F650 Sprag Clutch FAQ

compiled & edited by Kristian #562
Please read the Disclaimer before attempting any work in this FAQ.
Last Updated: 11 June 2006, by Winter #1935

• Introduction
• Sprag Clutch Misc
  • Symptoms
  • Advice
  • Sprag Clutch Replacement Instructions
  • Questions
• Sprag Clutch Minced

For other related FAQs:

• For Details of Flywheel Cover Removal, refer This FAQ
• The Rotax 655 Troubleshooting FAQ contains an (older model) diagram of the starter motor including the sprag clutch.
• Also the Engine Misc. FAQ

The sprag clutch acts as a one-way clutch to allow the starter motor to drive the crankshaft and thus start the engine. Once the engine starts, the crankshaft is turning faster than the starter motor can, so it basically "free-wheels" (BMW manuals typically call the sprag clutch the freewheel). Think of the sprag clutch like the rear hub on a bicycle, the freewheel (the part that clicks) as you coast down a big hill with your feet (the starter motor) remaining still. On the left is Flash's pic of the flywheel, the starter Gear and the Gears in between. The sprag clutch is in the gear behind the rotor/flywheel, for the sake of the starter. VERY few instances of problems with this item recorded. (thanks to norbrat and malsin)

The hub, rotor and sprag clutch are secured by about a dozen bolts. This special hardware has a rated shear strength, so that they will shear off rather than damage the woodruff key, crankshaft, or possibly snap the drive chain. These bolts & nuts should be secured with Loctite threadlocker so they don't work their way loose, catch on the stator and shear off while you're out on a nice morning breakfast run and can't figure out why your bike isn't getting any spark. kevmeister

• I bought a 1997 classic last summer. It seems that the sprag clutch engaging the starter has some serious problem. If for some reason the engine doesn't fire up on the first or second turn the starter just slips away. I can hear the electric motor running but the engine stands still. The other indication of trouble is that when I stop the engine and the crankshaft turns probably just a couple of degrees backward it sounds like something just breaking in the engine. I checked the three gears between the starter and alternator rotor and they are OK. Unfortunately, I didn't find anything about the sprag clutch in the FAQ. BMW Manhattan gave me a price for the roller-holder assembly as $121.03 (12-11-2-343-296) and the gear as $233.04 (12-11-2-343-294) I also was told that they do not sell tools like the rotor puller (12-5-510). My questions. Did anyone have similar problems with the sprag clutch? Any comments are welcome. Jenoe
• I work on the BMW F650 for the Police Department in Barbados. I had problems with the one-way Starter Clutch (Sprag Clutch) on Three motor cycles. When I dismantled the Clutch from the Flywheel I saw that the Spring around the one-way clutch was broken. I replaced the spring with one from and old oil seal and it worked perfectly for a short time. I haven't dismantled it as yet to see what is the current problem. I am currently searching for a one-way clutch from an automatic Transmission which could replace this. I believe it should be stronger than the original one-way clutch. Shawn737
• Hi, I received a lot of information from your site which I found very useful in servicing The Police Fleet of vehicles in Barbados. I currently have a problem with a bike that occasionally cranks when trying to start. The starter is fine because I cranked the engine with alternator cover off and I saw the gear spinning Independent of the Flywheel but occasionally would crank. I found the problem was the Sprag Clutch. Shawn. The Royal Barbados Police Force
• If you're bike still starts and runs, it's NOT likely the same problem. It can't hurt to pull the right side cover and have a look. Look & feel around in there to see if you find any stray hardware. Check the 8 bolts that secure the freewheel gear to the alternator shaft rotor. Behind four of those are nuts that secure the cover over the sprag clutch. To check those, you'll need an alternator puller tool ($25). I have one I can mail you, if you'll send it back right quick. kevmeister #1715

