Making a Custom Motorcycle Exhaust System

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

Custom builders often run into a problem as a bike project develops: the stock exhaust system does not fit and there are none available for this particular bike. At this point, the obvious solution is to fabricate a custom system which looks easy enough on one of the many reality TV shows, but in reality takes a lot of work and some special skills. That being said, a competent fabricator with no exhaust system experience can create a system that will be the envy of all stock bike owners.

 
First, a note about the size of the system. The design of a system, with regards to the pipe diameter and length, is outside the scope of this article which deals mainly with the actual making of the system. There are many excellent web sites dedicated to the theory and design of pipes for various applications (from single cylinders to 4 into 1s), but it is important to remember that every system is an individual and therefore not of the “one size fits all” type. In addition, modifying a stock exhaust design will generally affect the jetting


Having decided on making a unique exhaust system for your bike, and calculated the optimum sizes, it is time to lay out the basic shape. One of the simplest – and cheapest – methods of laying out a system is to use aluminum welding rods (1/8” or 3-mm in diameter) and some fender washers of the correct outside diameter of the proposed system. In essence, the fabricator will shape the welding rod to run from the exhaust port to the opening of the muffler (assuming a muffler will be used, as against 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. 


During the design and layout stage, the fabricator must consider some important aspects.

For example,
1) Heat
2) Clearances
3) Complexity
4) Tools and equipment


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 etc. If at all possible the fabricator should avoid wrapping the pipe to protect the surrounding area as this is generally a band aid solution to the basic problem of a poor design. (Note: The fabricator must allow for heat transfer rearwards—airflow will send heat backwards, which can be dangerous if a brake line or similar is in line with the rear of the pipe.)


Clearances


Besides the problem of heat transfer, 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%.


Complexity


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

 


Tooling


Needless to say, the home based fabricator or mechanic will not have a computerized tube bending machine, but that does not mean he or she cannot make an efficient, attractive exhaust system. Some basic tools will be necessary though, these include the following:


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

5) Air tools


Taking into consideration all of the above guidelines, the fabricator will know the diameter and length of the system he is looking to produce and the outset. The next phase is, therefore, getting the basic lay out. Initially this can be done by using a flexible pipe such as those found on a vacuum cleaner.

With the basic shape in mind, the fabricator can now use the aluminum welding rod to more accurately estimate the shape of the pipe. However, before starting to shape the aluminum rod, the fabricator can 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). 


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 know. They can supply most grades of stainless, pre-rolled tubes (‘U’ bends), and transitions.


Each time a new piece of tube is cut it will be slid up the aluminum rod toward the exhaust flange where it will be tacked into place. Three small tacks equally spaced around the tube will hold the joint in place. (Note: do not have any electrical components fitted during any welding, and if at all possible do not have any stainless braided brake hoses on the bike. The battery should also be removed.)


Cutting the stainless tubes must be done accurately. Of particular importance are cuts though 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 as once they have been tack welded into place, any burrs on the inside will give a rough welded join for the gases to traverse. 


Once the entire exhaust system has been tacked together, it can be removed from the bike ready 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.