This page is about the BMW motorcycle models from just before WWII and later, including the /3 models and the R50, R60, R69, R50/2, R60/2, R50S, R69S, R50/US, R60/US, R69US, R50/5, R60/5, R75/5, R50/6, R60/6, R75/6, R90/6, R90S, R60/7, R75/7, R80/7, R100/7, R100S, R100RS. The singles have a similar clutch, but smaller in diameter.
Table of contents
A. Introduction to the BMW dry clutch
B. Prewar and /3 information
C. /2 55-69 clutch
D. Measuring the backing plate
E. Measuring the friction plate
F. Measuring the diaphragm plate
G. Measuring the spring
H. Clutch actuating arm and bearing
I. Clutch pushrod
J. Clutch bolts
K. Flywheel bolts
L. Clutch installation
M. /2 transmission alignment
N. Fixing a stuck clutch
O. Modification for easy clutch pull
A. Basic BMW motorcycle clutch information
On average, we found that the /5 clutch would last around 60 k miles and the /2 a bit less. Some people rode very hard and would burn one up in short order and some would get more than 100 k miles. As it wears, it will usually state slipping at high speed. Then the speed at which it slips will gradually lower until you just aren’t going anywhere. It was not uncommon for a person to ride only in the city and have a badly worn clutch without any symptom. Then one day the rider would have some reason to get onto the freeway and discover that he/she couldn’t even keep up with traffic, due to clutch slippage. Once it starts to slip, it is all over. Assuming that it is adjusted properly, the clutch must be removed for inspection. It could be oil soaked or just worn out.
This is an attempt to clarify some of the clutch/flywheel issues that have cropped up. It is by memory and fortunately I still have a bunch of NOS and old clutch parts to show for examples. I did so many of these that I can’t even remember using measurements, but an individual owner has no way to get that experience. I have included some measurements as examples. Sorry, but I have no metric micrometer or caliper, so I had to use inches. I have no idea of the official BMW specs, I only know what works. Please do not fail to suggest corrections, or ask questions. I am forgetting some important things, as you will see below.
B. The prewar and postwar clutch and flywheel
This is only partial information about the late 30’s bikes and the /3 models.
This shows the flywheel that is in a R61 from about 1939. The clutch parts “slide” on the six posts that are pressed into the flywheel. As the power gets applied to the clutch, the parts tend to wear a ridge into the side of the post. One must carefully examine the posts for any wear in the form of minor ridges. These ridges tend to “catch” the plates and not allow “smooth” clutch operation. The holes in the parts also tend to become elongated. That shows as a bit more slop in the driveline.
These are some of the parts that apply to the post type flywheel. The friction plate may not look exactly like that one. This one is similar to a /2 plate.
It could look like this one and may be very thin
This is the /3 type flywheel. The large holes are for coil springs.
One of the springs nested in the flywheel. They are hard to hold in place during assembly.
This shows the other end of the spring where it rests in the forward clutch plate. Similar to the /2 diaphragm plate
Here are all of the parts, except the coil springs, that make up the clutch. The toothed parts have an alignment mark on them. Many of these parts have a balancing hole(s) in them. The hole is only drilled partially through.
The clutch pushrod has a square end on it so that it always turns with the flywheel.
There seem to be no hard and fast rules for these clutches. Maybe the after market companies made up so many slightly different parts that we can’t exactly figure out any rules for dimensions. The clutch friction plates have been found with a wide variety of thicknesses. One often needs to know and understand the operation of the clutch and make many measurements to get the right parts together.
The parts wear in much the same way as the /2 parts that are described below.
Modifying the pre-war BMW motorcycle clutch
The R61 clutch text and photos above reveal that it was a rather poor clutch when compared to the later /2. The /3 is easy to modify, just use a /2 flywheel and clutch parts. This R61 was a bit harder to modify, as the taper on the crank is different. The later flywheel wouldn’t fit, so the original one must be made to adapt to allow a /2 clutch assembly.
This is not a step by step recipe for this procedure. This is just a general outline of a flywheel/clutch that Joe Groeger adapted. It is necessary for one to understand how both clutches work and have access to a machine shop.
Rear main seal
The flywheel has a surface where the rear main seal rubs. The R61 had a spiral groove cut into it and was “wiped” by a felt seal. The felt had been replaced by a modern seal with a lip. The spiral groove had not been removed. The seal wore out almost instantly.
