Sunday, October 9, 2011

Velocette Venom belt drive

1955 Velocette Venom.
Sometime around a year and a half ago my Velo's clutch started slipping. It was on a vintage run sharing the route with a number of machines more suitable to gently pottering than the mighty Venom so much deliberate slippage of clutch was required. So much so that after twenty miles or so the slippage became involuntary.
Not such a problem you might think, just carefully follow the approved Velo procedure of taking the drive sprocket cover off with machine on stand, insert adjuster peg through hole in aforementioned sprocket and, in gear, roll the rear wheel forward slightly. Problem was that no amount of correct adjustment would cure the issue.

So, next step, dismantle the clutch to check. Friction plates badly worn. Replace and try again. No improvement. Metal plates are noted as worn. Buy new and fit. Same problem. Hmmm... for that matter the basket is also quite worn. Replace but still no improvement. This was all getting rather silly. The time came to stop wasting money on new parts and check it all through carefully. After much head scratching it turned out that the different plates and the basket had come from different suppliers and though all may have been just within tolerances, together they just didn't match up. The main problem being that the tangs on Velocette plates are cut with an angled face. The angle on the friction plates was sharper than that on the basket meaning that the plates would never sit snugly into the basket and would remain always very slightly lifted. Just enough to cause slippage under higher load but also mal-fitting enough to cause drag too.
KTT Service belt drive conversion.
With a significant amount of time already off the road not too mention much gnashing of teeth it seemed like the bike needed a complete new clutch and one that would definitely work. I recalled the Kevin Thurston (KTT Services - Kevin Thurston Transmissions) belt drive conversion that I had seen at a show. The conversion uses Yamaha R1 plates in a new basket but using the Velo face plate and spring holder. It has the nice feature of retaining the original Velo cush drive mechanism on the engine pulley and better still has an outrigger crank oil seal that bonds on to the cases to do away with the constant seepage of oil into the primary cover and the consequent leaks from the tin cases.

So far from this sorry tale of mechanical woes and wasted money the only advantage to me had been that I was now very familiar with the workings (or lack thereof) of Velocette clutches. Velo afficianados seem to go to great pains to defend the wonky engineering of the Hall Green clutch. Though my Venom is probably the favourite in my small stable my mind still boggles at quite how they came up with such a design. Drag is inherant in its design, it is fiendishly complex to manufacture, a b**** to assemble and set up and is housed in a vintage tin case. To speak up for it it does work (but so do conventional push rod designs and often much better) and is very light...

I got in touch with Kevin and shortly after placed an order. Kevin was very helpful, took a deposit and quoted delivery in a couple of months. In the end it took a little longer, not a big problem as it wasn't as if I had too much spare time and my only bike off the road.

When the kit arrived it was all very nicely made and initial fitting seemed easy. It came without the clutch basket bearing race so although the kit is reasonably priced, the race (not supplied by Kevin) is not, so it pushed the overall cost up a fair bit. I guess normally you would use the original chain basket race but in my case it was shot.

1955 Velocette Venom drive side view.
I'd like to say that fitting was a doddle but sadly it didn't work out so for me. The clutch itself was very easy but getting it all to fit inside the case and not rub and scrape was a different matter. The first point to rub was the inner primary case. I gave Kevin a call and he suggested removing the o ring that sits between the crankcase and primary case, if this didn't work he would make a spacer for me to sit inside the engine pulley. Taking the o ring out worked. Then kicking the bike over for a test run the kick start spring went. Another insight into the oddity of Hall Green engineering as I stuck two nails in the vice to pre-load the spring in the sping holder....

Putting the outer case on it was evident that the Velo wouldn't play ball and give up without a fight. First obvious point of contact was the felt seal holder that faces against the clutch. I made up a new double thickness cork gasket and drilled the rivets out that held the felt seal holder in and removed it, as it was going to run dry it didn't seem like the lack of a felt seal would cause any problems. Re-assembling I cursed the Velo tin case with its multitudinous screws holding it all together. The part of the outer case that I would describe as a small shelf under the insection cap was now obviously rubbing on the engine pulley. As I have no idea what its logical function is I took the step of cutting it off. Reassemble and still some rubbage on the pulley. To counter this I made a new central spacer approximately 1/8" over standard to push out the centre of the primary outer cover. It all now seemed good. However cracking the engine in to life caused something to move and it came to a sudden stop. The pulley had rubbed against the new outrigger oil seal. Nothing for it now except to make a spacer to sit inside the pulley. For good measure I also chamfered off the internal right angle edge of the pulley. The spacer made up was 1/16". Back together again and finally bingo, all good.

