BMW motorcycle /5, /6 transmission (gearbox) service and repair
I highly recommend Blaise for transmission work. See more detail on the /2 transmission page.
Of the 4 units (engine, transmission, driveshaft and final drive) in the BMW motorcycle drive train, the transmission is the least reliable. It is typical for a BMW transmission, using natural petroleum hypoid gear oil, to need repair in 50-75 k miles. Most make it to 50 k, and most are opened up by 75 k miles. Yes, you can find the occasional transmission that violates these numbers, but I am giving the average. You don’t need to believe me, ask any BMW motorcycle transmission re-builder about his experience. If you take a look at the sizes of gears and bearings and consider the horsepower, the unit should last far longer. It should last nearly forever. Most cars and trucks with a manual transmission go to the junkyard without ever needing a re-build. Why not our beloved BMW?
Metal in the oil of a BMW motorcycle transmission
Forever “they” have said that it is normal for the BMW transmission to have metal flakes in the oil. Does that make sense to you? I didn’t think so. Ask a bearing engineer how much metal “should” be in the lubricant. I think that you will get a strange look.
Any metal in the oil is bad, but in the BMW motorcycle transmission, it is common. The source of the metal is from more than one place. The amount and size of the particles are important in predicting failure. While you can’t completely stop the metal from getting into the oil, you certainly can do something about it.
Drain your oil into a clean container and carefully examine the oil in the sunlight. You will see the metal swirl around and produce a “sheen”. The magnetic drain plug (sport models only) will have “fuzz” or sludge on it. The rule of thumb is to pinch as much of the fuzz as possible between two fingers and rub them together. If you can easily feel the fuzz as fine sand, that is bad. If the fuzz feels smooth, then that is good. Well, maybe not good, but as good as it gets, and the bearings aren’t coming apart soon. Record your findings in your logbook.
BMW motorcycle transmission shifting “bench test”
When you do any work on a transmission, you should test it for proper shifting before installing it on the bike. It may save you lots of work and time. You might want to test an unknown unit. You may want to test a unit before having it repaired to get a feel for what you have. Here is how we did it. (You may need a friend for that “third” hand.)
1. Mount it down to the bench somehow. You could tie it down to a milk crate and then put your foot on it to keep it in one place. We had the official transmission/engine stand for this.
2. Figure out a way to turn the input shaft. We modified an old scrap clutch friction plate. We mounted a knob on the plate to use for turning it. You need to turn it as quickly as you can. Hand speed is ok.
3. You need to put some resistance on the output shaft. We used a piece of leather pushed/held against the output flange to get resistance.
4. Then shift it up and down through the gears. The first time that we did this, it was a surprise to us, but you can “feel” every defect or characteristic of shifting.
It would be crazy to install a transmission that hasn’t gone through a simple bench test. It is so easy to do and takes no special tools. I am showing it this way to show you that you don’t need any fancy tools to do this test.
You will need to rotate the input shaft for this test. I use the clutch friction plate. If you haven’t removed the clutch yet, maybe this is an excellent time to inspect the clutch. To avoid removing the clutch to get the plate, rig up something to turn the input shaft. I turn the clutch plate by using the clutch rod in one of the holes. That is plenty fast enough. A rubber hose of about 1″ diameter and clamps will allow you to use your cordless drill. Just be sure to run it slowly.
The BMW motorcycle transmission to be tested must be held down. You may make a mounting bracket from angle iron and clamp it in a vise, but this shows one way to improvise. I use 30 + year old straps that mounted the motorcycle in the crate. Any old milk crate should work well. The only requirement of the ohmmeter is that I have alligator clips on the probe ends to fasten to the neutral switch. The clutch plate is mounted the same as on the bike.
You will need two people to test the BMW motorcycle transmission this way, but I manage with just myself. I use my left ankle to apply pressure against the output flange. Without that pressure, the shifting may be difficult, and therefore, the transmission shifting test is invalid.
