Flowmaster American Thunder User Manual

409S
®
Stainless Steel
Removal:
1) Raise the rear of the vehicle and support securely with jack stands.
2) Remove the existing system from the header collector or manifold down pipes back.
Installation:
1) Locate the center carrier bearing cross member; this is where the front hangers will be mounted.
Measuring in 9-5/8” from the frame toward the center of the cross member there should be a factory hole.
Using a drill and bit, open the hole up to 3/8”. Then measure in 1.00” toward the center of the vehicle and make
a mark. This is where you will drill a second 3/8” hole. Mount one of the HA572 rubber hangers to the bottom of
the cross member using the 3/8” hardware provided, do this for both sides of the cross member. See photos
below.
2) Take the right and left H-pipe assemblies, 86074S & 86075S and connect them together at the center
cross-over connection. Place them up into position and connect the hangers on the pipe to the rubber mounts
installed above. The hanger rods should stick through the rubber block on the HA572 hanger approximately
1.00”. Support the front of the assembly with a stand. Place the provided 2” clamp at the center H-pipe
connection.
3) Locate the rear spring mount cross member, measure in 11 & 12 inches from the edge of the spring
mount and make a mark. This will be on the forward channel of the cross member. Once you have verified the
measurements are correct, center punch the two holes and drill a 3/8” hole in these locations. Mount one of the
HA572 rubber hangers to the bottom of the cross member using the 3/8” hardware provided. Do this step on
both sides of the cross member. See photos below.
Installation Instructions for:
SYSTEM #817654
1967-1972 GM C10 PICKUPS
2 ½” HEADER BACK SYSTEM

Flowmaster American Thunder Specifications

General

Muffler Included	Yes
Muffler Style	Chambered
Tip Finish	Polished
Sound Level	Aggressive
Muffler Quantity	1
Tip Included	Yes
Pipe Diameter	2.5 inches
Warranty	Limited Lifetime
Tip Style	Round, Angle Cut, or Dual Outlet (depending on the specific kit)

Summary

Vehicle Preparation and Exhaust Removal

Raise and Secure Vehicle

Safely lift and support the vehicle using jack stands before proceeding with removal.

Remove Existing Exhaust System

Detach the old exhaust system from the header collector or manifold down pipes backward.

Exhaust System Installation

Mounting Front Cross Member Hangers

Install HA572 rubber hangers on the center carrier bearing cross member for front pipe support.

Connecting H-Pipe and Front Hangers

Join H-pipe assemblies and connect their hangers to the rubber mounts installed earlier.

Mounting Rear Cross Member Hangers

Attach HA572 rubber hangers to the rear spring mount cross member at specified locations.

Muffler and Pipe Assembly

Connect muffler inlet pipes, mufflers, and over-axle tailpipes, ensuring proper alignment.

Rear Tailpipe Hanger Installation

Mount rear tailpipe hangers on frame rails for both short and long bed truck configurations.

Installing Exit Pipes and Final Adjustments

Fit exit pipes, adapt header connections, remove supports, and secure the entire system.

Exhaust System Installation Diagram