Replacing a front subframe can seem daunting, but with the right tools, preparation, and guidance, it’s achievable in a home garage. This guide provides a step-by-step walkthrough for replacing the front subframe on a Mazda Miata (NA/NB), outlining the necessary tools, parts, and procedures for a successful replacement.
Preparing for the Replacement
Before starting, gather the necessary tools and parts. You’ll need:
- Essential Tools:
- Hydraulic jack and four jack stands
- Engine hoist or engine support bar and chains
- Socket set (12mm, 14mm, 17mm, 19mm, 21mm) with extensions
- Wrenches (17mm, 21mm, or adjustable)
- Breaker bar or cheater pipe
- Torque wrench
- Hammer
- Long pry bar
- A helper
- Optional Tools:
- Ball joint/tie-rod end separator (pickle fork)
- Pliers
- 17mm line wrench
- Parts:
- New front subframe (NA and NB subframes are different)
- Optional: New steering rack, steering rack bushings, tie rod ends, cammed caster/camber bolts, nuts, washers, upper A-arm bolts, nuts, and washers. Consider replacing these parts if they show signs of wear or damage.
Step-by-Step Replacement Procedure
1. Safely Lift and Secure the Vehicle:
- Park on a level surface and securely lift the Miata using a hydraulic jack and jack stands. Place the jack stands under the reinforced areas behind the front wheels and in front of the rear wheels. Ensure the car is stable and at a comfortable working height (18-20 inches off the ground). Remove the front wheels.
2. Disconnect the Steering Rack:
- If replacing the tie-rod ends or the entire rack, remove them from the steering knuckles. Mark the position of the upper U-joint on the steering shaft before disconnecting it. This ensures proper alignment during reinstallation. For power steering, disconnect the pressure and return lines. For manual steering, simply unbolt the rack.
3. Detach the Control Arms:
- Loosen and remove the upper control arm bolt. You may need to loosen the rear sway bar mount bolt for clearance. Detach the lower control arms by removing the cammed adjustment bolts. Note the position of the cams for reinstallation. Be mindful of the brake lines to avoid damage.
4. Disconnect the Engine Mounts:
- Support the engine with an engine hoist or support bar. Remove the nuts securing the engine mounts to the subframe. Slightly lift the engine to clear the mount bolts from the subframe.
5. Remove the Old Subframe:
- Unbolt the subframe from the car’s frame. Each side is secured with two 19mm nuts under the upper arm mounts and two 17mm bolts at the rear. Remove the rearward nut and front bolt first, then the remaining fasteners. The subframe should now be free to remove.
6. Prepare and Install the New Subframe:
- Transfer any necessary components from the old subframe to the new one, such as a subframe brace or power steering hose mount. Position the new subframe under the car and loosely install the front nuts and rear bolts.
7. Reinstall Engine Mounts and Control Arms:
- Lower the engine, guiding the mounts into the subframe holes. Tighten the engine mount nuts. Reattach the upper and lower control arms, torquing the bolts to specification after the suspension is loaded.
8. Reconnect the Steering Rack:
- Secure the steering rack to the new subframe. Reconnect any disconnected lines and ensure the steering shaft is properly aligned and secured to the U-joint.
9. Final Steps:
- Reinstall the tie-rod ends if removed. For power steering systems, refill and bleed the system. Reinstall the front wheels and lower the vehicle. Get a wheel alignment as soon as possible.
Torque Specifications (1999 Miata)
Refer to your service manual for specific torque values for your Miata’s year. Below are torque specifications for a 1999 Miata as a general guideline:
- Sway bar mount to frame: 13-20 ft/lbs
- Steering u-joint keeper bolt: 13-20 ft/lbs
- Tie-Rod castellated nut: 32-41 ft/lbs (turn to hole)
- Motor mount nuts: 42-58 ft/lbs
- Steering rack bolts (NB): 55-77 ft/lbs
- Lower control arm cam bolts: 61-76 ft/lbs
- Subframe bolts (NB): 69-86 ft/lbs
- Upper control arm bolt: 87-101 ft/lbs
- Subframe nuts (NB): 87-101 ft/lbs
Always consult your service manual for the most accurate torque specifications. Following these steps and taking necessary precautions will ensure a successful Front Subframe Replacement on your Mazda Miata.