My right foot is holding me up. My left foot is holding the crate down and also applying pressure sideways against the output flange. My right hand is the motive power and turning the clutch plate. My left hand is operating the shift lever. I watch the meter to see if it show the neutral switch operating at the proper times. This might be a bit awkward the first time, but one person can do it. This one tested good. This 1974 BMW motorcycle /6 transmission was known to be hard to find neutral, and that shows up in this test. When riding that model bike, it is far easier to shift to neutral while rolling the last 10 feet before the full stop.
This transmission passed every test. I would have no concern with installing this one and expecting excellent service out of it.
The gear dogs and slider plates
When removing the shafts, be sure to mark the slider positions. You will want to have it go back together with the slider in the same position/place. This will ensure that the worn in parts are still together.
That is because of the method of production chosen by BMW. The gears are cast, the teeth cut and the dogs are not machined, but left with whatever precision that resulted.
This is a typical BMW motorcycle transmission gear, and the 6 “dogs” are faced up. What are the dogs? They are the parts that engage the slider or shifting plate. The gear is free to rotate on the shaft and the slider “drives” the output shaft.
The gear teeth can be used to define a “center” of the circle. The dogs can also define a center. The two centers should be identical. They are rarely the same. Only the teeth get precision treatment. The dogs end up where ever they are cast. For being a casting, they good, but not good enough. A close examination of dogs on used gears will usually show that less than all 6 are being engaged. I have seen as few as 2 show evidence of serious wear. That means that all of the horsepower is going through only two dogs. That is bad enough, but the situation is far worse. With only partial dogs engaging, the gear is cocked off to one side. With the gear cocked, it wears far faster. That is a reason for some, but not all of the metal in the oil.
The BMW motorcycle slider plate
This is a “slider” or shifter plate found in a /2, /5, /6 and /7 BMW motorcycle transmission. It may have other names, but the name is not important. This part drives the output shaft and selects one of two gears, or neutral. The outer groove that the shift forks ride in has been machined. They have a center. The holes line up with the dogs on a gear, and they must match. By examining the holes, we can see the wear. The wear shows how well they match. It is common to find that less than 6 are driving the shaft. I have seen as few as two showing major wear. This will cock it off to one side, but in this case, it isn’t very important in adding metal to the oil. It may be important in shifting.
One could consider that if the dogs are “off-center” and the holes in the slider are also “off-center”, how do we know which is which? It is quite easy to tell. If the gear had only one dog “driving,” that dog would drop into the slider, and it would work. The dog would “visit” each of the 6 holes equally often over time. We would then see which of the 6 holes in the slider the most worn. The hole in the slider with the most wear is the one out of alignment. They should all show the same amount of wear.
Let’s consider the reverse situation. If the dogs were perfect and the holes in the slider were perfect, then we would see equal wear on all. If we have a perfect slider and imperfect dogs, then the holes in the slider would all get equal wear. Only the “driving” dogs would show wear.
Examination of the slider and gear will show if there is unequal wear, and therefore something is made off-center. The dogs can be so far off that they carry too much power and snap off. Then the power shifts to the next dog, and everything keeps going. It is possible to discover a broken off dog stuck to the magnetic drain plug. You can keep riding, but understand that the next dog is probably going to break off. I have seen three broken off, and the bike was still running. This and other parts can be found at the bottom of the transmission. We had a slender long flexible magnet to shove into the fill hole and swish around across the bottom of the transmission. It was common to fish out some part. We put the owner on notice of impending transmission trouble. This way, the owner could take care of it before it left him/her stranded.
BMW motorcycle transmission shift fork
The shift forks are well made. They are precision ground and usually well done. One can usually reuse the forks. BMW made tools to reach down into the transmission and bend them into alignment. I only did that once and just swapped them after that. It is a tough job, as the forks are made of super-strong metal. A close examination of the upper and lower tips on any one fork will often show some uneven wear. That uneven wear shows that the forks were not in alignment. Later the /5 came out with almost the same transmission, with one very important exception. The bushings that mount the forks are made non-concentric. You can see that the mounting hole is off-center. This means that you can rotate the bushing to move it from side to side and get the proper spacing.
The bottom end of the /5 bushing on the left and top end of the /2 bushing on the right. Sorry, but I mixed them up when taking the photo.
