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Discussion Starter · #1 ·
This article is obviuously biased towards the Ducati Desmoquattro engine, but it is applicable to any bike using a sprag starter mechanism (The majority of large capacity V-twins).
The sprag clutch in the early Desmoquattros is the same unit as employed in the BMW GS650 (possibly others), KTMs and Aprilias.
Here is a description of a sprag clutch, how it works, and how to shorten the spring on one should you need to.

Symptoms: You press the starter button, you hear the starter whirr, but it does not engage the motor.
The problem occurs when the detent spring used to hold the sprags in their basket weakens or breaks. Unfortunately, the OEM spring is very weak, and causes this common problem.

The first step is to drain your fluids.



Then remove the clutch cover, pressure plate and clutch plates.
Once you have removed the clutch cover, locate and remove your clutch springs, I have 4, you'll probably have 6.


Then slide off your pressure plate.


To help in the removal of the cluth plates, I have made up a couple of picks that I can use to hook the plates out with.



I like to place everything in the order that it is going back on.
i.e. left to right, face down.


Once that is all out, install your clutch holding tool.
I made this from some old clutch plates. If you make one, use high tensile bolts, and drill and bolt the handle too, don't weld it. If you don't you will shear the bolts or tear the plates were the weld attaches.
Note the tape to prevent scratching.
Some people use a penny between the gears to hold the motor whilst you undo the flywheel nut.
I don't.


Now undo the ancilliary attachments on the generator cover.
You need to remove the 3 coolant hoses, the clutch slave cylinder, the gear shift and undo the 2 crank angle sensor connectors.
Mark them so they go back on the same way.


Some people recommend a puller to remove the case, but a few taps with a hammer (especially behind the waterpump cover/spiggot) and it should pull off relatively easily.



Get a 30mm sprocket and a breaker bar whilst getting someone to hold the clutch tool.
These nuts are on tight, so they can take a bit of effort to get off.


You shouldn't need a puller to get the flywheel and generator off, I have never needed one.
Once again, lay everything out the way it came off.
On my bike, the "ducati" on the magnet faces towards the motor, and the washer is convex.


Around the back of the flywheel, you will find a big circlip.
Remove it.


You should then be able to remove your sprag clutch.
On mine, the spring had lunched itself and was preventing me from pulling the sprag out. As I had a replacement spring, I ripped it out and then the sprag clutch just slid out.


As you can see, spring has had it.


Here is a CR19778 seal from SKF bearings.
Luckily, it uses a perfectly sized spring for a Ducati sprag clutch. I decided against shortening the spring as the wire gauge is a lot heavier than the OEM spring, and therefore will hopefuly stretch less.
At this stage, you can either replace your sprag, replace the spring, or shorten it as in the link above.


I removed the spring, and then wrapped the sprag clutch in masking tape to stop all the sprags falling out, I then cut a slot in the paper where the grooves in the sprags are, and replaced the spring, making sure the spring was sat way down in the grooves, and all the sprags are seated correctly.



/snip (24 images)
 

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Discussion Starter · #2 ·
One thing to check is galling on the inside of the flywheel, the sprags themselves and the starter gear.
As you can see, my gear is buggered, I just can't afford a new one at the moment.
When I pull the bike down in a few months time, I'll replace the gear.


Here's what it looks like sans flywheel etc.



Re assembling the bike is pretty much the reversal of pulling it apart, just remember to line up the dot on your flywheel with the key way on your crank, or it won't start.
Also, I use a fair amount of blue loctite on the retaining nut, on my model, this is done up to 146 ft/lb, although YMMV.
These things have a habit of coming lose and flycutting the inside of the cases as well as throwing heavy bits around inside the motor.
Expensive stuff.


Also, when it's at this stage,

thumb the starter to make sure that it is all functioning.

For reassembly I use loctite brand 3bond, just a thin layer on one side of the casing,
I also find that removing the water pump cover makes it a lot easier to line up the drive pinion.


Remember to do up all your nuts, bolts and fasteners in a criss cross pattern, I also like to run a tap/die over everything before reassembly, but I'm a pedant.
HTH
Jim
 

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Nice writeup!

//amullo
 

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Discussion Starter · #4 ·
Thanks :)
The bike will be getting pulled down to the frame in a few months, once I have fixed the gearbox on the R1 :rolleyes:
I was going to go into stack height adjustment, clutch spring shimming and crank angle sensor adjustment, but I thought it was out of the scope of this document.
Wen I pull her apart, I will document absolutely everything I do on her.
I'm a pedant when it comes to my bike, so I'm hoping to have a pictoral "by the book" account of the rebuild.
 

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Thanks for the tutorial. I'm not a gearhead, but at least I'm interested in learning as much as I can about the inner workings of the bikes, I find it fasninating. Maybe I'm an apprentice gearhead then. You did what I like to see in these step by step presentations; lots of big pictures and superinposed arrows to point things out, especially when identifying parts.
 

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Ugh

My bike was a little gimpy starting and after I put a lightweight flywheel in and sealed her up it wouldn't start. It really sucks doing the same miserable job two days in a row but thanks for the write-up it helped me from going insane when all I heard was the starter motor whirring!
 

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Thanks for the write up. I was able to get my sp up and running with the new spring from the shaft seal. It only cost me $5...if you don't count the days of labor.

I managed to put the sprag in backwards the first time. Then my starter went, I already had a replacement since its been on its way out for a while. The replacement starter was used, and apparently rebuilt, from ebay. After installing the "new" starter, I hit the button to hear a familiar coffee grinder sound...

wtf...I was dumbfounded.

After throwing tool and curses around the garage, I took a closer look at the starter. The starter had been jammed together with the magnet body section rotated 90 degrees causing it to spin backwards. I fixed the starter and now the engine fires up.

Thanks again.
 

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Does anyone know of a UK equivalent to the seal mentioned here?

John
 

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Discussion Starter · #10 ·
Glad this writeup helped people. I love the Internet :D
 

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Crap...I just changed the fuel filter/pump/strainer/coils/spark plugs etc etc in hope that it was my problem then I ran into this...last thing I wanted to do is dig in my engine to enjoy my 1098 again....damn it.

Thanks for the tuto.

P.S.

So basically all I need is that seal from SKF and I`m good to go?
 

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CR19778 seal from SKF bearings is definitely too small for a 1098...it broke at its seam after a few start ups.

Just FYI.
 

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A good maintenance. Why did you choose the 1995 Ducati 916 engine, two-phase system for your bike's year 2000 to manufacture early Classical World SBK beautiful.
 

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Desmogod,

In your writeup it dealt with simply replacing the spring which is the focus of this.

What other items did you take care of along the way since you were already in there?

What items outside of the spring, loctite, clutch tool, is needed? (i.e. gaskets, o-ring, sealants, etc.)
 

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Do you have a 1098?

The pictures and the step by step instructions were perfect. But, unfortunately, replacing the spring fixed the sprag for exactly 191 miles. I'll be shopping for a new sprag now.
 
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