How to Install Flowmaster American Thunder Cat-Back Exhaust - Split Side/Rear Exit (07-08 4.8L Sierra 1500) on your GMC Sierra

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. Spray a penetrating lubricant onto the bolt or nuts behind the catalytic converter as well as all of the rubber isolators supporting the system. 

2) All models: Support the muffler with a stand. Using a hacksaw or sawsall, cut the stock tailpipe off just behind the muffler. Separate the wire hangers on the tailpipe from the rubber mounts on the vehicle and remove the tailpipe from the vehicle. This step is not mandatory but makes removal easier. 

3) 2007-08 Models: Using a 15mm wrench, loosen the two nuts on the flanged connection behind the catalytic converter. Retain the nuts, they will be re-used. Separate the hanger on the pipe from the rubber mount on the vehicle and lower the inlet pipe and muffler from the vehicle. 

4) 2009-13 Models: Using a 15mm wrench, loosen the bolt on the clamped ball connection behind the catalytic converter. Separate the wire hangers from the rubber mounts on the vehicle and lower the inlet pipe and muffler from the vehicle. You will need to remove the ball clamp from the stock system; it will be reused with the Flowmaster system.

1) 2007-08 Models: Place flanged adapter pipe assembly #738TBS into position behind the catalytic converter. Reinstall the two nuts and tighten enough to hold, but still allow for adjustment. Place a clamp onto the front of inlet pipe #26411S and place onto the back of the adapter pipe. Then place clamp hanger #473HA over the back of the pipe and connect to the rubber mount on the vehicle. Support the rear of the pipe with a stand and tighten the clamps enough to hold. The welded hanger on the pipe will not be utilized.

2) 2009-13 Models: Place adapter assembly #739TBS onto the ball connection behind the catalytic converter. Reinstall the clamp onto the ball connection and tighten enough to hold, but still allow adjustment. Place a clamp onto the front of inlet pipe #26411S, place the inlet pipe onto the back of the adapter and connect the hanger on the pipe to the rubber mount on the vehicle. Support the rear of the pipe with a stand and tighten the clamp enough to hold. The hanger on the pipe will not be utilized on 8’ bed models. 

3) If installing the system on a vehicle equipped with an 8.0’ bed, place a clamp onto the expanded end of extension pipe #730TBS and place onto the back of the #26411S inlet pipe. Connect the hanger on the pipe to the rubber mount on the vehicle. This extension pipe will not be used 5.5’ or 6.5’ bed models. 

4) Place a supplied 3” clamp over the inlet of the muffler, then slide the inlet of muffler onto the back of the inlet pipe. Use a stand to support the muffler. Tighten the clamp just enough to hold in position.

5) Slide rubber hanger #HA168 onto the end of 226HA, then using the supplied bolts, washers, nuts and backing plates #HA566, mount the assembly to the frame on the left side of the vehicle using the angled oblong hole on the frame rail. Note: the backing plates #HA566 go on both sides of the frame between the hanger, washers & bolts to cover the oblong holes so that the frame hanger won’t bend into the oblong holes. 

6) Place two of the supplied 2-1/2” clamps over the right and left outlets of the muffler. Place right tailpipe #86095S into position over the rear axle, and slide into the right muffler outlet. Connect the two hangers on the pipe to the rubber mounts on the vehicle. Pipe should be approximately 3/4” away from the shock when adjusted. 

7) Place left front tailpipe section #86094S into position over the rear axle and slide into the left muffler outlet. Place a supplied 2-1/2” clamp onto the back of the pipe. Place left rear tailpipe section #86096S onto the front section, and connect the hanger welded to the rear of the pipe into the rubber hanger installed in step 3 above. Pipe should be centered between the shock and the spare tire when adjusted. Tighten the clamps just enough to hold in position.

8) Slide the side exit pipes #86097S or the rear exit pipes #86193S onto the ends of both over axle pipes and place a supplied 2-1/2” clamp onto these slip-fit connections. Tighten just enough to hold in position. Place the two stainless tips #ST461 onto the exit pipes and tighten them enough to hold position. Rotate the exit pipes and tips to the desired distance from the body or bumper, so that the angle cut on the tips are in the desired location. 

9) Adjust the position of all pipes and muffler to provide a satisfactory fit. A minimum 3/4” clearance around all parts of the system must be maintained; while also keeping suspension travel and vibration in mind. 

10) Tighten all clamped connections down securely. Place the supplied 1/2” hanger keepers onto the end of the inlet pipe and tailpipe hangers. Slide the 7/16” hanger keeper onto the left rear frame hanger up to the rubber mounts, to firmly hold the system in place. 

11) To prevent the rear axle breather tube from contacting the left side tailpipe, and possibly being damaged, use the enclosed zip tie to fasten the breather tube to the brake line bracket located on the rear end. Be sure to allow hose enough slack for the suspension to fully compress and extend. 

12) For a more secure installation, we highly recommend welding all slip-fit connections.