• I doubt anything is wrong with the gear. Probably just the sprag clutch is dorked up. If I were you, I would get the puller and pull the rotor off, remove the sprag clutch and gear and SEE what is wrong, then order the parts you NEED. You might find that some foreign object is in there and simply extracting it will fix your problem.
• Jenoe wrote "It seems that the sprag clutch engaging the starter has some serious problem. If for some reason the engine doesn't fire up on the first or second turn the starter just slips away. I can hear the electric motor running but the engine stands still." It could be a couple of things (1) Your battery might be a little on the low side which would then not create sufficient power or inertia to throw the gear onto the flywheel starter ring after a couple of attempts. (2) The splined path that the smaller gear on the starter motor travels along may be dirty or corroded preventing easy or full travel of that gear onto the flywheel starter ring. I'd look at those before doing any dismantling. When I stop the engine and the crankshaft turns probably just a couple of degrees backward it sounds like something just breaking in the engine" That's normal - It's just the valve de-compression linkage clattering. You can sometimes hear it at low engine speeds. Trevor 999 UK '01GS
• Trevor, it seems that you have the same assumption that I had before I removed the right side engine cover. Namely, that starter on an F650 works somewhat similar then on cars with a small starter gear sliding into a bigger gear attached to the crankshaft. To my surprise it is not the case. In the sprag clutch assembly there is a bunch of rollers (just like in some bearings) running on smooth surface one side and on a faceted one on the other. Each segment of the polygonal surface slightly tilted into the direction they want the clutch to be connected. They create a kind of wedge where the rollers can stuck moving the smooth surface with them, therefore turning the other ring. I believe that in our case the smooth ring is inside and the segmented one is the outside. Jenoe.
• Once you pull the right cover, you should be able to spin the big gear on the rotor one way but not the other, if the sprag clutch is working properly. There should be no "graunching" feel or noises, only smooth operation in one direction and NONE in the other. Flash 412 (CO)

A Few Important points before you read this:

1. I recommend you have a Clymer manual on hand before doing this job,
2. I wish to inform you that I am 'not' a mechanic and have made the following observations thru carrying out this work myself.
3. As Mal has stated, "this is a precision piece and one which he recommends you replace with a new part".
4. Take Mals advice seriously, it was only after taking the scenic route (and wasting a lot of time and a starter GBP 93 + postage) in trying to fix the old one that I then heeded what he said, to anyone else save yourself time and do the same.

Parts and Tools

There is not a lot of info on doing this job however, you will need the following items to carry out the task,

1. New sprag clutch (freewheel) cost approx. 60
2. Replacement Sprag cover cost approx. 5, and while you're there get replacement nuts for the cover,
3. RHS cover Gasket and also some instant gasket may be required
4. Top dead centre bolt, not sure of cost as we made our own,
5. Pullers, once again not sure of cost as we made our own. Note: YOU WILL NOT BE ABLE TO DO THIS TASK WITHOUT THE PULLERS
6. Loctite or some similar nut locking compound (medium strength)
7. A good socket set
8. Well lit and clean working area

Procedure

1. Remove the sump guard, then remove the sump plug to drain the oil from the bottom of the engine. You can also drain the oil from the frame, however if its not due to be changed you can get away without doing this.
2. Next remove the right had side cover, I also disconnected the three yellow wires going back up to the regulator so I could get the RHS cover completely out of my way.
3. Remove the bung on the left hand side of the engine to insert the TDC bolt, make sure you have the crank in the correct position and insert the bolt to lock the engine in place.
4. When this is done you can remove the rotor nut and then set up the pullers to remove the rotor / flywheel. Before doing this you may wish to take note of the set up of the gears from the starter motor down to the fly wheel, also note the small washer and spacer, ensure these go back on as they have come off. Also when removing the rotor the flywheel may come with it and thus so will the two other gears. If this is the case just take your time and work everything off together.
5. Now with the rotor off, put it up on the bench, the fly wheel will just pull out of the rotor without much required force, and there it is your little sprag clutch (freewheel as BMW call it).
6. Remove the three nuts and take off the sprag cover, you should then be able to remove the sprag with your fingers. You may find upon removing the sprag that the outer spring (similar to an oil seal spring) has broken and thus the bearings within the sprag have come loose, and this my friends is that horrid noise that you've been hearing.
7. Now just do everything in reverse making sure to give all moving parts a rub of some fresh oil before and during assembly, this may avoid damage upon starvation of oil on start up. Insert the new sprag, you will find that it will go in both ways however, it needs a push to put it in one way (wrong way) and will just slide in the other way (correct way), put on the new sprag cover, a drop of loctite on the bolts and on with the new nuts.
8. Next you will need to take your extra couple of arms out of your pockets and put back on the rotor and gearing from starter to flywheel. It is here that you will be thankful that you took a note as to what order everything goes on. ENSURE THAT YOU GET THE WOODRUFF KEY INTO THE SLOT CORRECTLY. At this point see if you can move the flywheel, it should move freely in one direction and not move at all in the other. This is easier to check if the starter has been removed.
9. When the rotor is in place, reinsert the rotor washer and nut and tighten. Flash on the chain gang site has said that you do not need loctite on a taper, I have duly taken his advice.
10. When this is done put on your new gasket, perhaps also a small amount of instant gasket and reinsert the RHS cover. Instant gasket will not suffice on its own as the paper gasket is required for the correct gap between the engine and cover. DON'T FORGET TO RE-CONNECT THE THREE YELLOW WIRES GOING BACK TO THE REGULATOR.
11. Remove the TDC bolt, put back in the bolt, top the bike up with oil (make sure that bung is in), put back on the sump guard and well thats just about it.

