Friday, March 18, 2011

Norton gearbox strip down

Last Saturday I went to John Taylor-Leigh's house (well garage mostly) and he helped me pull the guts out of the Norton's gearbox. I met John through the Norton Owners Club here, he has an 850 same year as mine ('74) and he's owned it since new. It's the only bike he owns and he rides it daily! Anyway one of the big problems I had been having with the Norton was that when I'm accelerating out from a standstill, so in 1st gear, the kickstart was pulling itself down. I'd been riding around like this until I heard that it's a sure sign your gearbox is about to seize! Which would leave me in a bloody mess on the side of the road somewhere. The theory was, from a few different guys I talked to, that it would be a collapsed bearing on the layshaft... hence the bike not being ridden for ages, and now this — a complete strip down of the gearbox.

I took photos of the process as a reminder and thought I'd put em up here. This (above) is the gearbox with the outer cover removed. I'd done this much before, but hadn't ever gotten in any deeper than this before now.

The kickstarter pawl is pretty worn and I will replace it, but nothing to do with the problem according to John.

So this is the gearbox empty, and with all bearings looking just fine!?

Second and third gear have a bit of wear on them and we will replace these as well, but still haven't found anything that would obviously be causing the problem?

To get the mainshaft out we had to also pull the clutch basket out from the primary chain case. I've done a lot of work on this clutch but haven't had to remove the basket before. I've just been pulling plates out and cleaning them and whatnot. To get the basket out we had to remove the front sprockets, the alternator and everything.

That's the basket. I had the clutch center replaced about a year ago. I asked for a hardened steel center, but John seems to think this one might not be hardened?

There she is empty! One thing we found on this side that was 'odd', according to John, was a rubber o-ring on the mainshaft. He reckons that shouldn't be there, and looking through the exploded diagram of this area I sure don't see any o-rings there either. Could this be my problem? We won't know til we put it all back together. Which is the plan now... rebuild it to original spec, while replacing some worn parts (since we're in here now it makes sense), and then road-test it.

We're currently waiting on some parts to arrive from Norvil in the UK. And then as soon as they're here I'll be back to John's garage to put it all back together. Which I'm thinking will be a much bigger learning curve than pulling it apart! It's fuckin great that John's into this. Originally I thought he'd just take the bike, fix it and I'd get it back. it wasn't until after I'd dropped it with him that I asked if I could come back and actually go through it with him. I thought he'd tell me to bugger off, but actually he was stoked I was interested to see how to do it for myself. Great guy! I'm slightly worried about my bike being up on that workbench while we are having all these bloody earthquakes, but apart from that I'm pretty stoked to be getting my hands dirty again, but with some expert help this time.


  1. wire the bike to the stand... just in case!

  2. Hi Luke! Thanks for the post. It is being helpful to sort out the gearbox on my Dominator 88 (same AMC gearbox). Around October 2011 the gearbox stopped working. It wouldn't let me select gears. I heard a metal breaking sound while riding at about 20mph; I braked and the engine stalled. Since then, I have not been able to start the engine or change gears...
    I am now in the stage of your 3rd photo and I have a question for you. To be able to take out all the gears, bearings and check what is broken in there, do I need to open and clear out the primary chain case? Or can I avoid touching that side of the bike? It is the first time I work on that part of the bike and I am a bit lost.
    In case you haven't seen it, there is a Mick Hemmings DVD out there that guides you through a complete gearbox strip down and rebuild (done with the gearbox off the frame). I wish I had that heated garage, tools and excellent light...

    Thanks for any help you could give!

  3. Hey sorry this is late, my commenting thing didn't wanna work and I've had to run this through a new browser...

    But yeah you do have to dismantle the primary to get all the gears, bearings, and shafts out. It's a bit of a drag eh... I've just dismantled mine again for other reasons entirely!

    Good luck man,

  4. I've noticed this post is still getting views so thought it'd be worth putting an update here.

    In the end it turned out my gearbox problem was actually the the gearbox case itself. The bearing on the mainshaft had been spinning in it's location and had worn the casing enough so that the mainshaft was able to move and bind a little of the layshaft when under torque. It was good to finally figure it out, but the new inner case cost me a bit.

    So yeah if you're kickstart is pulling down under power it isn't ALWAYS the bearing itself failing!