Introduction: Why Custom Mounts Are Essential for an RB Swap

Swapping an RB series engine—whether an RB20, RB25, or RB26—into a Nissan chassis like the S13, S14, S15, or R32 is a proven way to boost horsepower and torque while gaining the legendary durability of the straight-six platform. However, the stock engine mounts designed for the original SR20, CA18, or RB engine were never intended for a cross-platform swap. Simply bolting an RB into a chassis designed for a different engine leads to misalignment, poor driveline angles, insufficient clearance for the oil pan or steering shaft, and vibrations that can damage components and ruin the driving experience.

Fabricating custom mounts solves these problems by providing a precise, secure, and vibration-dampening connection between the engine and chassis. Whether you are a weekend fabricator or a professional shop, understanding the design principles and fabrication techniques for RB swap mounts will save you time, money, and frustration. This guide covers everything from planning and material selection to welding, bushing choices, and chassis-specific considerations, ensuring your RB swap is reliable and performs as intended.

The Importance of Proper Engine Mounts in a Swap

Engine mounts are not just brackets that hold the engine in place—they control the engine’s position relative to the transmission, differential, and suspension. In an RB swap, the engine must be positioned so that:

  • The transmission output shaft aligns correctly with the driveshaft (pinion angle and centering).
  • The oil pan clears the steering rack, subframe, and crossmember.
  • The exhaust manifolds or turbochargers have sufficient clearance from chassis rails, brake lines, and wiring.
  • The engine angle (inclination) matches the original driveline geometry to prevent U-joint binding.
  • The weight is distributed evenly across the front axle to maintain handling balance.

Custom mounts allow you to achieve all these goals simultaneously. Off-the-shelf adapter mounts exist for some combinations, but they often compromise on clearance or engine positioning. Fabricating your own mounts gives you full control over placement, material strength, and vibration isolation.

Tools and Materials for Fabrication

Essential Tools

  • Angle grinder with 4.5-inch cutting and grinding discs—for cutting steel plate and cleaning weld joints.
  • MIG or TIG welder—MIG is faster for mild steel; TIG offers cleaner, stronger welds for aluminum or thin materials.
  • Drill press or heavy-duty hand drill with cobalt or titanium drill bits—for accurate bolt holes.
  • Measuring tools: digital calipers, tape measure, combination square, angle finder, straight edge.
  • Clamps: C-clamps, locking pliers, and welding magnets to position parts during fabrication.
  • Safety gear: welding helmet (auto-darkening), leather gloves, fireproof jacket, grinding shield.

Materials

  • Steel plate: 3/16-inch (4.8 mm) or 1/4-inch (6.35 mm) mild steel is common for most RB swap mounts. Hot-rolled steel is easier to weld and machine than cold-rolled.
  • Aluminum plate: 1/4-inch or 3/8-inch 6061 aluminum can be used for weight saving but requires TIG welding and more careful design.
  • Bushings: Polyurethane (70–80 shore hardness) for street use with some vibration transfer; rubber for maximum comfort; solid aluminum for race-only cars.
  • Bolts and hardware: Grade 8 or 10.9 zinc-plated fasteners. Never use stainless steel in high-stress mounting applications.
  • Flat stock and angle iron for reinforcing tabs or creating gussets.

Design Considerations Before You Cut Metal

Engine Position and Driveline Geometry

Start by mounting the engine and transmission as a mock assembly in the chassis. Use an engine hoist with a load leveler to adjust the position. Critical measurements include:

  • Fore-aft location: The distance from the firewall to the front accessory drive. Clearance for the radiator and fan also matters.
  • Vertical height: The oil pan must clear the subframe and steering rack. A common problem with RB swaps in S13/S14 is that the RB oil pan hits the steering rack; custom mounts can lift the engine slightly or require a modified pan.
  • Engine inclination: The RB engine should sit at roughly the same angle as the original engine (typically 10–12 degrees from vertical towards the passenger side). This ensures the transmission output shaft aligns with the differential pinion.
  • Side-to-side centering: The engine should be centered between the chassis rails to balance weight and avoid interference with the brake master cylinder or clutch lines.

