibration, and extend the life of critical components. Whether you are rebuilding a classic Nash engine or maintaining a vintage inline-six, proper balancing transforms how the engine feels under load. An unbalanced engine not only robs performance but also accelerates wear on bearings, seals, and the crankshaft itself. This comprehensive guide walks through the theory, tools, and step-by-step process to achieve a well-balanced Nashville NA engine, with practical tips for both DIY enthusiasts and professional mechanics.

Understanding Engine Balance: Why It Matters for Your Nashville NA

Engine balance refers to the cancellation of inertial forces generated by reciprocating and rotating parts inside the engine. In a typical inline-six engine like the Nash Ambassador's 1950s-era "Nash NA," the firing order and crankshaft geometry naturally balance primary and secondary forces to a degree, but manufacturing tolerances, wear, or modifications can introduce imbalances. When the crankshaft, pistons, connecting rods, and flywheel are not perfectly matched in mass distribution, vibrations travel through the engine block, mounts, and chassis. Over time, these vibrations cause fatigue fractures, oil leaks, and premature bearing failure. Balancing ensures that all reciprocating and rotating components move in harmony, reducing stress on the engine and drivetrain. For a Nashville NA engine built for reliability and smooth torque, balancing is not optional—it's essential for delivering the buttery-smooth power for which these engines are known.

Types of Imbalance in the Nashville NA

Static vs. Dynamic Imbalance

Static imbalance occurs when the center of mass of a rotating part (like the crankshaft or flywheel) is not aligned with its axis of rotation. This causes a wobble that worsens as RPM increases. Dynamic imbalance involves two or more imbalances in different planes, creating a twisting force that shakes the engine front to back. A Nashville NA engine, with its long crankshaft and heavy flywheel, is especially susceptible to dynamic imbalance if the clutch assembly or harmonic damper is not matched.

Reciprocating vs. Rotating Mass

The reciprocating mass (pistons, wrist pins, rings) moves up and down, while the rotating mass (crankshaft, connecting rod big ends, flywheel) spins. Both must be balanced relative to each other. Many older engines were balanced at the factory for a specific tolerance, but after rebuilds with new pistons or reground crankshafts, that balance is often lost. The Nashville NA is a non-counterweighted crankshaft design original, so aftermarket counterweights or harmonic balancers may need careful matching.

Tools and Equipment for Balancing Your Nashville NA

Before starting, gather the following items. Quality tools make a measurable difference in accuracy.

  • Engine balancing kit – includes a balancing arbor, knife-edge ways, and assorted washers/weights for correction.
  • Dial indicator with magnetic base – for measuring crankshaft runout and piston deck heights.
  • Digital scale with 0.1-gram resolution – for weighing pistons, rods, bearings, and fasteners.
  • Torque wrench (ft-lb and in-lb) – critical for consistent rod bolt and main cap torque.
  • Crankshaft balancing jig or v-blocks – to support the crankshaft for static balancing.
  • Assortment of drill bits, grinding stones, and tungsten putty – for adding or removing weight from crankshaft counterweights, connecting rods, or flywheel.
  • Ring file and piston ring compressor – if new rings are installed during balancing.
  • Safety glasses and nitrile gloves – metal dust and solvents require protection.

Step 1: Disassemble and Clean All Components

Begin by removing the engine from the vehicle, draining oil and coolant, and separating the transmission and flywheel. Completely disassemble the short block: remove the cylinder head, oil pan, pistons, rods, crankshaft, and main bearings. Clean every part thoroughly with solvent and a parts washer. Pay special attention to oil passages in the crankshaft and connecting rods. Any residual sludge or debris will affect weight measurements. Lay out all parts in labeled bins—mark each piston, rod, and bearing cap so they can be reassembled in the original orientation if you choose to maintain original balance for a baseline.

Step 2: Weigh and Match Reciprocating Components

The first balancing step is to equalize the weight of pistons, wrist pins, and rings within each cylinder set. Weigh each piston assembly (piston, pin, rings) on the digital scale. For a Nash NA engine, aim for weight variance of no more than 0.5 grams across all six cylinders. Heavier pistons can be lightened by removing material from the inside of the piston skirts (not the crown). Similarly, weigh each connecting rod big end and small end separately by balancing the rod horizontally on a rod balancer or knife-edge. Record the weights. Then, match the lightest rod to the lightest piston, etc. If any assemblies are more than 1 gram off, either swap components or machine excess material from the heavier part.

Step 3: Measure Crankshaft Runout and Journal Sizes

Mount the crankshaft in v-blocks or between centers on a lathe. Use a dial indicator to measure runout at the front and rear main journals. Total indicated runout should be less than 0.002 inch for a stock Nash NA. If runout exceeds 0.003 inch, the crankshaft should be straightened or ground. Also measure the diameter of each main and rod journal using a micrometer; note any taper or out-of-round condition. Out-of-spec journals will cause uneven rotating mass and can introduce imbalance even if weights are perfect.

