Motorcycle exhaust flanges are one of those little, unobtrusive parts that most people do not pay attention to. At least, they do not pay attention to them until the gasket goes bad or they are trying to replace their exhaust system. Then those flanges suddenly become important parts, which can frustrate and annoy many do-it-yourself motorcycle mechanics.
The exhaust flange performs a simple, but important, function. It is what attaches the exhaust system to the exhaust port on the engine's head. Without it there, the exhaust headers would not stay in place and the engine exhaust would vent right into the air or onto the rider's leg, creating a lot of extra noise and some burned legs as well.
Like most engine parts, exhaust flanges have to fit the motorcycle they attaching to exactly. If they don't fit exactly, they cause an exhaust leak, with the associated noise and hot legs. Even so, they can be customised. The trick is knowing which customisations are usable on which type of bike. Exhaust flanges can be purchased at motorcycle parts shops and on the retail website, eBay.
Motorcycle Exhaust Flange Design
Exhaust flanges are unique to motorcycles, ATVs, and racing cars. Passenger cars and trucks use exhaust manifolds that connect directly to the engine head instead, allowing connection of the exhaust pipe. Exhaust manifolds are not used on motorcycles, due to the increase in back pressure that they can cause. This back pressure lowers the engine's efficiency, by causing it to use some of the power that it creates in order to overcome the back pressure.
To lower back pressure, motorcycles use exhaust headers, which are individual pipes that connect to the exhaust outlets for each cylinder. They come together before reaching the muffler. Since there is no exhaust manifold to attach to the engine head, the exhaust flange is needed to hold the exhaust header in place.
There are several types of exhaust flange designs used. The most common works with exhaust pipes that are flared at the end. This flared end butts up against the exhaust flange, with a collar on the pipe to hold it in place. An alternate design is the ball-type, which has the exhaust header pipes bulged out just before the end. In this case, the bulged-out portion of the header pipe meets up with the exhaust flange on the engine.
Ball type exhaust flanges do not require any sort of gasket, as the engine side of the bulge mates directly with a tapered part of the exhaust flange, providing a seal. Standard exhaust flanges do require a gasket to function, as the flange, itself, is making the seal with the end of the header pipe.
As this is a high temperature application, the gaskets are normally made of a combination of soft metals, at times mixed with normal gasket material. If paper is used as part of the construction, it is treated to allow it to resist heat, without ignition. The gasket, itself, can take on a variety of forms including: steel rings, tapered rings, crush rings, and steel mesh rings.
The flange clamps attach to the exhaust flanges on the engine with two or three mounting bolts or nuts. The pattern of this mounting hardware, including its location, distance from the exhaust pipe, and size of hardware, are all particular to the engine design. Without the right flanges and flange collars, the exhaust system cannot seal properly to the engine.
Custom Exhaust Flanges and Collars
Motorcycle owners love to dress up their rides, some doing so extensively. Among the first modifications many make to their motorcycles is to replace the exhaust system with a "tuned" exhaust. The exhaust flanges, themselves, usually are not changed, but the more visible parts, the collar and bolts, often are.
The most common type of exhaust flange collar customisation is a ribbed collar. This can be found in plain cast aluminium, chrome-plated, and black chrome. Theoretically, the fins on the collar help with improving heat dissipation, which they may do to some small extent. However, the main reason for changing the collar to a finned one is aesthetics.
Custom Exhaust Flange Hardware
Chrome-plated exhaust flange bolts are a common upgrade, especially when installing chrome-plated exhaust flange collars. An alternative design is to use chromed covers over the existing bolts. These may be domed or "high crown" to make them more obvious.
Some motorcycles use studs for exhaust flange mountings, rather than using bolts. A stud is a bolt that is threaded on both ends. One end is threaded into the head casting, the flange is slipped into place, over the studs, and then nuts are placed onto the studs to hold everything together. In this case, the same types of chrome bolt head covers can be used to cover the nuts.
Replacing the Exhaust Flange With a Custom Flange
As already stated, the exhaust flange and collar is usually replaced when the exhaust system is being replaced, although it can be replaced on its own. To do so requires removing and reinstalling the exhaust system, which is why most people wait until they are replacing the exhaust system anyway.
Removing the flange is simplicity itself, if everything goes well. It is held in place with two or three bolts or nuts, which can be removed with a wrench or ratchet and socket.. The new part is set in place, with the gasket centred behind it, and the hardware is re-installed or new hardware installed in its place.