Chassis-Specific Clearance Issues

  • S13 (240SX / Silvia): The steering shaft runs close to the exhaust manifold. Many RB swap mounts require notching the frame rail or using a relocated steering shaft kit.
  • S14/S15: Similar steering shaft clearance issues but with slightly different firewall contour. The oil pan clearance is similar to S13.
  • R32 GTS/GT-R: The RB engine was originally designed for the R32, so clearance is better, but mounts from an RB20 vs RB26 differ. Custom mounts may still be needed if using a different transmission (e.g., RB25 with a different bellhousing).
  • Z32 300ZX: The engine bay is narrow; RB swaps often require custom mounts to fit the longer straight-six within the V6 bay. Tight clearance for turbos and downpipes.

Bushing Selection

The bushing material affects NVH (noise, vibration, harshness) and engine movement under load. Rubber bushings (OEM-style) isolate vibration best but allow more engine deflection. Polyurethane reduces movement and improves throttle response yet transmits more vibration. Solid aluminum mounts (spherical or solid) are for track-only cars because they transmit everything and can cause chassis fatigue. Most street RB swaps perform well with 70-durometer polyurethane bushings.

Step-by-Step Fabrication Process

1. Measure and Mock-Up

With the engine hoist supporting the RB engine and transmission at the approximate position, measure the mounting points on the engine block (typically two long bolts per side into the block). Then measure the corresponding mounting areas on the chassis. For many Nissan chassis, the factory mounting locations are a stamped steel bracket that protrudes from the frame rail. You will need to create a bracket that connects from that chassis point to the engine block mounting holes.

Create a template using cardboard or thin sheet metal. Fit the template against the engine and chassis while the engine is still in the car. Mark locations for bolt holes and any bends. Transfer the design to steel plate.

2. Cut and Shape the Mount Plates

Using your template, trace the shape onto the steel plate. Cut with an angle grinder or plasma cutter. For complex shapes with tabs for bushings, cut the tab separately and weld it on later. Bevel the edges to ensure good weld penetration. Smooth all sharp edges with a grinding wheel to prevent stress risers.

3. Drill Mounting Holes

Drill the engine-side mounting holes first. Use a drill bit 1/16-inch larger than the bolt diameter to allow for adjustment. For the chassis-side, wait until the mount is positioned in the car to drill final holes precisely. If using existing chassis holes, match the bolt pattern exactly.

4. Weld the Mount Structure

Clamp the pieces together using welding magnets or a jig. Tack weld first, then check alignment with the engine in the car. If everything fits, complete the welds. For steel, MIG with ER70S-6 wire is adequate. For aluminum, TIG with 4043 filler rod. Ensure full penetration on all load-bearing joints. Add gussets or triangular braces to reinforce corners where the mount attaches to the chassis bracket.

5. Install Bushings

Polyurethane bushings typically consist of a metal sleeve and a urethane ring. Press the metal sleeve into the mount’s bushing hole (often 1/2-inch ID for M12 bolts). Then insert the bushing into the mount’s housing. Some mounts use a two-piece design with a bolt passing through the center of the polyurethane. Ensure the bushing is captured so it cannot slip out under load.

6. Test Fit and Adjust

Place the engine back into the chassis with the finished mounts. Bolt everything loosely and check for binding. Verify that the transmission tailshaft angle matches the differential pinion (within 1–2 degrees). Also check clearances for the steering shaft, brake booster, AC lines, heater hoses, and exhaust. Make adjustments by grinding or adding shims until everything fits perfectly.

7. Final Installation and Torque

Once satisfied, remove the engine, paint the mounts with anti-corrosion paint or powder coat, and reinstall for the final time. Torque all bolts to factory specifications (typically 40–60 ft-lbs for M10 or M12 engine bolts). Use thread locker (blue Loctite) on chassis bolts. After final installation, start the engine and let it idle. Check for unusual vibrations, exhaust leaks, and contact noises. Road test gently, then retorque bolts after 100 miles.

