Exhaust Header Fabrication Article  

  For some popular engines, header manufacturers have precut flanges in stock.  Most commercial flanges are available in 1/4", 5/16" and 3/8" thickness.  3/8" is obviously the strongest and most resistant to warping but costs slightly more and can be a little harder to work with than thinner flanges.  Flanges can be made in one piece or individual pieces for each exhaust port.  If flanges are not available precut for your engine you may have to fabricate your own.  You can buy some mild steel in your desired thickness and start cutting, drilling, and filing by hand, but that's a lot of work and will likely result in a less than professional appearance.  You can cut flanges with an acetylene torch but you will have jagged edges that require a lot of finish work.  Some companies use a computer controlled torch or laser to cut flanges but these are not readily accessible to most of us.  An interesting alternative is having the flanges cut to shape with a computer controlled abrasive water jet.  The water jet cutter is a jet of 60,000 psi water that carries an abrasive media directed through a small nozzle.  The water jet can accurately cut nearly anything from ceramic tile to stainless steel several inches think. 

Mandrel bends come in many different radii and prices vary so shop around to get what you need.  The two most common gauges of mild steel tubing for exhaust headers is 16 gauge, which has a .060" wall thickness, and 18 gauge, which has .049" wall thickness.  16 gauge will generally last longer, is easy to weld, and is more resistant to cracking.  18 gauge is easier to cut, a little more difficult to weld but is fine for most applications.  Collectors can be manufactured in several different ways, and each has advantages and drawbacks.  Formed collectors are manufactured from a single large diameter tube and are welded over the end of the primary tubes.  They are the cheapest and can give good performance if you pinch the center of the primary tubes together before welding to eliminate the need for a metal plug in the center.  The forming process used to manufacture the collector reduces the wall thickness in some areas due to stretching so this type of collector may not last as long.  Ed Henneman, of Headers by Ed, has developed a fabricated weld-on collector similar to formed collector but made of consistent thickness material, it is more expensive due to the labor involved but it will last longer and is available in custom sizes that are not available in a formed collector.  Merged collectors, like those available from Burns Stainless, offer the smoothest transition for the exhaust gases and are fabricated using specially cut mandrel bends.  Merged collectors are more expensive and can be custom made for your application.  Merged collectors are designed to slip on the end of your primary tubes and must be held in place with metal tabs.

This header has one of Ed Henneman's fabricated collectors with a short extension welded on and O2 sensor bung.

This header has a merged collector from S&S Headers ( I think they went out of business last year ). The transition "venturi" is from Burns Stainless. This is a VERY effective header.

  You will need several tools such as:  fine point felt tip markers, thin tape measure, hack saw, round and flat files, tubing cutter, tubing expander, and a welder.  A MIG, TIG or gas welder will work; use what you are skilled with.  An abrasive disk cut off saw and disk/belt sander are highly recommended.  These two tools can make square cuts and truing tube ends much easier.    

  Once you have your parts and tools rounded up you need to remove your stock exhaust manifold, down pipe and any other exhaust parts that may be in the way of your new header.  Disconnect your battery and any computers in the car, welding can damage them.  Bolt on your new flanges so you can begin to mock up your header.  Check the port to flange fit to be sure they match up correctly.  Many aftermarket heads or heads that have been ported will not match production flanges.  You can mock up your header with PVC pipe or some hose with a similar diameter to your selected primary tube size.  A less expensive option is wire with cardboard disks attached every 6" or so.  Start with the tube that originates farthest from the collector and lay it out to your required length; this will show you where your collector should be positioned.  If you start with the first tube going to the collector in the outside upper position, it may make layout of the other tubes easier.  Layout the rest of tubes, and adjust as necessary.  This will give you a rough idea of how the finished tubes will need to be positioned.   Here are some tips for header fabrication: *  Practice welding on some scraps of the tubing you plan to use for
your header. 

*  Do not use too much heat and filler rod; over-penetration will intrude inside the tube and disrupt the gas flow reducing performance. 

*  All cuts MUST be square so there is no gap in the joint when you weld it, this will reduce the amount of filler rod required and the chance of over penetration. 

*  You want to have about one inch of straight tube at the flange before a bend if possible.

*   Form the tube to the port shape before welding.

*   Use a file or die grinder to debur the cuts.

*   Cuts on bends MUST be tangent to the radius. 

*   When mandrel bends are formed the tube diameter of the bend is lightly reduced and must be stretched to the correct diameter using a tubing expander.

*   No "cheating" bends!!  "Cheating" refers to using a segment of a bend with non-tangent cuts to save space.  This is fairly common on production headers     but will adversely affect the exhaust gas flow. 

*  You may need two tubes to turn a corner to the collector together, which required tubes with radii that would lay perfectly inside each other.  It will take a little research to find compatible bends and they may have to come from different suppliers.

*   If your header design requires a joint in a primary tube, I recommend using flanges where possible rather than slip joints.  Slip joints can leak, rust and freeze up, making disassembly difficult. 

*  After you cut a section of tubing, measure it on the centerline and write the length on the tube for future reference.

*  Bends are measured along the centerline of the radius.

*  When test fitting the piece to get an accurate placement, use a fine tip felt marker to make 3 witness marks at the joint so the piece can be replaced in the exact position for tack welding. 

*  Tack the piece in place using 3 small tacks.

*  Tack all tubes before final welding.

*   If a tack weld needs to be cut, use an abrasive disk in a die grinder or a file to cut the weld.  Be careful not to damage the mating surface or the piece will need to be trimmed to eliminate the damage and ensure there are no gaps between the mating surfaces.

*   Test fit all tubes to the collector before final welding. 

*   Final welding should be done carefully, using small welds instead of one solid weld around the tube.  By alternating where you are welding you will reduce distortion so you shouldn't have to cut it apart and rework it to fit correctly.

*   As long as your welds overlap slightly you don't need to worry about leaks.

*   If your car requires an oxygen sensor fitting, no problem, they are readily available.  Just drill a suitable hole in the collector and weld the fitting in. 

    Once your header is complete you may need to attach it to a muffler and tail pipe.  Use the skills you perfected in fabricating the exhaust header to build a high performance free flowing exhaust system so you can reap the maximum benefit from your new header.