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
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.