How to Install Headers on a 2018–2023 Mustang GT

Allow the exhaust to cool for at least 2 hours if the car has been driven. Lift the car on a 4-post lift or secure on jack stands — never work under a car on a floor jack alone. Disconnect the negative battery terminal before touching any O2 sensor wiring. Lay out all tools and have penetrating oil (PB Blaster) within reach.

Working from underneath, unbolt the 4 mid-pipe flange bolts connecting it to the factory headers. Support the mid-pipe with a jack stand before breaking the last bolt. Unplug both upstream and downstream O2 sensor connectors — the upstream sensors will be removed with the headers, the downstream sensors stay with the mid-pipe. Label each O2 sensor connector with tape so you know which is bank 1 vs bank 2.

On the driver's side, the starter motor blocks header removal. Remove the 2 starter mounting bolts (13mm) and carefully lower the starter — leave the wiring harness connected but folded out of the way. This opens up the clearance needed to angle out the primary tubes on the driver's side long tube header.

The lower steering shaft coupler runs directly above the driver's side exhaust manifold. Remove the pinch bolt (10mm), slide the shaft up and out of the way. This is often overlooked and is the reason many first-time installs stall out. Don't skip this — it's a 5-minute job that makes the whole driver's side header removal possible.

Using a 13mm deep socket and a breaker bar or impact gun, break loose all header-to-head bolts (8 bolts per side). Work from the outside in. Once all bolts are removed, wiggle the header down and out, rotating as needed to clear the steering rack and subframe. The passenger side header comes out easier — the driver's side takes more patience. Keep a rag over the open exhaust ports immediately to prevent debris from entering the cylinder head.

Use a plastic scraper and brake cleaner to clean the exhaust port mating surfaces on the cylinder head. Inspect for cracks or damage. Install the new header gaskets (included with most kits) — these should be metal or metal-composite, not paper. Never reuse old gaskets. Install the O2 sensor bungs in the headers now if they're separate; most aftermarket headers include these pre-welded.

Feed each header into position, reversing the removal process. Start all header bolts by hand before torquing any — this ensures they thread cleanly. Torque in a star/cross pattern to the manufacturer's spec (typically 25–30 ft-lbs for 3/8" studs, 18–22 ft-lbs for M8 bolts). Reinstall the steering shaft coupler and starter motor. Plug in the upstream O2 sensors — don't overtighten, typically 26–30 ft-lbs.

Bolt up your new 3-inch aftermarket mid-pipe to the header collectors. Install a new mid-pipe gasket — never reuse the old one. Torque the flange bolts to 25–30 ft-lbs. Reconnect the negative battery terminal. Start the engine and let it idle — check for exhaust leaks at every gasket and flange joint with your hand (carefully). You will likely get CEL codes for the O2 sensors immediately; this is normal until the tune is applied. Take the car to your tuner within the next 50 miles.