Older motorcycles may not be as simple; however, not because the design is more complex, but rather due to oxidation. Both aluminium and steel oxidise more rapidly when they are hot, and there is no place on a motorcycle engine that is consistently hot, more so than what is encountered at the exhaust flange.
If the hardware appears to be rusted or it is hard to remove, spray a little bit of light oil, such as WD-40 or BP Blaster. These penetrate into the threads, providing lubrication for removing the bolts. After spraying them, allow the oil to sit for a couple of hours, to give it a chance to soak in. Never spray oil on a hot exhaust, as this is a fire hazard.
Removing Broken Exhaust Bolts
If the corrosion is severe enough, the bolts may snap off, rather than loosen. In this case, the broken-off bolt has to be removed. Depending on how tightly it is set in place, there are a number of methods that can be used.
If the end of the broken-off bolt is above the level of the flange, it may be possible to grab it with pliers or locking pliers to remove it. However, if it is broken off below the level of the flange, this is impossible. In that case, it may be possible to insert a small screwdriver to catch on the broken end of the bolt and back it out.
Bolts that are broken off and tightly stuck in place are harder to remove. These require the use of a screw extractor,, commonly referred to as an "easy out". This is a hardened metal tool that is specifically designed for removing broken-off bolts. To use it, first make an indentation into the centre of the broken-off bolt with a centre punch. Then, drill a 0.5-inch deep hole into the broken bolt. Use the largest extractor possible for that sized bolt, as to not break the extractor. The package for the screw extractor explain which size of drill bit to use with each size of extractor. This varies from manufacturer to manufacturer.
Once the hole is drilled, insert the screw extractor and tap it a couple of times with a hammer to seat it well. Use an adjustable wrench to grab the end of the extractor and turn it counterclockwise to remove the bolt. Spraying penetrating oil into the hole or applying heat may help for stubborn bolts.
Drilling and Installing a Heli-Coil
In cases where the broken bolt cannot be removed, the bolt hole has to be re-drilled and tapped. The easiest way to do this is with a threaded insert by Heli-Coil.. Heli-Coil inserts come as a kit, with the installation tool and the correct size drill bit included. Drill the new hole exactly centred on the old bolt, to a depth at least the length of the Heli-Coil to be inserted. Note that this hole is wider than the diameter of the bolt that could not be removed.
After drilling the hole, it has to be tapped to receive the threaded insert. The kit comes with the right sized, specialised tap. This is not a standard sized tap, but rather a special one for use with the Heli-Coil. With the hole drilled and tapped, the Heli-Coil can be installed. Some Loctite thread locking compound should always be used on the outer threads of the Heli-Coil, when installing. The installation tool threads the insert into the threaded hole, breaking off the installation tang when fully seated.
Buying Motorcycle Exhaust Flanges on eBay
eBay is an excellent source for all types of motorcycle parts and accessories. eBay sellers list both new and used parts and accessories for all brands and models of motorcycles. You can buy with confidence, knowing that you are getting your parts for the lowest possible price.
When choosing motorcycle exhaust flanges and collars, be sure to check your motorcycle make, model, and engine size closely. These are very specific parts, and while they may fit more than one size engine that a manufacturer produces, you cannot be sure of that, unless the manufacturer has listed it that way. The seller should be able to provide accurate information about which engines their parts fit.
Flange kits generally come complete with all necessary parts and hardware. However, when buying just flanges or flange collars, the hardware is usually not included. In these cases, it is advisable to buy new hardware, unless you know that the old hardware is not corroded and is still serviceable.
As part of the customisation of a motorcycle, the owner may decide to replace the exhaust flange or exhaust flange collar. This is usually done as part of a exhaust header or total exhaust system replacement. If it is not done as part of a larger replacement, the exhaust system has to be removed, in order to make it possible to replace the flange.
These unassuming parts can be purchased as OEM stock parts, or custom parts. Customised exhaust flanges and flange collars are usually finned, supposedly adding to the cooling capacity of the engine. While they do accomplish that to some minor extent, the fins are mostly for appearance. Custom flanges and collars are available in cast aluminium, chrome-plated, or black chrome-plated.
Many owners replace the hardware, when replacing the exhaust flange or collar. Chrome-plated hardware, or hardware caps, help to dress up the exhaust flange, making it look more customised and "finished."