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Timed crank case breather

Discussion in 'Norton Commando Motorcycles (Classic)' started by Gilesy, May 12, 2018.

  1. Fast Eddie

    Fast Eddie VIP MEMBER

    Oct 4, 2013
    It seems a tad ironic that many of us with holes in the timing side are plugging them up whilst those without holes are drilling some!

    When it comes to breathers, I’d remind folk that unless you have the early frame where the cross tube is in the way, fitting a Comnoz sump plug breather is as simple as simple can be. They work even if you don’t plug the timing chest holes, but if you don’t have any holes, then you’ve got the perfect plugging!
  2. Deets55

    Deets55 VIP MEMBER

    Oct 3, 2013
    I think the difference is the location of the reed valve. With a crank case mounted valve the best option is to block off the timing chest. With a timing case mounted valve perhaps the more holes from the timing chest into the crankcase might work better.
  3. jseng1


    Nov 26, 2009
    If you are connecting the one way breather to the left side of the case via the hollow cam vent with the rotary disc removed or if you drilled the sump and are connecting the one way valve to the lower rear case - then you should also plug the holes that vent into the timing chest - otherwise you will blow some air back & forth between the timing chest instead of pumping it out through the one way valve (reducing its effectiveness).


    Removing the rotary disc etc on a 750 and installing a one way reed valve breather at the original left case vent and plugging the timing case vents definitely works. This is the setup I use as shown in the vid.

    Or you can leave the holes in the timing chest and mount the breather as oOnortonOo has shown in an earlier post above.
    Last edited: May 16, 2018