Joe cut off the rest of the spiral grooves to make up a new seal surface.
Joe cut the surface down to fit a modern and available seal. This is the number and size.
The transmission bulge
In the center of the photo is a cover with 4 fasteners. That cover sticks out beyond the front surface defined by the front of the transmission case. That means that it would hit the /2 clutch assembly. The entire assembly must be recessed into the flywheel cavity a bit. The finished modified clutch must clear the transmission. It is not a lot, but must be taken into account.
The metal between the arrows was removed down to what you see. This accomplished a few things. The clutch assembly is sunken into the flywheel more. The recess for the spring is still too small for the /2 spring. The spring had to be cut down about 3/16″ or so in diameter to fit this place. More metal could have been turned out, but Joe did it this way. The material taken out just up to the posts is about as far as one can go and keep enough metal to support the posts. To get the clutch assembly to mount deeper into the flywheel, the posts had to be pressed into the flywheel more. The distance from the top of the posts to the bottom where the spring rests, must be exactly the same as the /2 flywheel.
A close up of the post after it was pressed into the flywheel.
To keep the posts in place securely, they are spot welded on the back side of the flywheel. Then the back side of the flywheel must be turned down to cut off the posts evenly.
Here you see the slightly smaller spring laying in the flywheel cavity.
Here are the parts laid out. Top to bottom; flywheel with spring, old style backing plate, spacer ring, friction plate and diaphragm plate. The spacer ring would not be needed if the later (better) part was being used.
The clutch parts have been installed in the flywheel and held with 3 bolts. One must test the function of the clutch to see if the dimensions are correct. The flywheel and clutch assembly are laid on a drill press bed. A round arbor is used to allow the press to apply pressure the same way that the clutch pushrod does in actual operation. When compressed, the friction plate is completely loose, proving that the clutch will release fully.
This shows the original R61 clutch rod above and the modified /2 rod below.
Here you can see the details of the clutch rod bearings. The modified /3 part was used here with the original /2 parts and it works just fine.
C. The /2 clutch, late 55 thru 69
The clutch for the singles and twins is the same system. The single’s clutch parts are just smaller in diameter. All other info applies.
To remove the clutch parts from a /2 requires no special tools. The two 8X1 bolts that mount the tank from underneath are perfect to use as clutch removal tools.
Since the advent of the Earles fork models in 55, through the models mentioned above, the clutch system has stayed the same, although it has evolved and improved. The later models, /5, /6 and /7 use the same system and the parts are often interchangeable. I can’t remember exactly when certain changes were made. So many of these old bikes have been altered and modified over the years that one can seldom be confident of originality.
An early /2 clutch assembly
The same parts spread out. From left to right; Diaphragm, clutch plate, spacer ring and the “early” backing plate. These parts are all in good condition. The clutch plate is almost new and shows very little wear.
You can see two important items in this photo. First, see the gap between the spacer ring and the backing plate? That allows the clutch dust to escape from the assembly. It is very important to allow the dust to escape. Without an escape space, the dust will get behind the diaphragm plate and pack it up so that it won’t flex. When it gets that bad, it will no longer disengage. It is very important to clean a diaphragm plate that you are going to reuse to get any dust out from behind it. I immerse it in my solvent tank and flex the plate until clear solvent comes out from behind it. This may take some time.
See the hole that is partially drilled into the backing plate? That is for balance. Each part is assumed to be balanced and the parts can go in any order. There is no reason to mark them or pay any special attention to that aspect. Remember KISS?
This spacer ring is to be used only with the early backing plate with the 6 tabs for mounting.
Here you see the same assembly with the next version of backing plate. I call it the “mid” one. It also may have one or more holes drilled into it for balance. The advantage of this plate is that it has no tabs to bend. The early backing plate was famous for warping and the tabs only accentuated it. This “mid” version combination won’t work well as it has no place for the “clutch dust” to escape. The clutch operation will be fine for awhile, but eventually it will fail to disengage. The spacer ring should be left out and the thick washers should be used in its place. This made a good combination. This assembly with washers wasn’t very popular because they made it harder to assemble and install the clutch. It is worth the extra trouble to use the washers. The spacer ring was sometimes used because it was easier to install. I only found a few clutches that wouldn’t disengage and it was due to the build up of dust. I have no way to know the history, but it could be that the clutch had been replaced once or twice and the diaphragm wasn’t cleaned. The bike would get the new backing plate and the installer would use the spacer ring, rather than the washers. Be safe and don’t use this combination of parts. Use washers.