Chassis-Specific Notes for Common Nissan Platforms

S13 (1989–1994 240SX / Silvia)

  • The steering shaft clearance is the biggest challenge. Many builders use a steering shaft relocation kit or modify the factory shaft to a two-joint design.
  • The RB oil pan often contacts the steering rack. Options: use a modified RB26 pan, a custom mild-steel pan, or lift the engine with taller mounts.
  • Fabricate mounts using 1/4-inch steel plate. Attach to the frame rail where the original mount bracket was located. Many S13 owners also weld a reinforcement plate into the frame rail.

S14 / S15 (1995–2002)

  • The engine bay is slightly wider than S13, but the steering shaft still requires attention. Use an eccentric spacer or move the steering rack slightly.
  • The transmission mount must match the RB transmission (e.g., RB25 transmission has a different mount pattern than SR20). Often requires a custom crossmember.
  • Clearance for the RBs front accessory drive (alternator, A/C compressor) against the passenger side frame rail is tight. Plan mount placement accordingly.

R32, R33, R34 Skyline

  • Because these cars came with RB engines from the factory, swap mounts are simpler. However, if swapping a different RB variant (e.g., RB25 into an R32 GT-R), the mounts differ. The RB26 in R32 uses a unique mount that is not identical to RB25.
  • If using a GT-R crossmember versus GTS, the mount location changes. Custom mounts can correct the engine position for optimum weight distribution.
  • Consider using adjustable solid steel mounts for track use, but remember NVH tradeoffs.

Z32 300ZX

  • Very tight engine bay. The RB26 is long and pushes the oil filter close to the steering rack. Custom mounts are nearly mandatory to also allow room for turbo inlets.
  • Use aluminum mounts to reduce weight, but TIG welding aluminum requires high skill.
  • Some builders fabricate tubular subframe mounts that bolt to the frame rails instead of using the original V6 mount pockets.

Common Mistakes and How to Avoid Them

  • Ignoring driveline angles: Even if the engine bolts in, an incorrect angle will cause driveline vibration and premature U-joint wear. Always measure with an angle finder on the transmission tailshaft and differential pinion flange.
  • Weak weld joints: Hobbyist welds that lack penetration can crack. Practice on scrap, or use a professional welder. Grind off paint before welding.
  • Overtightening bushings: Polyurethane bushings require specific torque. Over-tightening deforms the bushing and reduces vibration isolation.
  • Not using a jig: Fabricating symmetrical mounts (left and right) without a jig leads to uneven engine positioning. Use a fixture or build one side, then mirror it.
  • Neglecting heat clearance: Turbo RB engines generate immense heat. Ensure mounts are at least 1/2 inch away from exhaust headers. Consider heat shielding or wrapping.

Safety Considerations

Fabricating engine mounts involves welding, cutting, and handling heavy components. Always wear appropriate PPE: welding helmet with correct shade (10–13), fire-resistant gloves, and a grinding shield. Work in a well-ventilated area to avoid welding fumes. Secure the vehicle on jack stands or a lift when working under it. When lifting the engine, use a load-rated hoist and never get under the engine without safety supports. After installation, double-check all bolts are properly torqued before starting the engine.

Conclusion

Custom engine mounts are the foundation of a successful RB swap. By designing and fabricating mounts that precisely position the engine while maintaining proper driveline geometry and clearance, you ensure reliability, performance, and a professional finish. The process demands patience, accurate measurement, and solid welding skills, but the results are worthwhile. Whether you are building a street-driven S13, a time-attack R32, or a track-focused Z32, investing time in mount fabrication pays dividends in driving enjoyment and component longevity.

For further reading and community insights, visit Zilvia.net forums or RB Motorsport for chassis-specific mount templates and advice.