Step 4: Static Balancing the Crankshaft and Flywheel

Place the crankshaft on a balancing arbor or knife-edge ways. Allow it to settle—the heavy side will rotate to the bottom. Mark the heavy spot. To correct static imbalance, drill shallow holes into the counterweight on the heavy side, or add weight (e.g., Mallory metal) to the light side. Remove material slowly and re-check frequently. A general rule: remove no more than 5 grams at a time before re-checking. When the crankshaft comes to rest at random positions, it is statically balanced. Repeat the same process for the flywheel/flexplate if not already balanced. Many aftermarket flywheels for the Nash NA are pre-balanced, but stock flywheels often require correction.

Step 5: Dynamic Balancing the Rotating Assembly

Dynamic balancing requires a specialized balancing machine that spins the assembly at several hundred RPM to measure imbalance in multiple planes. For a Nashville NA, the rotating assembly includes the crankshaft, flywheel, harmonic damper, and clutch assembly. If you do not have access to a dynamic balancer, many engine shops offer this service for around $100–$200. The machine will tell you exactly how much weight to add or remove at specific angular locations on the counterweights. Follow the machine’s instructions precisely. A well-done dynamic balance will reduce vibration to near-zero, especially at cruising RPMs of 2000–3500 where these engines spend most of their life.

Step 6: Balance the Connecting Rods (Reciprocating + Rotating)

Each connecting rod must be balanced for both the reciprocating (small end) and rotating (big end) portions. Use a rod balancer that separates the two. Weigh the big end and small end of each rod. The big end weights should be within 1 gram of each other; the small ends within 0.5 gram. If any rod is heavy, you can remove material from the balance pad on the rod (usually the large boss near the big end). Be cautious not to weaken the rod. Once all rods are matched, assemble the rods and pistons onto the crankshaft with new bearings and check that the rotating assembly spins freely without binding. Torque the rod bolts to spec using a torque plate if possible.

Common Mistakes to Avoid When Balancing a Nash NA

  • Forgot to include the harmonic damper weight – The Nash NA damper can add up to 5 lb of mass that must be balanced as a unit with the crankshaft and flywheel.
  • Ignoring piston pin weight variations – Even quality aftermarket wrist pins can vary 2–3 grams; always weight and sort them.
  • Mixing rod big-end and small-end measurements – Never assume all rods are identical; each rod is unique.
  • Using worn bearings during balancing – Bearing wear introduces radial play that skews weight distribution; always balance with new bearings of the correct clearance.
  • Skipping the final assembly check – After balancing, rotate the assembly and feel for any tight spots or scraping that indicates an undetected issue.

Benefits of a Properly Balanced Nashville NA Engine

A balanced engine delivers several tangible benefits beyond reduced vibration. Fuel efficiency improves because the engine does not waste energy fighting inertial forces. Throttle response becomes crisper, and the engine pulls more smoothly through the RPM range. Bearings last two to three times longer, and valve train wear decreases. For a classic Nash Ambassdor or Statesman, a balanced engine transforms the driving experience from a rattly tractor to a refined touring machine. On a chassis dyno, a balanced engine often shows a 3–5 horsepower gain at the wheels due to reduced parasitic losses.

When to Seek Professional Balancing Services

While static balancing can be done at home with careful measurements and a digital scale, dynamic balancing requires a machine that spins the assembly at high speed. If you do not have access to a dyne, consider sending your rotating assembly to a reputable shop that specializes in vintage inline-six engines. Some shops even offer a “balance and blueprint” service for the Nash NA, which includes matching piston weights, balancing the crank, and aligning the flywheel. The cost is typically $300–$600, which is a small price compared to rebuilding an engine damaged by vibration. Also, if your engine has been bored oversize or uses custom pistons, professional balancing is strongly recommended.

Maintenance Tips for Long-Term Engine Balance

After installing a balanced engine, follow these practices to preserve the balance over the engine’s life:

  • Use a high-quality harmonic balancer (damper) that matches the Nash NA’s original frequency; a loose or slipping damper introduces imbalance.
  • Keep the engine mounts in good condition; worn mounts amplify vibrations even from a balanced engine.
  • Re-torque the flywheel and pressure plate bolts after the first heat cycle.
  • If you ever replace a piston or connecting rod, rebalance the affected cylinder assembly.
  • Avoid drastic changes to the rotating mass, such as a lightweight flywheel, without rebalancing the entire assembly.

Conclusion

Balancing your Nashville NA engine is a rewarding process that yields smoother power delivery, longer engine life, and a more enjoyable driving experience. By following the steps outlined here—from weighing and matching components to performing static and dynamic balancing—you can achieve a level of refinement that matches or exceeds the factory specifications. Whether you undertake the work yourself or enlist a professional, the investment in time and tools pays back in every mile. A balanced Nash engine is a testament to the engineering of an era, delivering power with the grace of a much more modern powertrain.

For further reading, consult the Nash Car Club engine balancing guide or the detailed discussion on the AACA Nash forum about balancing inline-six crankshafts. A professional balancing service like BalanceTech offers specialized support for Nash NA engines.