This is the front or the friction side of the “mid” backing plate. This one is discolored because it has been in my storage for the past 25 years. It is in really good condition otherwise. Don’t worry about a little rust, it will be gone after a few shifts. The thing to worry about is the taper or deep grooves. As it wears, it gets tapered. This part becomes thinner at the center. The friction plate gets thinner at the outer edge. All three friction parts become tapered. I understand that if you order a backing plate for the /2, you will get the one that was introduced for the /5. See it below. It has no washers, but has a cast on part that replaces the washers. If you use that far superior plate, be sure to not install the washers. If you install the washers with that plate, you will find that the engine and transmission are locked up. Nothing will turn.
This is the washer compared to a dime. It is .131″ thick and .630″ in diameter.
D. Measuring /2 BMW motorcycle clutch backing plate
This is a “worn” backing plate. The original thickness is .234″
Thickness at the outer edge .229″
Thickness at the inner edge, .225″ and this one is still useable. It is not possible with this micrometer to get down in the bottom of the “worn” grooves for a really accurate measurement.
This backing plate is the last one for the /2. See how the old separate ring is part of the plate? It has no way for the dust to get out, but it is the strongest one and easy to install, as it doesn’t have those 6 washers to fall off of the bolts. Photo by Bernd Kupper, thanks.
This is the latest /5 backing plate and it really can’t easily be measured due to the irregular back side. It is thicker overall and this shows .255″ This plate could be measured by using a depth micrometer and parallel bars of the exact length that fit down into the friction area, if you really cared enough.
Latest clutch backing plate for the /5 BMW motorcycle
The friction side
Here you can see why it is so much stronger and resists warping. It does not work on most /2’s without a small modification to the ribs at the front of the transmission case. Do not install it with the spacer washers.
BMW motorcycle backing plate wear
A perfect used “mid” plate.
It shows some wear, but it is still useable.
This is an early type backing plate with some shallow grooves. While it looks awful and I would try to replace it, this one isn’t very deeply grooved. I don’t have a “bad” one to show. This could be taken in and ground off, but it really isn’t worth it. If the customer were short of funds I would reuse this one. The disadvantage is that the grooves must wear the friction plate to “bed in” and that means a shorter life. It is common to remove a perfectly working clutch and find grooves much worse than this.
E. Measuring the /2 BMW motorcycle clutch friction plate
Two types of friction plate are available. A “cheap” version has no spring in the middle. It was an after market part. The original /2 clutch friction plate is a bit hard to measure because because of the spring. The spring just naturally spreads the friction parts apart a bit. I use calipers gently and a new one is about .350,” or more, in thickness. It is often referred to as a 9 mm plate.
Photo by Bernd Kupper, thanks.
See the slightly curved metal in the middle of the sandwich? That is the spring. All of the motorcycles were originally supplied with this type of friction plate. It has a softer action than the cheaper one. I have used both and slightly prefer the one with the spring.
It can only wear down to the rivets and then it is eating up the diaphragm and backing plates. I would replace it at about .020″ from the rivets, as it is just too much trouble to get to it and the risk to the plates is high and they are really expensive. The wear will be greatest at the outer edge. The friction material on the /2 plate can get saturated with oil and I know of no way to extract it out.
The /5 plate starts out at .240″ thickness. It is bonded, has no rivets and the “useable” wear amount is actually greater. I have pulled them out at under .200″ at the outer edge and they weren’t slipping yet. Oil can be washed off easily.
F. Measuring the BMW motorcycle diaphragm plates
The wear on a diaphragm plate is much harder to measure. The sheet metal on the back side is not flat, so a micrometer won’t tell you much. We never worried about it and just guessed, by comparing it visually to the backing plate. Deep grooves and/or extreme tapering are both bad.
Two types exist and this is how I measure them to see which is which.
The /2 one on the left is .536″ deep and the /5 one on the right is .417″ deep.
G. BMW motorcycle clutch spring
This is a typical clutch spring used from late 55 up thru the early 80’s. See the red spot of paint? See more below.
