Hesketh Owners Club - Changing The Sprag - 2022 Update



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 and only start once you're 100% certain that you have both the tools and the skills required to finish the job satisfactorily

ALL of the pictures shown here are 'thumbnails' and if you 'click' on them a new page will open up showing a full-sized version of that picture.



To replace the sprag, 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".  These are often found in automatic gearboxes and quite a few electric-start motorcycles have similar starter mechanism designs (KTM, Aprilia, Triumph, Suzuki, to name a few...) You should be able to source a sprag using the Part Number above from any good engineering supply merchants. Failing that, a Austin-Mini (NOT BMW) gearbox specialist may be able to help, as it started out originally fitted in some of the auto boxes fitted to that model. If at all possible, get more than one quote, and don't forget to ask if VAT and shipping are included, prices and terms vary alarmingly..!


  • Drain the oil into a clean container, for re-use (unless you wanted to change it anyway).  Most V1000s have two engine drain plugs

  • Whilst the oil is draining, MAKE CERTAIN THE BIKE IS IN GEAR and then release the gearchange pedal from the gearchange shaft.  Then remove the drive-side footrest and mount from the frame and put the whole assembly safely to one side. (There may be a small metal key between pedal and shaft - if there is don't lose it!  Most V1000s have a splined shaft and pinch bolt assembly , so the key may not be there on your bike).

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

  • Remove the thirteen M6 allen bolts securing the drive-side 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 cap and insert a "lever" between the clutch and the casing. Whatever you use, gently does it and careful with those threads..!!

  • 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 the bike 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.  Refit the rotor nut, just a few threads, to prevent the rotor disappearing into the depths of your workshop as it flies past your ear...!

  • Once the rotor is loosened, remove the nut and rotor and make sure that the rotor shaft woodruff key is removed and retained for refitting.




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

  • Remember which way up the thrust washer is fitted and how close it is to the starter gear... Take pics if this job is going to be undertaken over any period of time...

  • Behind this is the sprag, which is a push-fit in its housing...  On the vast majority of V1000s the sprag is retained in the housing by a circlip.  This has to be removed in order to remove the sprag..



(I can't show a photo of this circlip, my V1000's 'Production Development' engine doesn't have the circlip - yet..!  But I do have the engineering drawings (Thank you David) showing the assembly):



  • (LEFT:) 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 leaning toward the right... This is WRONG and 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.

  • (RIGHT:) Old Sprag (LHS), New Sprag (RHS). The new one is supplied in a plastic former to prevent it becoming deformed or distorted in transit.


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...)


  • When actually refitting 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 drive side) and the sprag MUST lock up when the starter sprocket is rotated clockwise, and 'free-wheel' when the starter sprocket is rotated anti-clockwise, thus allowing the crankshaft to 'free-wheel' 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 - Here's an MP4 video (RIGHT) of me clumsily showing the correct fitment of the sprag:


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

  • use the drawings above to ensure you have the parts assembled in the correct order

  • 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.

  • Check the alternator rotor for cracks and damage. There is much recent evidence of old Lucas alternator rotors coming apart and the parts fitted to Hesketh V1000s are no exception to this. If you find that you need to replace yours, you may need to add the scribed ignition timing marks yourself, so don't throw your original away in haste..!

  • Don't forget to fit the alternator rotor woodruff 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 KTMs, 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.


  • ALL of the general service items, including; Ignition Timing, static and dynamic settings, valve clearances and carburettor balance;

  • Fuel condition, battery condition and charge level, battery terminals and starter cable condition

These are not foolproof checks, guaranteed to prevent your V100 from 'spragging', but they're well worthwhile and might help. If not, at least they'll help it run better when you get it started......