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.