Here are two springs laying on a flat table with the edges against each other. This is to compare for wear. One must rotate them around a bit to get the average. The plane of one side isn’t necessarily parallel to the other side. In the shop, time is money, your money and this is a quick way to measure a spring. We always had a new one around for a comparison. Neither of these are new, but one is nearly perfect and the other fairly worn. When the difference in height of the two springs is the thickness of the spring, then we replaced it. This one is almost to the wear limit. The spec height is 17.7 mm. Just measure down to the surface of the table, in a few places, and see if it is still in spec. It should take 165 kg to press it down to 11 mm in height. We never did that.
As a general rule, we found that with normal riding, one could reuse a spring once and sometimes twice with each clutch friction plate replacement. The wear on a spring is in two places. Most of the wear is on the ends of the finger tips. Some is on the backside, but not much. Just being compressed for years will make it flatten out.
These tips are nearly perfect.
These tips are badly worn and replacement should be considered. Measure it.
In the first spring picture, one can see some red paint that is a +. Here is a close up. This mark shows that it is the heavy duty spring.
This spring has two clips wrapped around the outer edge. This is the “top” side.
This is the bottom side and the clip looks homemade. I wasn’t able to find out what this is about. If it were to make the whole spring thicker, to extend its life, then I would have thought it would have several clips added, not just two. Has anyone seen this before?
H. BMW motorcycle clutch actuating arm and bearing, 1956-67
The overall condition of the /2 clutch could originally be seen from the outside. If the bike had 40-60 k on it and the original clutch, this would be an accurate indicator. Now, with an unknown history, it is anybody’s guess.
See the very small “free space” between the lock nut and the adjuster bolt? If that space is the amount of the thickness of the lock nut, or less, then the clutch “system” is fairly worn. The wear could all be in the plate or any one part, but is usually a combination of wear of all of the parts. I have seen the lock nut removed to get more “adjustment” out of the clutch. I have seen the bolt get replaced with a longer one to get the “adjustment.” It is all futile, except as a very temporary measure to get home. The risk of rivets grinding up the two plates isn’t worth the “savings” of more than the few more miles to get to a repair place.
This clutch adjustment (1955-67) is an indication of a healthy clutch. Both use a 10 mm wrench.
The clutch actuating arm in 68 & 69 BMW motorcycles with the “throw out” bearing.
The clutch adjustment bolt that you see in the picture is the one that was changed in 68. It is a larger diameter and the head is 8 mm, the same size as the bolt shaft. The arm has a larger hole in it to accommodate the larger diameter bolt. The nut is of course larger too, 13 mm, but is still quite thin. More threads show with this arrangement than the older one. Can you see that between the locking nut and the head of the bolt, there is room for about two of the nuts? This “free space” shows that the total of all three plates is still large enough for one to be confident of decent clutch life. At the same time that the clutch arm adjustment bolt was changed, the rest of the transmission was altered, as was the flywheel. Click here for more about the clutch and other controls. This is how it looks if you could “see thru” the transmission case. The black colored item is a seal. This one is shot. The item on the shaft just just about an inch to the right of the bearing mechanism is a felt seal. It is supposed to soak up any slight amount of oil that creeps past the seal. It does little good and only for a short time.
Here are the parts for the BMW “throw out” bearing. From left to right; clutch arm, bearing rear race, balls and retainer, bearing front race and the clutch rod.
The clutch actuating bearing is sometimes called a “throw out” bearing. It is very reliable and rarely needs to be replaced. I can only remember seeing a couple of bad ones, usually from water rusting the parts. This is a rare example of the bearing after the tranny has had water sitting in it. These parts are junk. The water gets in because the rubber boot that covers the speedometer cable is cracked and allows water to run down and into the case. The case can fill up with so much water that it completely fills the case. It is a good idea to make sure that the boot is in good condition. Replace as necessary. Additional protection can be had by filling it with grease. Rotate it once a year to check for cracks.
BMW provided only one type of clutch rod. It is “captured” and can’t be removed without removing the transmission. In the early days of the /2, the transmission wasn’t vented and oil could escape and flow along the inside of the input shaft. It wouldn’t be stopped by the felt ring on the clutch rod and would migrate along the rod and onto the clutch. Earl Flanders invented a three piece clutch rod. The advantage is that it had an “O” ring that worked much better to seal the oil from migrating forwards. One could remove the original clutch arm from the rear of the tranny and pull the clutch rod out a bit. It could be cut off and pulled out further and cut off again. Finally it would be all of the way out. The three piece rod could easily be inserted into the input shaft.
