Building a Custom Motorcycle Exhaust System

Exhaust Collector, 4 into 1.
John H Glimmerveen. Licensed to About.com

There is nothing more aggravating to a custom motorcycle builder than discovering halfway through a project that the stock exhaust system does not fit—and that there are none available for this particular bike. At this point, the obvious solution is to build a custom exhaust system. Sure, it takes a lot of work and some special skills, but even without exhaust system experience, an otherwise competent mechanic can create a system that will be the envy of all stock bike owners.

A Note on Sizing 

We'll touch briefly here on system design before getting into the actual method of fabrication. The finer points of the workings of an exhaust system, however, is outside the scope of this article. Before you actually begin to lay out the design and start the build you should research the theory and design of exhaust systems for various applications (from single cylinders to 4 into 1s). It is important to remember that every system is unique and not “one size fits all." In addition, modifying a stock exhaust design will generally affect the jetting, so it's good to have some knowledge of engine timing as well.

Design Considerations

Heat Transfer: Obviously, a running engine will produce heat. This heat will transfer to the air flowing over it, or to anything that comes close to it, for example a rider’s leg or a fiberglass fairing. If at all possible, you should avoid wrapping the pipe to protect the surrounding area as this is generally a temporary solution to the basic problem of a poor design.

You must also allow for heat transfer rearwards—airflow will send heat backwards, which can be dangerous if a brake line or similar component is in line with the rear of the pipe.

Clearances: Exhaust systems must have sufficient clearance to allow for expansion and movement. The amount of expansion will depend on the performance of the engine (more power equals more heat), and the material from which it is made.

The fabricator should allow for a size expansion around the header pipe of approximately 20 percent.

Complexity: The old adage to “keep it simple” very much applies to exhaust systems. Complex, ever-changing pipes and tight corner radii will adversely affect performance and should be avoided where possible. 

Tooling: While you likely will not have a computerized tube-bending machine, that doesn't mean you can't still build an efficient and attractive exhaust system. Some basic tools will be necessary, though, and include the following:

  • A quality hacksaw with new stainless-specific blades
  • Files (flat for tube ends, and round for de-burring inside tubes) 
  • MIG or TIG welder (to tack together the various parts) 
  • Vice to hold tubes during cutting (it will need round jaws to evenly clamp tubing) 
  • Air tools


Laying It Out

Taking into consideration all of the above guidelines, you should now know the diameter and length of the system you are looking to produce. The next phase is to set up the basic layout. Initially, you can use a flexible pipe, such as those found on a vacuum cleaner, to lay out your basic shape. Once you do that, go on to lay out the system with aluminum welding rods. One of the simplest—and cheapest—methods of laying out a system is to use aluminum welding rods (1/8” or 3mm in diameter) and some fender washers of the correct outside diameter of the proposed system.

In essence, you will shape the welding rod to run from the exhaust port to the opening of the muffler (assuming a muffler will be used, as opposed to a straight-through pipe). The aluminum wire will be shaped to give the optimum curves (keeping them to a minimum so as not to interfere with the gas flow) and the washers will be used to ensure there is sufficient clearance around the engine etc. 

However, before starting to shape the aluminum rod, mark off the optimum length on the rod when it is still straight. The aluminum rod should be fixed to the cylinder head at the header pipe flange then shaped to follow the desired contour toward the muffler (where fitted). 

Building the System

The next phase is to cut various pieces of stainless tubing to fit over the aluminum rod. There are many suppliers of stainless tubing.

Burns Stainless in California are probably the best known. They can supply most grades of stainless, pre-rolled tubes (‘U’ bends), and transitions.

Each time you cut a new piece of tube, you'll slide it up the aluminum rod toward the exhaust flange and tack it into place. Three small tacks equally spaced around the tube will hold the joint in place. Do not fit any electrical components during the welding, and, if at all possible, remove any stainless braided brake hoses as well as the battery. 

You must cut the stainless tubes accurately, particularly the cuts through bends. These cuts must be perpendicular to the centerline of the pipe or tube. A simple method of ensuring the cut is perpendicular is to slide a tight fitting rubber ring onto the tube. The rubber ring will try to conform to the smallest circumference, and in so doing will create a perpendicular edge to follow with a marker pen. 

Once the tube pieces have been cut, they must be de-burred. Once they have been tack welded into place, any burrs on the inside will give a rough welded join for the gases to traverse. 

Welding the Components

Once the entire exhaust system has been tacked together, it can be removed from the bike for final welding. Ideally, the exhaust system should be TIG welded by a professional welder. Although weld appearance is important, a professional welder will also be aware of the need to minimize distortion during the welding process—there is no point making a perfectly fitting system when tacked only to find it does not fit the bike after being fully welded.