How to Repair Corvette Exhaust Manifolds

Gen I, II & III Corvette cast-iron exhaust-manifold repair and remedies

Chris Petris Aug 24, 2004 0 Comment(s)

Only a few things are so aggravating that I would rather let my Corvettesit in the garage than drive it until it's repaired. Exhaust leaks areat the top of the list. Repairing exhaust leaks requires patience andperseverance when dealing with broken manifold studs.

Finding The Leak

The first task is finding the exhaust leak. A minor leak will benoticeable at startup and quiet down as heat builds. However, theseminor exhaust leaks just get worse with time. The old-fashioned way tofind an exhaust leak is to pour a pint of automatic transmission fluiddown the carburetor, which creates intense white smoke, then plug theexhaust pipes to see where the smoke exits at the manifolds. Doing thistype of smoke test today will likely mean a visit from your local firedepartment and/or irate neighbors.

Today, most automotive repair shops use a smoke machine that injects aharmless smoke into the exhaust system to aid in leak detection. Yourlocal repair shop should be able to provide a leak test that finds allthe leaks at one time, and it's well worth the money.

Exhaust leaks will eventually cause damage at the point of the leak. Asthe exhaust gas exits through a leaking gasket, a track forms thatetches the metal mating surfaces. If this etching from leaking exhaustgases is not corrected quickly, the mating surfaces may requiremachining with a surface grinder.

Are Yours Factory?

All small- and big-block Chevrolet cast-iron cylinder heads that usedcast-iron exhaust manifolds did not use exhaust-manifold gaskets fromthe factory. Factory-exhaust head pipes utilized a sleeve to center theexhaust-manifold-to-head-pipe seal (donut). This helped prevent burnoutof the fiber seal. Most exhaust systems do not have the sleeve to centerthe fiber seal, and the fiber seal just floats around, ready for blowoutat any time.

Passenger-side exhaust manifolds came from the factory with a flatsurface to provide a seat for the heat-riser valve, except for somespecial high-performance applications. The heat riser and passenger-sideexhaust manifold were also a metal-to-metal seal. The heat-riserassembly stops exhaust gases from exiting the passenger-side exhaustmanifold during engine warm-up, forcing the exhaust gases to circulatethrough the cylinder head to the intake manifold, then out thedriver-side exhaust manifold. This heats up the engine quicker andallows correct choke operation. The heat-riser valve assembly uses abimetal spring that loses tension when heated, thus opening the valveand allowing exhaust flow through both exhaust pipes.

Exhaust Manifold Repair Kit

* PB B'laster penetrating fluid

* PreLube 6

* Oxygen-acetylene torches (for stud removal)

* Propane torch

* Antiseize thread lubricant

* Thread tapping lubricant

* Air or electric impact wrench (if available)

* 3/8-inch drive ratchet (avoid using a 1/2-inch drive; seasoned exhaustfasteners tend to break)

* 9/16-inch-deep and regular-depth sockets

* 14mm regular depth socket (for worn manifold bolts)

* 3/8-inch extensions, 6 and 12 inches long

* Stud-removal tool or Vise-Grip pliers

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The engine should be at ambient temperature before removing the six9/16-inch bolts that retain the exhaust manifolds. It's typical to findthe exhaust-manifold bolt heads corroded away. If a 9/16-inch socket isslipping, a 14mm six-point socket will sometimes grab a corroded or wornbolt.

When removing exhaust flange nuts, a good soaking with PB B'laster orPreLube 6 rust penetrant works extremely well at loosening frozenfasteners. An overnight soaking of penetrant works best, then anair-impact wrench set on low can save considerable time and aggravation.The air-impact wrench utilizes an internal hammer to slug the nutsloose. Keep in mind that if you use the impact wrench on high torque, itmay break the exhaust studs like stick candy. If the air-impact wrenchwill not remove the nuts or if you do not have an impact wrenchavailable (air or electric), a propane torch will apply enough heat tohelp nut removal. Apply heat directly to the nut, avoiding the stud asmuch as possible. Keep a 9/16-inch socket, extension, and ratchet handyto loosen the nuts immediately. Watch the fuel hoses and lines whenapplying heat, and remember where you put the hot nuts after removal.

This is what a 30-year-old manifold looks like after numerousexhaust-head pipe flange removals. If the studs are not broken, theybecome bent from overtightening. The black soot around theexhaust-manifold-to-cylinder-head sealing surfaces indicate irregularsurfaces, requiring surfacing or exhaust gaskets.

The right side of this photo shows an accumulation of scaly metaldeposits. The scaly deposits are layers of corroded exhaust-manifoldiron casting. Scaling occurs when corrosive exhaust gases andcondensation seep out of the sealing surfaces. A torch and a smallhammer work well when removing the loose, scaly corroded material. Whileapplying heat to the sealing surfaces, tap lightly with a small hammerto pop off the scaly deposits. The scales are extremely hot and stickimmediately--long-cuff gloves will save a lot of scarring.

After removing the scale from the sealing surfaces, a high-speed grinderwith a coarse 3M Scotch-Brite disc is used to do a final cleanup beforeinstallation. After cleanup, a straight-edge should be used to check forsurface warpage. An exhaust manifold is not as critical as cylinder-headsealing for long-term sealing, though 0.010 inch should be the maximumwarpage allowed.

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