Each one flipped over, but still the opposite ends.
The /5 bushing with the “nut” end for the adjusting wrench on the left. The /2 on the right. Can you see that the hole is “off-center” on the /5 bushing? As one rotates it, the fork is moved from side to side compared to the sliding plate. This allows one easy and exact adjustment of the plate.
The shift forks are ground “off” a bit during manufacture, and they tend to get worn down, during use, to fit. It is most common to see one tip show far more wear than the other one. I never found that to be a problem. There is a test for misalignment, and if the forks pass that test, then I have never seen a problem shifting.
This photo shows a tip of the BMW motorcycle transmission shifter fork riding in the groove of a slider. If the fork is out of alignment, then one tip is off to one side, the plate will be cocked to one side, and it will bind up and make hard shifts. Over a lot of time, it will get ground off and may work better.
The path through the gears of a BMW motorcycle /5 transmission
This photo shows the gears as they are located in the BMW motorcycle transmission case. I have removed all of the unrelated parts possible, for clarity. The lower shaft is the input shaft. The splines are on the left end and stick into the clutch plate. The middle shaft is one piece and is called the “cluster gear” and the “intermediate shaft.” The upper shaft is called the “output shaft”.
The sliders are not engaged with any gear. That is because this is the neutral position.
The output (upper) shaft isn’t shown as it isn’t needed or desired for this demonstration. This is the path for first gear. The “right” slider has moved over to the right to engage first gear.
This is second gear. Now the “right” slider has moved left and has engaged second gear.
This is third gear. The right slider has moved back to neutral and isn’t shown. The “left” slider has moved over to the right to engage third gear.
This is fourth gear. The “left” slider has moved over to the left to engage fourth gear.
BMW motorcycle transmission bearings
The BMW motorcycle transmission uses C3 bearings. That means “loose” fit. Check the “play” of the gears on the output shaft, as they are quite loose on the bushings. All of this “looseness” is necessary to allow the gears to “center” themselves and try to distribute the power equally through all 6 dogs. The uneven wear is evidence that it wasn’t “sloppy enough” to work. The metal in the oil is evidence of wear. Where exactly does this wear come from?
The most wear is usually on the helically cut 4th gear on the input shaft. It gets all of the horsepower, all of the time. It is also 4th gear, which is the one used most of the time. The other 3 gears are straight cut. Over time, the input gear on the input shaft (4th) becomes undercut. Then it really makes noise and sheds metal. The next source of metal is the other gears. Next is from the kick starter sector gear. See those very fine “wires” that show up in the oil? They are bits sheared off of the second tooth. More info below.
Change the oil often
The best single thing that you can do for your transmission is to change the oil often. That flushes out the metal. Use synthetic oil, as it seems to double the bearing life. When you have it rebuilt, have it done properly. The factory has changed the spec for shaft end play. In my opinion, the later spec (.004″ cold) is still a bit loose. The /5 and later transmissions are not tolerant of end play. Lots of people can change bearings, but few know how to rebuild one of these units. Don’t try the stupid method shown in the factory shop manual for measuring the shaft end play. More below.
/5 BMW kick starter gear information
Owner abuse causes damage to the kick start sector gear. Two forms of abuse are common. Often, the rider “jumps” on the lever to start the bike. Avoid the need for this by keeping the bike in good tune. One in good tune can be started by using only your hand on the kick lever.
The gears may not be meshed, hit, and lock up. This chips off a tiny piece of metal that are now in the oil and may circulate in the bearings. The tooth finally wears away too. The correct way to use the kickstart lever is to gently engage it and push the engine through until it gets on compression. Then lift the lever up a bit, but not to the top, keep the teeth engaged, then give it your best kick. The second form of abuse is neglect. It is normal for the /5 kickstart lever to bottom on the footpeg rubber.
See the first tooth, the one on the right? It starts as a partial tooth and gets worn down even more. You can even see some damage to the second tooth, but not much. This sector gear is still in a rather good condition. I have seen them with the second tooth looking almost like the first one and the third one damaged too. That damage makes a lot of metal in the oil. Learn the proper kickstart procedure.
