Point-Seven-five wrote:The sprag locks onto the smooth part of #9.  

The friction between the sprag cams and the smooth part of #9 cause them to lock onto the smooth surface.  They grip the smooth part the same way the wheels of a train engine grip the rails under it.  Because of the way they are shaped, the more friction there is the harder they grip.  Once the engine starts the sprag is moving faster than the #9 gear and the cams release.  

Have you ever seen the type of door stop that flips down and holds the door?  The harder you pull on the door the harder it grips on the floor to hold the door open.  The sprag works on the same principle.  Each one of the cams in the sprag is like that door stop.

Sorry for my total ignorens

And Viking (I don’t need no BS)

Do the teeth on my picture in last post, with the 3 red arrows lock on the blue arrow or the red arrow part on the drawing??????

And i ask in this rude no BS way, because I just don’t understand many of the technical explanations (sorry my bad).

But keep it KISS. Blue arrow or red arrow?????

The red arrow part.  The teeth of the blue part are connected to the starter.

The sprag is being driven by the smooth part of #9 to turn the shaft that drives the alternator and the crankshaft. That shaft runs through the center of #9. Once the engine starts the sprag floats allowing the #9 gear to stop turning and thus causing the starter motor to stop turning when the power is disconnected.

Point-Seven-five wrote:The red arrow part.  The teeth of the blue part are connected to the starter.

The sprag is being driven by the smooth part of #9 to turn the shaft that drives the alternator and the crankshaft.  That shaft runs through the center of #9.  Once the engine starts the sprag floats allowing the #9 gear to stop turning and thus causing the starter motor to stop turning when the power is disconnected.  

I’m actually so sorry but I just don’t get it.

I understand the teeth on #9 join the starter, but still have absolutely no idea where these teeth go (see picture)??”

And PLEASE don’t explain in technical terms just post a picture (even better with an arrow showing where the teeth on my picture in this post lock on to???”)

.

Danish biker wrote:
I’m actually so sorry but I just don’t get it.

I understand the teeth on #9 join the starter, but still have absolutely no idea where these teeth go (see picture)??”

And PLEASE don’t explain in technical terms just post a picture (even better with an arrow showing where the teeth on my picture in this post lock on to???”)

Here's a little something to expand on daveyson's description.

Those are not teeth at the points of your red arrows, so quit calling them teeth and quit thinking of them as teeth. They don't bite into metal. It is the curved surfaces of the sprags that wedge against the hub of Gear #9 and start it rotating Gear #3. Gear #3 rotates the crankshaft to start the engine. Sprags are designed to pivot inward hard against the hub when the starter rotates Gear #9 and pivot away so they just contact the hub when the engine starts. Their curved design allows them to exert force and relax force. 

It's like pushing a large truck with tall wheels out of the mud. There are guys in the rear pushing on the frame and guys on the sides pushing on the wheel tires. The guys on the side pushing the wheels are the sprags. They aren't biting the wheels; they are pushing on the tire tread and sidewalls trying to rotate the wheel. When the truck gets traction, those guys fall away from the wheel into the mud and then get up to walk alongside the truck to push again if it gets stuck. Likewise, when the engine starts to crank, the sprags fall away from the hub by pivoting to create less friction but they are still in light contact with the hub if the engine stalls and the starter uses them to push against the hub to rotate the gear assembly again.

We're failing here using picture with circles and arrows to explain this to you. Certainly, the Danish Army should have experienced mechanics to explain this in your native language but I'll try a couple of photos. These are from Alby's brilliant post. I linked you to this post way back in Post #17 of this thread when I reckoned you'd end up here sloshing an ocean of solvents at the problem.

Here's something else to look at before looking at a complicated clutch. 

daveyson wrote:
.

That is the best freaking drawing I’ve ever seen

That really clarifies e everything.

My brain just could not understand why my sprags have “cuts” (teeth) if they where not supposed to lock down (with the cut) on a teeth of a gear. In my wildest imagination I could not understand it would lock down on a total round (flat) smooth surface. I thought they where actually engaging something that with “clicks into the fine cut.

Sorry for my ignorance but until a few days ago I didn’t even understand it was the outside of the sprags that was turning the outer ring. I thought it was the inside of the sprags that turned something.

Laitch wrote:

You guys are incredible

As mentioned above my problem was I didn’t (till a few days ago) understand that it was the outside part of the sprags that in fact turned the outer ring and thereby starting the engine. I thought the inside turned something.

So I’m assuming that my spinning can be caused by:

1. Ware on the outside of the sprags.

2. Ware on the inside of the sprags.

3. Ware on the inside of the sprags cage.

4. Ware on the surface of the hub.

5. All of the above or more then one of them.

If I’m right I should get a new complete sprag but also #9 so the hub would be without ware.

Wow!
I have not disassembled or rebuilt a sprag clutch but with all the relevant descriptions, pictures and diagrams in this thread I am now well informed.
Thanks to you all for the detailed Sprag Clutch Technical Thread.
Shep

Update

I’ve been riding 3.000 km with out any failure

So what has changed?

The 75% diesel fuel and 25 % engine oil in the bike running at 2.000 rpm until the fan kicked in, didn’t do anything (when reading this I am thinking I must have been a wild man to dare try this).

The riding for 50 km, with 3.000-4.000 rpm, with 50% diesel fuel and 50% engine oil didn’t do anything (still thinking “wild man”).

So after taking all advice and trying it out no matter how dangerous it sounded, I decided to do one last think before taking everything apart and spending lots of money on a new sprag.

I took the sparks plugs out.

I took the crankcase cover off. Turned the crank till the first of the 3 holes in the sprag housing was visible. 

