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How to install a Flowmaster Force II Catback Exhaust on your 1999-2004 GT Mustang

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 do not 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. Remove the stock tailpipes from the vehicle. It is usually necessary to raise the vehicle at the jack points to allow for removal and replacement of the tailpipes.

4. Support the stock mufflers with stands, then separate the steel muffler hangers from the rubber mounts at the rear of each muffler. A lubricant will usually ease removal. 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 #52558-129 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 nuts onto the studs. Repeat for the left side using muffler #52558-130.

3. Slip the right side tailpipe #16251 over the rear axle and into the muffler outlet. 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. Repeat on the left side of the vehicle using tailpipe #16250.

4. Adjust the position of all muffler, pipes and tips to provide a satisfactory fit. Maintain a minimum of 1/2" clearance between the Flowmaster system, the drivetrain and chassis components, while also keeping suspension travel and vibration in mind.

5. Tighten down all clamped connections securely. After the system is secure, slide the 7/16” hanger keepers onto the ends of the hangers located at the rear of the mufflers and tailpipes. This will prevent the hangers from slipping out of the rubber mounts.

6. For a cleaner appearance and more secure installation, we highly recommend welding all slipfit 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.


QtyDescriptionPart #
1Right Muffler52558-129
1Left Muffler52558-130
1Tailpipe, Right16251
1Tailpipe, Left16250
1Parts KitPK342
22 1/2" Saddle ClampsMC250S
47/16" Hanger Keepers

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