As I said earlier I am not a mechanic, and thus this job took me a long time, however I reckon that if you've got all the above ingredients at the ready and also know the difference between a ring spanner and a socket set you should be able to complete this job in about 1 - 2 hours and thats taking your time!! Hope this gives you an idea as to what is involved in replacing the sprag clutch (freewheel). collo_man

The Spring from my sprag clutch is broken

When I removed the sprag clutch the spring was broken and I replaced it with a spring from a crankcase oil seal off a mini. This spring was very slightly wider than what had been on the sprag but I thought that maybe I'd get away with it!! Could this cause the non-release of the starter when the bike fires up? collo_man

• Of course. If you saved the old original spring, you might drop round to an industrial supply place and see if they have something that better matches the original. Or... you might spring 150 punts or so for a new sprague clutch ("freewheel" according to BMW). Flash 412 (CO)

What are the differences between the GS and Classic?

• There is a difference in the Classic & GS/Dakar engines where the sprag clutch and alternator are concerned. The Classic has a 280 watt stator and the hub, rotor, and sprag clutch are secure by 8 M6x16 bolts & 4 M6 nuts. The GS/Dakar has a 400 watt stator and a bigger hub, rotor, and sprag clutch secured with the bigger M8x16 bolts & 4 M8 nuts. kevmeister

I decided to pull the right-hand side engine cover to visually inspect the contacts of the ignition sensor and the stator.

Huh? I watched Flash pull the alternator on the F650 Issue 2 DVD, and the sprag clutch housing should not be out yet. Before I even got the cover off, one of the sheared-off bolts fell on the ground. Uh-oh.

Looking under the sprag clutch housing, I find all the other sheared-off bolt heads, along with a bunch of chewed-up insulation from the windings.

The sprag clutch housing comes off easily enough revealing a pretty chewed up stator. [Ed note: You can see the three yellow wires for the stator / generator on the right of this image]

Pulled the nut that secures the sprag clutch assembly to the rotor shaft. It came off with a good amount of effort. There was plenty of threadlock residue, a good sign.

This nut fell once the housing came loose.

Here is the back side of the sprag clutch housing. There are only two LOOSE nuts attached the sprag clutch cover. The other two were completely off. All those nubs are the sheared-off bolt studs. I don't find any evidence of threadlocker on any of these bolts.

The Clymer manual has this to say about the sprag clutch bolts in step 2 of the Starter Clutch Disassembly (p. 145): "The freewheel cover nuts are secured with a high-strength threadlocking compound and may be difficult to loosen". And again in step 3 (p. 146): The eight Allen bolts have been secured with a high-strength thread-locking compound and may be difficult to loosen... These fasteners are NOT difficult to loosen. The remaing two nuts are completely loose.

Here's another angle

So, making my list of replacement parts, I have a damaged magneto, obviously.

And a dinged up hub

Here's the lineup of suspects. Which of these is the most serious offender?