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Flowmaster American Thunder Catback Exhaust System ('99-'04 GT/Mach 1/Bullitt) - Installation Instru

Installation Time

2 hours

American Muscle
Shop Parts in this Guide



1. Raise the vehicle up on a hoist or rack to working height. If you don't have access to a hoist or rack, raise the vehicle and support securely with jack stands.

2. Using a hacksaw or sawsall, cut the stock tailpipes off approximately 2" behind the muffler case.

3. Separate the steel tailpipe hangers from the rubber mounts at the front and rear of each tailpipe. A lubricant will usually ease removal. Then remove the stock tailpipes from the vehicle. It is usually necessary to raise the vehicle at the jackpoints to allow for removal and replacement of the tailpipes.

4. Support the stock mufflers with stands.

5. Separate the steel muffler hangers from the rubber mounts at the rear of each muffler. A lubricant will usually ease removal. Then remove the two nuts at the muffler to catalytic converter connections and remove the stock mufflers from the vehicle.


1. Before installing anything, prep the clamps supplied in the hardware kit by removing the nuts, and applying a thick lubricant such as white grease or anti-seize to the threads.

2. Position muffler into the stock location on the right side of the vehicle and connect the hanger rod at the rear of the muffler assembly to the rubber mount on the vehicle. Slide the flange on the pipe up to the flange on the head pipe and reinstall the stock onto the studs.
Note:Loosening studs may make installation easier.

3. Repeat step 2 on the left side using muffler

4. Slip the right side tailpipe over the rear axle and into the right muffler outlet. Then connect the hanger rods on the tailpipe to their stock rubber mounts on the vehicle. Install a provided 2 1/2" clamp and tighten enough to hold in place but still allow for adjustment.

5. Repeat step 3 on the leftside of the vehicle using tailpipe.

6. Adjust tailpipes and mufflers for proper alignment, tighten all connections. Maintain a minimum clearance of 1/2" between the drivetrain and chassis components. After all slip fit connections are secure, slide the provided 7/16" hanger keepers onto the ends of the hangers located at the rear of the mufflers and tailpipes. Slide them up to the rubber mounts. This will hold the system securely in place.

7. For a cleaner appearance and more secure installation, we highly recommend welding all slip-fit connections. If you live in a geographical area that has harsh winters or sees a great deal of precipitation, the use of high temperature paint over the welded areas can help to prevent surface rust and premature corrosion.

Installation instructions provided by Flowmaster

This guide applies to Vehicle years: 1999, 2000, 2001, 2002, 2003, 2004 and Submodels: Bullitt, Mach 1, GT

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