Here is another view of the same gear. You can see that the second tooth has some metal taken off of the tip. Not good.
This gear is the one that the sector gear engages. It can get ruined and isn’t available from BMW. I don’t know about availability in 2017. See the shaft on which the gear rides? That is the shaft that comes loose and falls into the transmission. More info below.
This is what a rear cover looks like just after it has been removed. Check the shaft to be sure it is tight. Some do a preventative procedure. Either drill a hole in the center of the shaft on the outside and mount a bolt and washer. That prevents the shaft from falling into the transmission case. A few like to weld the washer onto the shaft.
A big-time “no-no”
It is possible to mess up the transmission by loosening certain bolts. These bolts are under the air cleaner. On the top of the /5 transmission are two bolts that take an Allan wrench. They are recessed into the case and are about flush. These hold the shifting forks in place. The only time that these can be adjusted is when the transmission is being repaired. If either or both of these are loosened while the transmission is together and the bike is ridden, you will have big trouble, and safety could be an issue. The shifting forks could come loose enough to allow the bike to get into two gears at once, and things will come apart in rather dramatic ways. It is even possible, but not likely, to crack the case. It is likely that the transmission would be turned into junk because it may not be cost-effective to rebuild.
This shows the two bolts that mount the shifter forks on the inside of the transmission.
The /5 transmission
This shows the two shifter mounting bolts. See the horizontal sheet metal piece and its mounting bolt in the center? It holds the two halves of the air cleaner box down.
The famous “clunk” of BMW transmissions
The clunk happens when the two parts that must meet are going at different speeds. As one part grabs the other part, we hear a nasty noise and even feel it. Don’t let anyone tell you that that is normal or OK. It is common, but it isn’t the best thing and can be reduced. Let me ask you this; if you have a piece of metal and you hit it as hard as you can with a hammer, will it stay as nice as long as if you had given it a gentle tap?
There are a few factors. If you pull the clutch lever and “wait”………….. then shift, it may be in the proper range of time. One part begins to slow down and eventually gets in the ballpark of the speed of the other part and presto a “click” rather than a clunk. If you “wait” too long, then it may not even shift at all, or it may feel as if it shifted and quickly pops out of gear. It that case, it was never fully in gear. With experience, you will learn to “feel” and make a good shift.
One factor in the “wait” is the condition of the clutch. I have seen clutches that didn’t fully release, and then the “wait” is useless. It will cluck, no matter what. It may be badly out of adjustment and wouldn’t release. It is about the same as not getting “pulled in” far enough. Check the free play. The end of the clutch lever should have about 3/8″ of free play. Cold transmission oil has more drag, or friction and as the temperature increases, the “waiting time” decreases.
It may help to “pre-load” the shift lever before going into first. Move the shift lever down until it “hits” and hold some pressure, pull the clutch, “wait” and add pressure. It helps a lot with the other shifts. It’s even possible, on most BMW transmissions, to shift without the clutch and get the “click” if one gets good with the timing and pre-load.
An important /5 “fix”
The /5 has a trait that isn’t very nice. The shaft that the kickstart idler gear rides on (photo above) has a tendency to come loose. It is a shrink-fit (it was cast in) in the aluminum cover and shouldn’t come loose. Soon after it comes loose, it may fall “into” the transmission. This can really cause trouble. As usual, the solution is to fix it before it locks up the transmission and ruins it. A common fix is to remove the transmission, drill a hole in the center of the pin and Locktite a bolt and flat washer in the hole. The washer must be larger than the pin and keeps the loose pin from falling in. At this point, the kick starter should be used only in an emergency.
Simple BMW motorcycle transmission tool for spacing the shafts.
Don’t ever try to space the shafts by the method shown in the factory shop manuals. Eventually, the factory came out with a tool to hold the shafts. I think that you can still buy one from Cycle Works.
