Flowmaster American Thunder Installation Instructions

409s stainless steel

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409S

Stainless Steel

Removal:

Raise the rear of the vehicle and support securely with jack stands.

Remove the existing system from the header collector or manifold down pipes back.

Installation:

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.

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.

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

Page 1 Installation Instructions for: SYSTEM #817654 1967-1972 GM C10 PICKUPS 2 ½” HEADER BACK SYSTEM 409S Stainless Steel ® Removal: Raise the rear of the vehicle and support securely with jack stands. Remove the existing system from the header collector or manifold down pipes back. Installation: Locate the center carrier bearing cross member;...
Page 2 Take the 86076S muffler inlet pipes and place a supplied 2½” clamp onto the expanded end, place them onto the back of the H-pipe assembly’s 86074S & 86075S. Rotate them up into their approximate position and tighten the clamps enough to hold, but still allow for adjustment. Take one of the 842548-Z mufflers and place a supplied clamp onto the inlet, slide the inlet onto the back of one of the 86076S inlet pipes with the outlet of the muffler in the center of the vehicle.
Page 3 It will be necessary to adapt from your existing headers or manifold downpipes to the Flowmaster H- pipe assembly. Due to the many different variables in header/manifold down pipe locations, we have included 2 #86084S adapter pipes to aid in the fabrication of the connection between these two areas.
Page 4 Installation diagram for: SYSTEM #817654 1967-1972 GM C10 TRUCKS 2½” HEADER BACK SYSTEM 409S Stainless Steel ® EXHAUST SYSTEMS FLOWMASTER PERFORMANCE EXHAUST...