The clutch was extremely easy to adjust. I even got it right first time - after all that I felt that I was due some luck. Giving the bike a test ride it worked magnificently. The clutch action is extremely light and it will hold stationery in gear for a good while longer that the normal 3 or 4 seconds the standard clutch gives before drag and stall. So finally after all the effort it was well worth it. If it seems like I encoutered a lot of problems fitting it, well I did! But really it's perhaps not too much of a surprise given the very narrow proportions of the Velo clutch and the individual nature of each Velo. So, certainly the KTT belt conversion isn't a buy one day and fit that evening upgrade but it is worth it and I feel has made my Velo a good bit more pleasurable to ride.

Grove Classics roll on centre stand for Velocette.
As a footnote I picked up a second hand Grove Classics roll on centre stand halfway through the fitting of the clutch saga so decided to fit it at the same time. Along with the belt drive I can certainly recommend it too as a boon to the useability of the Velo. The standard centre stand is a bit of a beast to use with nothing roll-on about it at all, it requires a massive heft and taking it off the stand usually involves skidding it along forwards several feet in a hernia-inducing lunge on all but the roughest of surfaces. The new roll-on stand is just that and requires a lot less effort to get on and off the stand. The only criticism is why didn't they fit a tag on it to catch with your boot to lower the stand as it can be a bit tricky to get a purchase on. This way body weight could be used to far better advantage to get the bike on to the stand too. I sense a home modification coming on..

Velocette Venom 171.
And finally, just a little about the bike. It's a '55 model. Though the Venom was officially from the '56 model year onwards a few were produced in '55. Starting with engine number 101 mine is 171. Certainly not the oldest survivor as both 101 and 102 survive. It was originally finished in the rather horrible 'dove-grey' option which a previous owner perhaps sensibly overpainted in black. As far as I can work out the only difference in spec between a '55 model and '56 is in the saddle which is the same type as fitted to MSS models, a little more padded and rounded that the later Venom ones.


  1. I have had a world of pain with this belt kit in the anorexic width of the chain cases. I have introduced a 5mm spacer around the chain case (large), new central spacer and retainer, and produced a new secondary drive sprocket with increased depth. Result is no rubs and horrible noises.
    However, as the project reaches the point I may drive it, the clutch won't let go. Adjusts perfectly, light as a feather and drag like a b*******d. Oh good grief. I'm trying to work out if this needs to be wet,

    1. As I guess you have read, I feel your pain! The chain cases are truly dreadful. Worth persevering though as the belt / clutch conversion is a good piece of kit. Maybe you've got the Thruxton clutch? Actually and unbelievably designed to drag.

    2. ps - It should run dry just fine.

    3. RCT is more or less rocking now, althought the clutch is not a moment of love still. Looking at the modified actuator pon mod, I dropped the pivot of the actuator arm by 5mm to increase the throw. I'm using a 1 inch lever throw, clutch is heavier, still first gear clonk and poor idle in gear.


    4. Good to hear that you are nearly there. I think the pivot centre to nipple centre on a Velo should be 1 1/16. See the Velobanjogent:
      Good luck in sorting it out.
      By the way even though I reckon the clutch on mine is now pretty good I still always try to find neutral before coming to a halt and definitely don't attempt to hold the clutch at traffic lights...

  2. Had similar rubbing problems, I've given up ,dam thing is just too thick to easily fit. Or am I too thick to find a solution. At the mo it's running open, not satisfactory.

    1. Yep, the tight clearance is pretty annoying. Should be solvable with a packing gasket.It's worth bearing with it as the conversion is a decent bit of kit.

    2. Hi Mates
      Barry from Australia anybody had experiance of Velo Venom lining swelling when warm/hot, mine seem to be, just put in new bonded / plain plates and inserts in the chainweel, when hot the free play goes to 12mm (1/2")when cools done it's back to 1/16", any thoughts, the bonded ones came from Austria ( sim' things from India)

    3. Hi Barry, you can see from above that I've had a boat load of woes with pattern Velo clutch parts. If I were going to stick with a standard clutch from what I know now I would probably get the original plates re-lined at a decent local clutch specialist. I've always found that Velo clutches drag and heat up pretty quick if held at a standstill on the clutch and have adjusted my riding to find neutral before coming to a stop. Could be that your plates will bed in when properly soaked in oil and with a few more miles. Let's see if anyone else has got any experience.