Finally BMW solved the original problem by venting the transmission. Then, some of the oil could come out of the vent hole. They fixed this by machining a spiral groove on the speedometer drive gear to “work” the oil back downwards.
Here you can see an example of the old gear below and the new upper one with the spiral cut. Photo by Bernd Kupper, thanks.
I. The BMW motorcycle clutch push rod
The purpose of the clutch pushrod is to apply pressure to the diaphragm plate. This disengages the clutch for shifting. The rod must be long enough to do the job. As the rod and all other clutch parts wear out, the play must be taken up with the adjustment.
On top is the BMW clutch rod. The BMW rod has the felt seal near the rear end. The felt seal is to stop oil from traveling along the rod and getting onto the clutch plate. That would cause it to slip. The /2 plate can’t be easily cleaned up and reused once it is saturated in oil. The /5 and later can easily be washed off and reused. Oil doesn’t saturate it. The best way to insure that oil doesn’t get on the /2 clutch is to make sure that you have the newer type speedo gear and the vented bolt. The felt can be replaced, as a new one has a slit along it for installing it. The clutch rod can be easily inserted from the front of the transmission. Never overfill the transmission with oil. Best to only fill to the lowest thread, or even a bit lower. Lower is better than higher.
The lower one is the 3 piece Flanders /2 clutch rod. I only mention this as I suspect that a few of them are still out there. You might find this thing with the ball in the middle to be a bit confusing. The Flanders rod has an “O” ring, rather than the felt seal, to stop oil migration along the rod. I never saw a problem with the three piece rod. I have seen many that were a bit rusted up, so I know that they weren’t getting accidentally oiled by leakage. That is a good finding.
This wonderful 3D sketch was created by Chris, thanks so much.
I have seen the tit be about one mm long. That means that the effective length of the rod is that much shorter. The corresponding hole in the diaphragm plate must also be inspected for excessive wear. That hole can get wallowed out a bit. That also makes things “shorter” like the worn pushrod. I have no pictures of a worn one, sorry.
Just back about 1/2″ from the tip is some odd wear. It is slightly smaller in diameter than the rest of the rod. It was caused by the input shaft front bearing loosing it’s ball retainer. The balls are sort of loose in the space and allow the shaft to move around a lot. The common reason for that bearing to die is because the /2 rear engine main has slightly failed and is loose. The input bearing, tiny by comparison, can’t take the load and fails. I have seen a rod have it’s diameter reduced by 1/3. This rod is still good and the damage is only cosmetic. The rust is because when they are removed they are shiny and very clean. Exposure to the humidity in the air causes them to rust. It is harmless. In theory they aren’t supposed to turn, but evidence to the contrary is common. Inspect the small throw out bearing for rust and wear. Rust on the bearing will make it fail eventually. It can be inspected without removing the transmission from the bike.
J. BMW motorcycle clutch bolts
If you decide to replace the older type beveled clutch bolts with the newer Allan head bolts, be careful. The earlier ones are flush with the surface of the backing plate. The later ones stick up a few mm and can hit the webbing on the front of the transmission. That will either make noise or cause the engine to lock up. When you first bolt the transmission to the engine, make sure that the engine turns without hitting anything. If you wait until the job is finished, you may be very unhappy to find that you have to do it all over again. At least this way you won’t have wasted a lot of time.
K. BMW motorcycle flywheel bolts
The photo shows a sheared off flywheel bolt on a 1974 R90/6. The rider was just riding along in traffic and all at once the bike revved up and wouldn’t go. Inspection showed that the 6 flywheel bolts had backed out and they finally broke off just between the flywheel and the crankshaft. Had this been opened up before the failure, the bolts would have been found to be quite loose. The holes are wallowed out a bit and that is proof that it was loose for some time before shearing off. The flywheel is actually useable, but since they are so cheap, I would prefer finding a good used one. The more serious damage is to the crankshaft. Those holes are even more wallowed out.
