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)

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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