BMW motorcycle exhaust pipes and finned nuts
In 1970 BMW came out with a new series motorcycle series called the /5. From then on, all exhaust pipe fasteners to the head were of one kind. They copied the very successful former “sport” model system. The exhaust pipes go into the BMW head about an inch (25 mm) or more and are held in by the exhaust nut. It is finned for cooling and is cast out of aluminum.
BMW motorcycle exhaust nut from 70-71 and tool provided in the BMW tool kit. See the holes for the tool?
Exhaust nut typical of the late ’70s with “shorter” squared-off fins.
On the left is a /5 nut with the longer rounded fins. This is the later one without the holes for the pin wrench. On the right is the late 70’s type, also shown in the photo above. The one on the right has just been glass blasted, and the one on the left has not.
To remove the nut takes a special tool. The tools have been made in several varieties, and most work. The one supplied in the BMW toolkit was nearly useless.
This tool is from a BMW motorcycle toolkit, which works poorly to remove the finned exhaust nuts. The curve is wrong and will accidentally lift the pin out of the hole. When the round pin is only partly in the hole, it easily tears out the metal. One can grind the inside of the curve to get a proper fit.
The “Matra” tool that BMW provided to motorcycle dealers. The curve is correct, unlike the one in the toolkit, but it still applies all pressure to one hole. That is what can rip the hole out.
The early /5 BMW motorcycle had exhaust nuts that still had 6 holes for a pin wrench similar to the ones shown above. That had been standard for many years. The BMW motorcycle tool kit wrench curve is wrong for the /5 and later nut. In the first days of the /5, we just ground our tool to fit, and it “sort of” worked. It is the wrong way to go and will pull the aluminum away from around the pinhole. A new tool soon came out, and then the nuts had no pin holes to fit the tool in the toolkit. BMW had a tool made out of cast iron. It was long and heavy. Many aftermarket tools are made of aluminum and are lighter but often break.
This is a “short” version of an aftermarket tool. Photo supplied by Lonnie Walker, thanks.
This BMW motorcycle exhaust nut tool is made by Ed Korn, and it works very well. It is the best that I have used. One needs a 3/8″ drive handle.
Ed’s tool in operation
Make sure that the teeth are fully meshed and tight. You may have to use a lever to “straighten” one or two fins so that the tool will fit properly.
To crack a nut loose, one has to hold the tool well engaged in the teeth with pressure in the direction of removal. It may come free with some arm pressure. If not, then one should use a plastic rebound-less hammer. After it is loose, the nut must only be removed by hand and without the use of a hammer. There is a great risk that the threads will gall. They are aluminum against aluminum. If it seems to get tighter while rotating it, stop. To continue is to risk ruining the threads on the head. To replace an exhaust nut is cheap, but fixing the head is not cheap. The nut must be cut off with a hacksaw. Cut the nut at two places (on opposite sides) and only saw down “to the threads” of the head. Now use a chisel in the saw cut to break the two pieces off. When you get one cut finished, try to spread it apart and get the nut off. Often it will come off at that point. If not, then make the second cut.
This is what happens when one cuts too deeply into the exhaust nut with the hacksaw.
Installing the BMW motorcycle exhaust system
The best method is to install the exhaust pipes and then the mufflers. Run the exhaust nut onto the head by hand to ensure that the threads work. Clean up as necessary to get it to go on smoothly. The threads on both sides should be prepared with some anti-seize compound before installation. That will increase the chance that the nut will come off easily the next time. Clean as necessary. Make sure that the ( sports type) exhaust pipe goes into the head easily. Be sure that you have the compression rings. If one or both have been lost, get more of them. On the forward surface of the thicker ring where it mates with the exhaust nut, I use some anti-seize there too. It reduces friction and allows it to turn more without stressing the threads. Tighten the nut up loosely.
On the /5 and later, the crossover(s) should be mounted first and the assembly mounted to the heads.
Now attach the clamps to the motor mounts. It may require tapping the long through bolt a bit from side to side to get them both on. However, that takes two persons. By myself, I just used adjustable pliers to grab the clamp and lift it up to the motor mount bolt. Leave the nuts loose.
Mount the mufflers last and leave the fasteners loose. Now, tighten each fastener, starting at the front. The motor mount nuts need to be reasonably tight, as they play a dual role. The engine is a structural element in the frame design, so don’t let the engine “move” around. Loose motor mounts may allow excessive engine vibration too.
Clean the entire system of grease before starting it up. Foreign material such as grease may discolor the nice new chrome.
Once the bike is started up, listen for exhaust leaks around each side. Now tighten the exhaust nuts as necessary to stop the leaks. You only want to tighten the nuts just enough to stop the leaks. The tighter you tighten them, the greater the chance of the threads galling later on. Do not exceed the book torque settings. If they leak after reaching maximum torque, then start over and carefully examine each part. Some of the anti-seize may run out as the system gets hot. Just wipe it off.
Some believe that an owner should remove the exhaust nuts yearly to regrease them and ensure that they will come loose. I disagree, any removal can gall the threads, so why run that risk? Wait until you need to remove them. Buy and keep a spare exhaust nut for your model of BMW motorcycle. You will probably need it eventually.
BMW sports motorcycle exhaust nut sealing rings
This is what the exhaust rings look like.
This is the order of the rings.
Stripped BMW motorcycle exhaust threads
You may remove the head and send it to a shop that is set up to repair the threads. Some weld it up and machine the threads down. Some cut off the old threads flush, bore out the hole, insert a pre-made piece, and weld it in. Both procedures seem to work well. The same shop will replace any missing fins for a small sum. At the same time, you will want to do a valve job. Soon you will have hundreds of dollars in that head. All because you were too cheap to cut the nut off in the first place.
I highly suggest that anyone doing some of their mechanical work should be prepared for it. Buy a good BMW motorcycle exhaust nut tool and at least one spare nut. They are very cheap.
Thread file for use on BMW motorcycle exhaust threads
A metric thread file is also very useful.
A close-up of a thread file. The teeth on the very end are made to go in and clean out “inside” threads.
This thread file is American and shows the “threads per inch” as I couldn’t find my metric file for the photos.
Updated 15 July 2022