It is easy to make a tool from an old cover to use for holding the shafts and bearings in place. It is usually easy to find an old cover that has the clutch arm bosses broken off. First, clean off the surface that mates against the case. Check it for burrs and any defect that might be found on that surface. Second, have the rear of the old cover machined off to make the tool. Hand sand the bearing mounts a bit so that the bearing is a slip fit.
The input shaft is “stiff” after reassembly
Sometimes the input shaft will feel kind of stiff. This can be because the shaft and bearings are not seated perfectly. Solution; warm it up and give the input shaft a medium hit. You will/should feel the change instantly. The problem would have taken care of itself once the bike got up to temperature, but who wants to risk that it might not free up later? Find out before you install your newly rebuilt transmission.
The vent bolt on the /5
The vent bolt mounts the ground wire from the battery, and holds the speedometer cable into the bushing, and provides a vent for the transmission. Since it must do a few things and the bushing is a bit odd, the result is often less than desired.
This shows the two types of BMW motorcycle speedometer bushing used over the years. They are interchangeable. I think that the one on the left with the “square groove” is for the /5. The one on the right has the “circular groove” and is for the /2. I prefer the one on the right as it is more deeply cut and allows for more error in bushing installation.
This is how it should look as it goes into the BMW transmission. It must usually go all of the way down. The bushings vary in fit. Many are finger loose, and some are quite tight. It is the tight ones that really cause the problems. I will address them in detail. This photo is really “telling,” as it shows several things. Look closely at the hole for the vent bolt. See that it has no threads? They are stripped out. That is one of the reasons for writing this information. I want to show “you” how to prevent this from happening. See how the bushing is rotated so that as the bolt goes into the transmission, it will miss hitting the bushing? That is what you want.
Look carefully at the far side of the bushing, and you can barely see where the end of the vent bolt has hit it. If that happens badly enough, it can cause the hole in the bolt to be mashed against the bushing and be covered up. It won’t vent. If you think of the bushing from the top, consider that if it is rotated slightly counterclockwise, then the bolt could hit it as it gets started in. That will damage the bolt threads. If the bushing is rotated slightly clockwise, then the bolt can hit the far side. That is what happened here, but only to a small degree. The bushing can be turned even more clockwise and block the bolt from hitting the bottom easily. What the person installing the bolt will notice is that the ground lug isn’t tight when the bolt is. The person then tightens the bolt even more, and that will strip out the threads. I install the bushing so that the vent bolt doesn’t touch the bushing at all. Of course, it is possible that a previous owner or mechanic overtightened the bolt and ruined the threads, like this one.
Correctly or incorrectly installed, it will look like this. I have shown no battery ground wire or speedometer cable, as this transmission is out of a bike.
A few of us took transmission oil temperature measurements. On an eighty degree day and an hour of riding, a /5 will show a temperature of around 160-165 degrees F, or 70-75 C. The temperature was taken with a probe style thermometer. Other BMW transmissions are similar.
Protect your /6 transmission clutch boss from breaking off.
The /5 clutch arm pivot pin was held in by a cotter pin. These models of transmission didn’t have a problem with breaking off. With the advent of the /6 models, BMW elected to retain the pin with a sheet metal clip, or keeper.
That is a view of the transmission cover before it breaks. See the pin that holds the clutch arm onto the cover? The keeper falls off, the pin comes loose and slides down a bit. Soon it is only sticking in the lower part. Next time that you operate the clutch, the pressure breaks the boss off. The arm falls down and off of the bike. Sometimes the bearing parts also fall out. You will be riding without a clutch, or walking. You will also be paying for a very expensive repair job. It can all be avoided by fastening the pin so it won’t fall out if the little keeper disappears. I don’t have a picture of the original type clip that fell out, but it wasn’t flat. The later one is a real clip of spring metal and is flat. It stays in place. It is very hard to see on the bike, but one of the good ones is in the photo. Sometimes both ears would break off. This is when you will be very glad that you used safety wire to hold the arm from falling onto the roadway. In those cases, it is impossible to say what exactly happened or what to do to prevent it. The broken boss(s) can be repaired by a welder. The transmission must come out for the welder to weld it properly.
