Changing the Sprag Clutch
in the Starter Mechanism:

  • NOTE: This Information is provided in good faith, but no responsibility can be taken by the Hesketh Owner's Club for any damage or loss suffered, howsoever caused, should readers choose to use this information.  Please read this through before you start.  Only start once you're 100% certain that you have both the tools and the skills required to finish the job satisfactorily.

  • Drain the oil into a clean container, for re-use (unless you wanted to change it anyway).

  • Whilst the oil is draining, MAKE CERTAIN THE BIKE IS IN GEAR and then take the LHS footrest and mount off and release the gearchange pedal shaft from the gearchange shaft. (There may be a small metal key between pedal and shaft - if there is don't lose it!).

  • Disconnect the alternator wiring from the main harness (Three bullet connectors under the battery tray.)

  • Remove the thirteen M6 allen bolts securing the LHS engine casing ("Drive Side" casing) and remove the casing.  (Don't worry about the circular generator cover, or the oval clutch adjuster cover, they'll both remain attached to the engine casing.)

  • You may find it useful to remove the oil filler and insert a "lever" between the clutch and the casing. Whatever you use, gently does it and careful with those threads..!!

To replace it, you'll first need to get one... It's a Borg-Warner part: BWX1310003 known as a "freewheel" or "one-way clutch" or "sprag clutch". Hesketh Motorcycles will almost certainly have some stock. (Failing that, a Mini (Austin) gearbox specialist may be able to help, as it started out originally fitted in some of the auto boxes fitted to that model).
Once the casing pops off, you'll see (from left to right), the starter pinion, the starter reduction gears, the starter chain and sprocket (behind the alternator rotor) and the clutch.

There's a big nut holding the rotor on and it's a LEFT-HAND THREAD!! Get a handy mate or spouse to hold their foot hard on the back brake pedal and undo the nut.

(This is why it had to be in gear...)

Now the alternator rotor may or may not easily come off...  If it's reluctant, you'll need a puller.

Make sure that the rotor shaft key is removed and retained for refitting.

 

Behind the rotor is a thrust washer, then the starter sprocket, which will lift away with the chain and last of the reduction gears...

Remember which way up the hrust washer is fitted and how close it is to the starter gear...

Behind this is the sprag, which is a push fit in its housing...


This is the broken sprag, sat on top of the starter sprocket.  If you look really closely, (Click to open a full size image) you'll see that the "rollers" in the sprag are all leanng toward the right... This is because these rollers have all been forced to rotate past their normal working position.  This can only happen when the tolerances between the sprag, the inside of the sprag housing, and the outside of the starter sprocket are wide enough to allow it.

Clean and Check everything before installing the new sprag

Borg-Warner give the following dimensions:

Sprag housing I/D is 66.383 +/- 0.013 mm

Shaft diameter (O/D of starter Sprocket): 49.721 +0.008 or - .005 mm.

If either the housing or the sprocket doesn't meet specification, don't bother fitting the new sprag... The part which appears to be most commonly subject to wear is the Starter Sprocket... It's also the easiest to replace, but you'll need to be sure that the replacement meets spec as there is evidence of widely varying manufacturing tolerances... (Yes, really...)
 

 

Old Sprag (LHS), New Sprag (RHS).

The new one is supplied in a plastic former to prevent it becoming deformed or distorted in transit...


When replacing the sprag, remember that it can be fitted either way around, but only one way is correct..!  The engine turns clockwise when it's running (viewed from the clutch side) and the sprag must lock up when the starter sprocket is rotated clockwise.

(Thus allowing the crankshaft to "freewheel" inside the sprag when the engine starts and accelerates to a higher rotational speed than that of the starter sprocket.)

  Test  for correct operation by fitting the starter sprocket without the chain and reduction gear.
 

I'm converting some MP4 videos into MP3 or WAV files...  More ASAP

 


**  Reassembly is very much the reverse of disassembly, but here are a few useful tips to help you along the way  **

 

When fitted correctly, there should be no gap between the sprag and the thrust-washer behind it.
 

Refitting the starter sprocket/chain/reduction gear can only be done as an assembly
(unless you dismantle the primary starter reduction gears).
It's fiddly, so deep breaths and plenty of patience required.
 

Don't forget to fit the rotor key.
 

The alternator rotor nut holds it all together and should be torqued to 50 lbs/ft2
 

Starter sprags wear out (unless you never start your bike).
This problem is not specific to the V1000, check on-line for forums or blogs discussing the problem on Aprilias, Ducati's and other well-known makes... The sprag is an optimal solution for the problems of starting a large pistoned engine, with a small electric motor.  But, they wear out...

It's progressive wear though and the eventual failure of the sprag is normally precipitated by some sort of "catastrophic" event - such as an enormous backfire (like the one that finished my sprag off).  So, after you've spent all this time changing your sprag, please don't simply hit the starter button and let another huge backfire ruin all your good work.

Things to check:

** Ignition Timing - Static Setting **

** Carburettor Balance - Mechanical Balance **

Not foolproof, but well worthwhile...

Once you have the bike running again, check the ignition timing dynamically and use vacuum gauges to balance the carbs ASAP.