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Dodgy sprag

Discussion in 'Norton Commando Motorcycles (Classic)' started by Blackchicken, Oct 30, 2017.

  1. Blackchicken


    Oct 28, 2017
    I’ve just put in a new starter motor into my Mk111 Commando. When I try to start the bike the starter motor turns & the sprag clutch won’t engage. I’ve pulled the sprag out & cleaned it, it’s only been recently fitted & doesn’t appear to be damaged in anyway. Once everything is reassembled the electric start will often work for 2 or 3 starts then just won’t engage. If I kick start the bike & ride for say 1/2 hr & things are nice & hot, the electric start works perfectly with the sprag engaging, but once cooled down we’re back to non-engagement. Any ideas on what could be causing this?
  2. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Nov 20, 2004

  3. kommando


    May 7, 2005
    I copied this from a post made a few years ago which may be of use.

    This is slightly different to the Ken Merrick mod outlined on this website ages ago This mod increases the angle of the sprags so it has no chance of 'flipping' and consequently wrecking the sprag/drive gear/sprocket Obtain a LRT354220 hardened bearing inner race from your local bearing stockist (should cost only a few pounds, I got two for less than a tenner) Turn/grind the drive gear spigot (what engages with the sprag) down to 1.450" diameter. make sure you have a good radius tween the spigot and gear i.e. NO UNDER CUT You can do this with the needle roller still in place, just plug the bore turn/grind the bore of the your new inner race so it has an interference fit of 0.002" (1.448") chamfer the bore so it will sit over the radius Press the two together ,remount in the lathe and turn to length. Turn/grind the OD down to 1.632" (this is an increase of 0.007" over standard) radius the end so it is an easy fit in the sprag. Job Done Ken Merrick did his mod a little differently, he glued the sleeve to the drive gear with loctite retainer. He also internally ground out the engine sprocket to remove the sprag dig in marks (approx 0.010") and increased the size of the sleeve accordingly. I have found that it is not necessary to do this. Even if you have to pay someone to do the machining for you it should work out cheaper than replacing crunched bits for new ones. The drive gear and bearing sleeve can be turned with sharp carbide tools but I used a toolpost grinder to finish the diameters (Same toolpost grinder i used to reface the cam followers when it replaced my camshaft)
  4. dobba99


    Jan 25, 2014
    Hello Blackchicken
    I am the author of the post copied by Kommando.
    It sounds very much like your sprag is not gripping the engine sprocket. i have come across this before, albet on a Kawasaki Z1300, same symtoms, starter revolving and not engaging. I quick strip down of the starter sprag (needing a puller making to get something off, cant remember what that was, alternator maybe) an emery up of the sprag engagment race, reassemble and viola engaged every time. the engagment race was too shiny/polished causing the sprag to slip.
    What oil are you putting in the primary drive? Try a different oil, ATF instead of 20W/50. I find the heavier oil dosn't help with clutch slip. Or if you have it in bits again, take some 80 grit emery tape to the inside of the engine sprocket and drive gear snout.
    I have sleeved a number of drive gears for MK3 owners who have had sprag problems. Cheaper than replacing sprag, sprocket and drive gear. I have also reset the overload device as i have found that these were set too low from new.
    Anyone who has reset their overload device will tell you it is not an easy task to get correct. It sounds straight forward in the manual but when using a torque wrench to get the 50ftlbs setting you will find the adjusting nut revolves when checking the breakaway torque leading to false setting etc.
    I overcome this by machining off the staking flange and making a thin locknut for the adjusting nut. Much much simpler to adjust and set the device.
  5. Blackchicken


    Oct 28, 2017
    Thanks for the responses, a few things here for me to try so will work my way through for a solution.
  6. jimbo

    jimbo VIP MEMBER

    Sep 25, 2004
    I assembled mine with real good grease and it wouldn't work, had to clean off the grease to get it to grab:confused:, no more problems in several years
  7. Biscuit


    Sep 10, 2012
    I would not use ATF in the primary as dobba99 suggested because I don't think the hydraulic chain tensioners used in the MK3 would work correctly with the lower viscosity oil. I would clean the primary using Brakleen ®, or another degreaser type spray, and air to get rid of whatever you are using in there and use a bike specific 20w50 oil. I use Mobil 1 V Twin. These oils do not contain the friction modifiers that may be allowing your sprag to slip.