I prefer to replace the pin with a bolt. The bolt must have a long part that isn’t threaded. The bolt (pin) must be installed so that the head is at the upper end. Even if the Nylock nut falls off, the head will prevent it from falling through and off.
Kick starter on the /6 and later
The /6 came out in 1974 and had a kick starter. In 1975 it was dropped from the line, except as an accessory. The European models may have had them for longer than the ones made for US delivery. The question keeps coming up about how to add the kick starter to an existing bike. Don’t bother, as they didn’t work well. To kick over a 900 cc motor, the engineers had to give the kick starter a bigger mechanical advantage. The result is less throw. It turns the engine far less of one revolution. Everything must be perfect, including the rider’s technique, to kick-start one. If it is used on a bike that was in very good tune and the rider knows how to use it, only then can it be useful.
The transmission case was not strong enough for the impact of repeated kicks. Even the “easier to kick” /5, had lots of trouble with the rear cover. Save yourself lots of expense and grief and use the electric start.
Comments by John Falconer
The five speeds are good boxes but definitely had bad years and foibles. The first ones were notoriously bad, and if you have one from ’74 or so, then I’d consider getting rid of it. The shift cams were flimsy, and the case is not as strong as later models, and many parts are NLA. From about ’76 through ’80, you get a box that is much better than the first five speeds and one which is still compatible with the flywheel/clutch assembly on your /5. The shift cams are improved and in ’79 (I think it was ’79) that the case was substantially stiffened. From ’81 on, the input shaft was changed to accommodate the lightweight clutch so you can’t use these boxes on your /5 unless you fit the lightweight clutch assembly in toto (and this poses a problem since the mounting hardware is a larger diameter than the threaded holes on your /5 crank).
A good change to any pre-82 box is to fit the latest (third distinct design) of shift cams. In the late ’80s, BMW omitted a circlip, which can allow a shaft to move in its bearing – this can be an expensive problem but shouldn’t affect you if you’re running the /5 clutch since those boxes won’t bolt up anyway. If you can’t trace the lineage of your gearbox, a disassembly would quickly reveal it, and would also give you a chance to address one of the most common and easy-to-fix failures, that being the early wear of the plastic roller that follows the outer track of the large shift cam. You can replace it with the roller fitted to K75 gearboxes. If the roller is worn, some of the symptoms you describe can ensue since the cam plates don’t get properly located to where they need to be with each gear selection.
BMW motorcycle transmission oil
Our experience with bearing life was that most transmissions failed between 50 k and 75 k miles. This has been hotly disputed on one of the motorcycle lists by more than one owner. One’s own experience is important and part of a large database. A shop sees a larger part of the whole database. I asked Rick Weber about this issue. He has been working on BMW motorcycles for the past 25 + years. He was starting as I was getting out of the business. Here are my questions and his answers, directly copied and pasted.
On Tue, 19 Feb 2002 08:49:41 -0800 “Duane Ausherman” writes:
The question came up recently on the /5 group list. “How long do the transmissions last?” I found that we were opening some of them at 50 k, and most had been opened by 75 k miles. One guy responded by saying that in 250 k, his had never been opened. I guess it could happen, but we never saw anything close to it. He also said that he got 225 k on his first set of valves. That is hard to believe. Did you get an impression of how long they lasted? A general idea of the range of miles. I didn’t count the factory errors of the too loose input shaft in 70 and 71 when we had to open almost all of them. We are thinking about bearing life mostly. Any words you might have would be appreciated, thanks. Sincerely, Duane
The secret to long tranny life is using synthetic gear lube, Spectro is the one I’ve used and had the most experience with. Usually, 50k to 75k seemed average life span for the bearings getting regular oil changes with petroleum-based lubes. You can at least double that with synthetic. I had one customer who had used nothing but synthetic and wanted me to inspect his gearbox that had 135k miles on it and never had been opened. I swear it looked like brand new inside, no gray sludge, no metal on the drain plug. The 4-speed gearboxes seem to go longer than the 5 speeds, so I’d think the 4 speed could have a longer interval between overhauls.
Valves seem to last 60k to 80k miles.
Updated 6 Nov. 2019