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.

Fastener Fascination

Factory exhaust manifolds used 3/8-inch-diameter studs with 16 threadsper inch of various lengths that are available from most automotivesupply stores. Various 3/8-16 nuts have been used over the years toconnect the head pipe to the exhaust manifold. We prefer to use Stoverlocknuts, as this style is crimped at the top, allowing good retentionwith easy removal. Apply antiseize thread lubricant to the exhaust studsbefore installing the 3/8-16 Stover nuts. Brass exhaust retaining nutswith lock washers will not stay tight and this can be a real problemwith an exhaust-manifold-to-head-pipe fiber seal (donut). The fiber sealcrushes as the head pipe moves around because of the loosening nuts. Theloose nuts keep getting looser, doing further damage to the fiber seal.The Stover locknuts should be retightened an additional time afterwarm-up to seat the fiber seal properly. Stover locknuts will grip aworn stud when a regular nut will slip. These locknuts are availablefrom your local hardware or fastener supply store.

Gaskets Or Not?

Should you use exhaust-manifold gaskets? Gaskets are not necessary ifthe manifold and cylinder-head surfaces are flat. The problem is, themanifolds will almost always require surfacing if gaskets are not used.Using a good-quality gasket and retightening the exhaust-manifold boltsafter a few heat cycles will keep the exhaust leaks away. There are manyexhaust-manifold gaskets available. We prefer Fel-Pro No. 1444 for thestandard exhaust-port size; larger port sizes are available for modifiedports. The gaskets require no extra coatings and allow for hot and coldcycling.

Tap the cylinder-head-bolt threads to allow proper exhaust-manifold bolttorque. The exhaust-manifold bolts should be torqued to 35 lb-ft. Ifyou're replacing exhaust-manifold bolts, watch the length carefully. Thebolt lengths are in 1/4-inch increments; hardware-store bolts are eithertoo long or short and cylinder head damage can result. There should beno more than 1/4 inch of bolt protruding through the exhaust manifold.Most Corvette mail-order suppliers have OE or reproduction bolts, heatrisers, and exhaust manifolds available. Protective eyewear and glovesshould always be used when working with exhaust-system pieces.

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Here, a torch is heating the exhaust manifold near the stud so it can beremoved without breaking off flush with the manifold. The trick is toget the manifold hot with minimal stud heat. Keep the torch movingaround the manifold near the stud. If the stud gets red hot, it almostalways twists off flush, which will force you to drill out the remainingportion of the stud. So if you see the stud turning orange, let it cooland try again. Have a pair of Vise-Grip pliers ready to grab the studwhen the area around the manifold is just turning orange. If the studdoes not move, heat the manifold again and watch the stud for excessiveheat. Once the stud is moving, it will tighten up as the manifold coolsoff. Reheating will allow complete stud removal. Remember, everything ishot and will leave a lasting impression! Allow the manifold to coolwithout any aid--do not cool it with water.

If the stud breaks off, drilling is the easiest way to get out thestubborn piece. When the stud breaks off flush, it means corrosion hasmade the stud and manifold become one. If the broken piece is not flat,use a handheld grinder to flatten out the stud; this allows centerpunching. Carefully drill completely through the broken piece of studwith a 5/16-inch drill bit. If the drill bit is centered carefully, acenter punch can break out the remnants of the stud. Tap the threadswith a 3/8-16 tap. An "Easy-Out" stud or bolt removal tool can be used,but in a situation like this they almost always break off and things getreally messy. Easy-Outs work when there is minimal thread resistance,and in an exhaust system there is no such thing as minimal threadresistance.

Use a 3/8-16 tap to clean the threads before inserting new studs. It's agood idea to use cutting oil and patience when tapping the threads. Themanifold has gone through many heat cycles, making the cast-ironmanifold brittle. A tap can break in an instant and ruin a perfectlygood afternoon if it grabs unexpectedly.

This manifold should be machined with a surface grinder. Your localmachine shop can machine the surface with the same surfacer-grinderthat's used for flywheel surfacing.

This heat-riser valve has a large buildup of scale and corrosion on theflat sealing surface. Using a gasket will stop an exhaust leak for awhile, but as the fiber seal wears, the gasket becomes loose and theleaks begin. If you're using a gasket be sure to specify it for theexhaust heat riser. Some gaskets do not have the metal ring that keepsthe hot exhaust gases off the fiber gasket material. Replacementheat-riser valves are available and reasonably priced.

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