Why did this happen? We can’t know for sure, but I suspect that they weren’t properly torqued down. Had properly tightened, but defective bolts sheared off, the holes wouldn’t be wallowed out. We preferred to use new bolts each time, as they are cheap when you consider the potential damage. This failure would have been so expensive to repair that instead, it is now salvage.
L. BMW clutch installation
The various books call for a clutch alignment tool to be used when reassembling the clutch. That tool is available from Cycle Works and saves a bit of time. It is nice for a BMW shop to have. For the occasional BMW mechanic it is unnecessary to have that tool. Hand tighten the 3 long clutch bolts so that one can still grab the friction plate hub and move it with some effort. Then use the actual transmission to align the clutch. Just slide the transmission into place and move it around until it fully seats. Then very carefully slide it back so as not to move the clutch friction plate. Now, tighten up the 3 starting bolts enough to start the 3 shorter bolts and tighten them. Remove the 3 starter longer bolts and install the last 3 short bolts. Now tighten all 6 screws. The transmission should go back in easily. If the /5 and later transmission “seats” in place, then all is aligned and you are done.
Some mechanics just use a mirror to hand center the friction plate and then tighten the bolts down.
M. Transmission alignment on the /2 BMW motorcycle
Sometimes a /2 BMW motorcycle will have quite a rattle in the transmission when hot and at an idle. Pull the clutch lever in and the noise will go away. That rattle may be indicating a normal situation, a serious problem or a transmission that is out of alignment. The /2 transmission must be aligned after it is installed. The noise is coming from the error between the input shaft and the clutch hub splines. In time, this can wear the splines on both parts. I have received several emails from owners that did this alignment procedure and greatly reduced the rattle at idle. The /5 and later transmissions do not require alignment. The transmission case has a dowel locating pin and so the case can’t move around.
Here is how to align a /2 transmission. Slightly loosen the 4 fasteners of the transmission to the engine. Keep them close to finger tight. You only want the transmission to be able to move freely. As a test, grab the transmission, by hand and move it around. It should move sideways about 1/2 millimeter or so. Start the engine and run the rpm up to between idle and mid range. Pull the clutch lever in several times. With the clutch lever held in and the engine running, reach down and tighten up a couple of the fasteners. Now it is in alignment. It centers itself. Tighten them all up. As usual, they don’t require much torque. They only hold the transmission in alignment. Test it for noise again. If it is still there then you may have a more serious problem. A poor state of tune will cause you to hear this same noise. One reader recently reported that he had to actually ride it around the block and then tighten the bolts. That fixed the noise.
N. Your BMW motorcycle clutch is “stuck”
It is not uncommon for one to take a BMW motorcycle out of storage and to discover that the clutch is stuck. The lever will pull in, but the clutch doesn’t disengage. This can happen in less than a year of non-use. It happens more in humid climates. I have removed a few stuck ones and find lots of rust and crud that sort of glues the parts together. One can remove a clutch that is working perfectly and see the rust marks from an earlier “stuck clutch.”
The solution is very simple. Ride the bike. You are thinking, “How do I get it into first gear?” and that is a good question. The same as you would if your clutch cable is broken and that is on my page about cables and controls. Basically, start the bike up and let it get warm. Point it in a direction where there is nothing to hit. With the engine at a low idle speed, take the bike off of the stand and get ready to ride. Move the bike forwards by foot and gently shift it into first gear. Now you are moving along. Shift up as you naturally would when increasing speed. Within a few shifts the clutch will break loose. The rust will quickly wear off and get thrown off of the friction plate.
O. Modification of the clutch for an “easy pull” lever.
The /2 clutch lever pull was always easy. The /5 was a bit heavier, but in 74 the new /6 clutch lever pull was excessive. Several modifications to the clutch pull mechanism have been devised. Google will help you find current mods for the /6 excessive clutch lever pull.
We all know that “There is no free lunch.” So, what is gained and what is lost? The “pull” required after this modification is reduced to about 1/2. That is good. The actual amount of travel down at the transmission is reduced by the same 1/2. That is bad. The fact is that the original design has way too much travel. Losing 1/2 isn’t important, but it is more important to keep it in perfect adjustment. The free play at the lever must never exceed the specified amount. At the far end of the lever, it should not move more than 1/4″ (6 mm) before the play is gone. The play will change from the time it is stone cold to really hot. In both cases it must have some free play, or you risk burning up the clutch.