Replace the exhaust system flex pipe on a 1970 AMC Rebel Machine with 390ci V8 engine to eliminate exhaust leaks and restore proper exhaust flexibility.

Warnings

Tools required

Parts

• Exhaust flex pipe assembly × 1 — 2.25" or 2.5" diameter depending on exhaust system
• Exhaust pipe clamps × 2 — Band clamps sized to pipe diameter
• Exhaust hanger isolators × 2 — Replace if deteriorated
• High-temperature exhaust sealant × 1 — Use OEM specification
• Penetrating oil × 1 — Use as needed

Preparation

1. Park vehicle on level, solid ground and engage parking brake
2. Place wheel chocks behind rear wheels
3. Allow exhaust system to cool completely (minimum 2 hours after last engine operation)
4. Spray all exhaust fasteners, clamps, and hangers in the work area with penetrating oil and allow to soak for 15-30 minutes
5. Raise front of vehicle with floor jack and support securely on jack stands at frame rails
6. Inspect exhaust system to locate failed flex pipe section (typically between exhaust manifolds and intermediate pipe)
7. Identify all exhaust hangers and mounting points that may need to be disconnected for access

Procedure

1. 1

   Locate and assess flex pipe damage
   Position yourself under the vehicle and locate the flex pipe section. On the 1970 Rebel Machine 390 V8, the flex pipe is typically located in the H-pipe or Y-pipe section after the exhaust manifolds. Inspect the entire flex pipe for cracks, holes, or separation. Use a marker to mark the cutting points approximately 2-3 inches on either side of the damaged flex pipe section, ensuring you leave enough straight pipe for the new flex pipe to clamp onto.
2. 2

   Support exhaust system
   Place a floor jack with a block of wood under the exhaust pipe behind the section to be removed. Raise the jack just enough to support the weight of the exhaust system without lifting the vehicle. This prevents the exhaust from sagging or binding during removal and installation.
3. 3

   Remove exhaust hangers if necessary
   If the flex pipe section is supported by rubber exhaust hangers, use a pry bar or exhaust hanger removal tool to disconnect the exhaust pipe from the hanger loops. Work the rubber isolator off the hanger hook by pulling downward and twisting. Inspect rubber isolators for cracking or deterioration and replace if necessary.
4. 4

   Cut out damaged flex pipe section
   Using a reciprocating saw with a metal cutting blade or an exhaust pipe cutter, carefully cut through the exhaust pipe at the marked locations on both sides of the damaged flex pipe. Make cuts as straight and perpendicular to the pipe as possible. Allow cut sections to cool before handling. Remove the damaged flex pipe section from the vehicle.
5. 5

   Prepare pipe ends for new flex pipe
   Use a wire brush to thoroughly clean the outside of both cut pipe ends, removing all rust, carbon deposits, and scale. Clean back at least 3 inches from each cut end. The pipe surface must be clean and smooth for proper clamp sealing. If pipe ends are severely rusted or crushed, trim back further to reach solid pipe.
6. 6

   Measure and verify flex pipe length
   Measure the gap between the two cut pipe ends with the exhaust system in its normal position (supported by jack). The replacement flex pipe should span this gap with approximately 2 inches of overlap on each end. Verify the new flex pipe diameter matches the existing exhaust pipe diameter (typically 2.25" or 2.5" on the 390 V8).
7. 7

   Test fit new flex pipe
   Slide one band clamp onto each cut pipe end before installing the flex pipe. Position the new flex pipe between the two cut ends, ensuring approximately 2 inches of overlap on each side. The flex pipe should slide on easily but fit snugly. If too tight, apply high-temperature exhaust sealant to the inside of the flex pipe ends. Verify the flex bellows section has freedom to compress and expand without binding.
8. 8

   Apply exhaust sealant
   Remove the flex pipe temporarily. Apply a thin, even coat of high-temperature exhaust sealant to the outside of both cut pipe ends in the area that will be covered by the flex pipe. Reinstall the flex pipe, ensuring it slides fully onto both pipe ends with equal overlap on each side.
9. 9

   Position and align flex pipe
   Center the flex pipe between the two cut ends, ensuring equal overlap on both sides. Rotate the flex pipe so the flex bellows section is positioned to allow for engine movement (typically oriented vertically or at a slight angle). Ensure the flex pipe is not twisted and that both ends are fully seated on the exhaust pipes.
10. 10

    Install exhaust clamps
    Slide both band clamps into position over the flex pipe connections, centering each clamp over the joint. Hand-tighten the clamp nuts, ensuring the clamps are properly aligned and not cross-threaded. Tighten clamps progressively in an alternating pattern to draw them down evenly. Tighten until clamps are firmly securing the connection but avoid over-tightening which can crush the pipe. The clamp should compress the flex pipe firmly against the exhaust pipe without deforming either component.
11. 11

    Reconnect exhaust hangers
    If exhaust hangers were disconnected, reconnect the exhaust system to the rubber isolators. Use a pry bar if necessary to align the exhaust pipe hook with the rubber isolator. Ensure the exhaust system hangs at the proper height with no binding or excessive tension on hangers.
12. 12

    Remove support and check clearances
    Carefully lower and remove the floor jack supporting the exhaust system. Allow the exhaust to settle into its normal position on the hangers. Inspect all clearances around the new flex pipe, ensuring at least 1 inch clearance from the driveshaft, transmission, floor pan, fuel lines, and brake lines. Manually move the exhaust system side-to-side and up-and-down to verify adequate flex pipe movement and confirm no contact points.

Reassembly

1. Verify all exhaust clamps are properly tightened and aligned
2. Confirm all exhaust hangers are properly connected and exhaust system is properly supported
3. Double-check all clearances between exhaust components and surrounding parts (driveshaft, fuel lines, brake lines, floor pan)
4. Remove jack stands and lower vehicle to ground
5. Remove wheel chocks

Verification

• Start engine and allow to idle. Listen for exhaust leaks at the flex pipe connections. A hissing or ticking sound indicates a leak requiring clamp adjustment or additional sealant
• With engine running, observe the flex pipe area for visible exhaust gases escaping from connections
• Rev engine moderately (2000-3000 RPM) while observing flex pipe connections for leaks under increased exhaust pressure
• Allow engine to reach operating temperature and verify no leaks develop as components heat and expand
• After initial test run, re-inspect clamp tightness after exhaust system has cooled. Retighten if necessary
• Test drive vehicle and listen for any exhaust drone, rattles, or unusual sounds indicating improper installation or contact with undercarriage components
• After 50-100 miles of driving, re-inspect flex pipe installation and clamp tightness

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