tech36

The How of Exhaust Systems
by Lynn Bennett

Over the last few years my motorcycle projects have included the
restoration or partial restoration of a BSA A65, two BSA B44’s, a 500
cc unit Triumph Rickman, a Triumph Trophy Trail, a 1975 CZ 400, a 1973
CZ 250, a 1987 CZ400, and a Rickman Montesa. Some or all of these
projects have required new or modified exhaust systems. Additionally, I
replaced the stock mufflers on my Ducati Monster. From this experience
and nearly 40 years of off-road riding experience I have gathered some
ideas and concepts for exhaust systems. Here’s what I know.

FOUR STROKE PIPES
If you are lucky enough your project bike will already have an intact
exhaust system. Sometimes it will need repairs like straightening or
reattachment of the mounting brackets. Sometimes you’ll have to start
from scratch. Ideally a mandrel bent pipe is best but finding someone
to do it for a reasonable cost is nearly impossible. (A muffler shop
pipe bender is not a mandrel bender and leaves stretch and shrink marks
on the tubes, making them unsightly and not of continuous diameter.)
You could spend hours making a drawing but the resultant pipe probably
would not fit correctly or you might find someone to trial and error
bend up the header pipes. An easier approach is to get “U bends” of the
correct bend radius and tubing diameter, cut and mix with straight
tubing, and weld up an appropriate header pipe. You basically build the
pipe from the head towards the end or muffler. For a four stroke
application the pipes can be TIG welded (butt welds), ground down
flush, polished and chrome plated. The welds become invisible if it is
all done right. An alternative is to use gas welding or even electric
arc welding but unless you get the penetration of the weld right,
grinding might make the pipe wall thickness minimum and subject to
later fractures. No grinding and heat resistant paint might also be an
alternative.

For mounting brackets I have found a hard mount to the engine mounting
plate bolt or a nearby frame point necessary as well as a rear point
for the clamped on muffler. The brackets on the pipe itself should be
brazed on rather than welded as this gives some flex to the joint for
vibration survival. The pipe may also include slip joints to make it
easier to install. Those slip joints need to be tight and held together
with springs. Where the pipe spigots into the head, it needs to be
expanded to fit tightly into the head to assure no exhaust gas leaks
there. Springs at his location are optional, but necessary if the first
hard mounting point is a long way from the head and it might be
possible for the pipe to slip out of the head.

I have found four strokes to be very unforgiving to excessive back
pressure created by too small (flow wise) of a muffler. And since I
have need for spark arrestor type mufflers for dual sport riding I find
that Super Trap spark arrestor disk type systems to be ideal. The
amount of noise and the gas flow goes up with more disks and down with
fewer. In general twelve disks is nominal, while less reduces noise and
flow while 18 acts like an open pipe, flow wise, but does some minimal
noise reduction over an open pipe. I have bought universal Super Traps
and also had custom mufflers made that included the adapter plate for
Super Trap disk use. Both of these approaches use a repackable insert
for noise reduction.

TWO STROKE EXPANSION CHAMBERS
Two strokes require a system that includes tapered, opening and
closing, cones with sections of straight or curved pipe in between.
Most installations require that the system be bent up like a pretzel to
fit on the bike. Lengths of the various cones and the other sections is
most critical and in today's world are generated by computers followed
by testing. Assuming the cone dimensions are know they can be
fabricated by rolling them from flat sheet metal, welding the seam,
then cutting and sectioning them together to get them to fit the bike.
This is not a task for the uninitiated. The welding of choice for these
system is gas welding.

Two stroke expansion chambers like to have flexible mounting, usually
rubber. Springs are used to hold the pipe to the head and to hold the
muffler section on, while several rubber mounts along the pipe help to
keep it from departing somewhere along the trail. The pipe should fit
the head tightly and the brackets should also be brazed on for the same
reasons that this works for four stroke systems. Loops can be brazed on
at critical points to act as anchor points for the springs. After
market repackable mufflers are available, both with and without spark
arrestors.

TYPES OF WELDING
As an aside we should probably touch on the various types of welding
that are available today and what they should be used for in exhaust
pipe fabrication.

The old standby is oxyacetylene or gas welding. Gas welding works by
heating the metals to be jointed with a flame until they melt and flow
into each other. Usually a filler material (a like metal) is added to
the pool of molten metal to fill any void in the joint that might
develop. The molten pool is just moved along the joint solidifying into
classical filets. Both steel and aluminum can be gas welded but the
oxygen in the air creates an impurity inside the aluminum weld that can
weaken it. For this reason a special flux is used. Aluminum gas welding
is hard to do since you get none of the indications of how hot the
metals is until it just makes a hole. At least with steel you can see
it go from red hot to slippery looking to molten so that you can move
the heat at the right instant. Steel gas welding works great for two
stroke exhaust systems.

Electric arc welding has several forms but all use an electric arc to
heat the metal to molten so that it will flow together. The home arc
welder available today uses a flux coated rod that is the electrode
through which the current flows to heat the metals. The electrode melts
and becomes a filler for the joint. A newer style is the wire feed
version (MIG) where the rod is in the form of wire on a reel that
automatically feeds to match the consumption of it as filler material.
The wire material and coating is matched to the parent metals to be
welded and an inert gas like argon or helium can be used to blanket the
joint to keep the air from reaching the molten metals. A third type is
the TIG welder (Tungsten Inert Gas) where the current flows through a
non consumable electrode made of tungsten. The appropriate filler rod
is dipped into the molten pool, as in gas welding, while a blanket of
inert gas like argon or helium is used to protect the joint from the
air. All of these electric welders have amperage and voltage controls
to regulate the heat, based on material thickness and other parent
material specifications. When the metal is aluminum and the inert gas
in helium it is called Heliarc welding. TIG welding is ideal for four
stroke exhaust systems.

Brazing is not really welding but a process similar to soldering like
plumbers do when they make up copper pipes or the electronic
technicians do when they work on your TV or computer. A gas flame is
used to heat the metal until the coated brazement rod just flows into
the joint. Brazing works best when the mechanical joint allows for a
small crack into which the brazement will flow by capillary action. The
joint materials’ micro crystalline structures intertwine with the
brazement making the joint, in some cases, stronger than the base
metal. Brazing is preferred for some metals whose physical strength can
be effected by the heat of welding, such as chrom/moly steel, but the
joint fit must be precise and tight or the joint strength will be
compromised. Brazing should be used for all bracket and spring loop
mounting.

I have done a little arc welding (very little) and some gas welding and
long ago learned that a professional does a much better job than I can
do. I do use my gas welding outfit to tack things together so I can
take them over to the real welder. By using a professional welder I get
tight fitting joints that don’t leak at the head and at any slip joints
(he swedges each joint to exactly fit). All the welds are near perfect,
and all the brackets and spring loops are brazed on securely. That is
the exhaust system answer.