Then sprayed and sprayed flammable (only type I could get) carburetor cleaner into each hole.

Crankcase cover on. Hit the starter, with plugs still out.

Plugs in. Ran engine at 1.500 rpm for 5 minutes.

Drained oil and took filter out.

Put in a thin engine oil. It is a multiple oil used both for diesel and gasoline engines I got from a friend of mine who is a auto mechanic but also ride bikes and know about them.

I’ll keep riding on this oil the rest of the season and next spring go for 15x50 Mobile 1.

OK now its my turn, My mates 1993 75s developed this problem, crankcase off, turned engine over to access holes in sprag clutch, ONLY ONE HOLE FOUND is this right or do i need new glasses , comments and advise welcome

moriarti wrote:OK now its my turn, My mates 1993 75s developed this problem, crankcase off, turned engine over to access holes in sprag clutch, ONLY ONE HOLE FOUND is this right or do i need new glasses , comments and advise welcome

Shoulda gone to Specsavers........ 2 other holes blocked?

Eye test booked and probing tool ordered thanks for the reply

Imagine my feeling, seeing NO holes......

Then found out early ones have no holes!! Change to mineral oil solved it.

moriarti wrote:Eye test booked and probing tool ordered thanks for the reply

 
 

Actually they (the holes) can be hard to spot. Two of the are actually harder to see, since they are placed further back as I remember.

92KK 84WW Olaf wrote:Imagine my feeling, seeing NO holes......

Then found out early ones have no holes!! Change to mineral oil solved it.

Yes that would have been “bad luck” for me. Luckily mine had the holes.

Thanks for the CLUE on finding them D/R. Being a Mining Engineer (retired) you would think finding HOLES was my forte  Thanks for all advise from you all and NOT one sarcastic note of humour, YET

Up date Found all 3 holes, sprayed loads of cleaner through to no avail, Stuck solid no joy. A few hours later sprag on bench, you would not believe the crud in this thing all  rollers stuck solid. SO SCRUB A DUB DUB needed, a good hour cleaning then overnight soak in THINNERS,now looks like new. The bike was due a new clutch anyway so not much of a bind to go that extra bit for the sprag. NOTE to all, would it be a good idea to do an annual clean of the sprag via crank case access. Thus deducing the risk of failure. The amount of work/expense to reach this little bugger is considerable to say the least. Milage on this 75 is 88000 miles.What do you think

I would guess that sometime before you got it, your bike went a l-o-n-g time without an oil change and probably was running on crap oil.  Just curious, what does it look like under the valve cover?  My bikes all have clean top ends and no sprag problems.  There is nothing in the sprag that will make it load up with sludge before the rest of the engine.

Bottom line, I am pretty sure you don't need to do anything special to service the sprag as long as you are doing oil changes in less than 10,000km.

I agree on that!

Frequent oil changes seem to solve the problem. My Ks about to get their end of summer oil change.

Indeed, just watch the video in Danish Biker's April 13, 2021 posting. I didn't bother taking out the starter. Rather, I just removed the crankcase cover; sprayed in brake cleaner where shown in the video; and re-installed crankcase cover. Pressed the button and all was well.

Now, as several of you suggested, I will replace the engine oil with a diesel engine oil. Bothersome to me is that I have faithfully used BMW brand (expensive) engine since buying my '91 K100 in 1996 with 5,000 miles on it. I would have expected the brand would have the appropriate detergents. I have changed my oil every 5,000 miles or every riding season, whichever comes first.

Thanks, All.

ArtK wrote:Bothersome to me is that I have faithfully used BMW brand (expensive) engine since buying my '91 K100 in 1996 with 5,000 miles on it. 

Look at it this way. If you hadn't been using that oil, you might be gyrating in the same kind of desperation that Danish biker had to overcome through grit, persistence and Google Translate.   Sprag assemblies can wear and gunk can form regardless of the depth of our faith. 

ArtK wrote:
Now, as several of you suggested, I will replace the engine oil with a diesel engine oil. Bothersome to me is that I have faithfully used BMW brand (expensive) engine since buying my '91 K100 in 1996 with 5,000 miles on it. I would have expected the brand would have the appropriate detergents. I have changed my oil every 5,000 miles or every riding season, whichever comes first.

Thanks, All.

I'd say it's probably not a problem with the oil you use, although detergent type oils are better in the long run, but that it's more a function of your cold climate and the amount you use the bike ie short runs vs longer trips and how hot the engine gets to burn off any accumulated condensation in the crankcase.
A strong detergent oil like diesel oil should give your crankcase a good clean out but you shouldn't need to use it all the time imo. There are plenty of good detergent type dino oils and of course synthetics available.
Then you'll need to watch for oil leaks as removing all the crud can open up worn seals etc

So, I ran into this problem. Bike doing fine, starts up great, then suddenly one day NOPE. Starter spins and spins. It's a high milage bike (100k) and it's been through the wars.

Through a friend I found the video about taking off the starter motor to check if it goes clockwise and counterclockwise, and then spraying carb cleaner back behind the smaller gear in the engine. Nada. Did it again. Nada. Did it again. Nada.

Changed oil and pouted a little bit.

Then read on here to bump start it, ride it a good while, and try again. Bump started on a hill easily (my first time, mind you) and then went riding in this 90-degree weather.

Home, parked, closed my eyes and said a little prayer. Hit the ignition, starter spun and then suddenly VOILA. Engine turned over. 

This forum saved me a huge headache and expense. I'll prob do another oil change just to capture whatever gunk finally freed up. If it happens again, I know I've got some additional solutions to try before any kind of tear down.

Anyway, thank you all!