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How to stop the carb manifold Allen screws from backing off?

Discussion in 'Norton Commando Motorcycles (Classic)' started by SteveBorland, Aug 7, 2019.

  1. SteveBorland


    Nov 15, 2010
    I find that the manifold screws have a definite tendency to loosen off, especially if the bike is used enthusiastically. Since they are Allen bolts, the little devils will be damn hard, which makes trying to drill a small hole rather difficult....

    Locktite doesnt work well since the heat from the head breaks it down.

    Other than frequent check tightening, are they any other ways of solving this?

    At Spa this year, one bolt came all the way out, resulting in a very lean mix (and piston damage, but more on that later).
  2. Steves


    Oct 16, 2016
    I find spring washers work. Get your self an old English pillar drill and some good bits then drill holes in everything!
    baz likes this.
  3. kommando


    May 7, 2005
    Mine have wiring holes drilled in them, but I never use wires or washers and they never come undone. I did spray them with a matt silver paint many years ago when I did the whole head in silver maybe the paint on the threads acts like a weak but heat impervious loctite. The silver paint on the head did not last and was removed but not off the allen bolts.
  4. Reggie


    Jul 24, 2006
    Cowboy Don likes this.
  5. toppy


    Dec 2, 2012
    I have stainless caphead screws from RGM an the fibre heat insulator which am sure some will say is a disaster in the making but i just put a dab of whatever gasket sealant i have open an never have a problem.

    Yes my bike runs at speed to its not for looking at

    I have the ham can type air filter with soft good condition rubbers between it an carbs. Am just wondering if your carbs may be vibrating loose for some reason or the screws you have are not thermally compatible with head an manifold. I know mine should not be according to many but they don't cause any issue so they can stay for me.
  6. Fast Eddie

    Fast Eddie VIP MEMBER

    Oct 4, 2013
    I used Wellseal on the threads to act like a not setting thread lock. Dunno if it worked, but I’ve had zero issues with them coming loose.
  7. maylar

    maylar VIP MEMBER

    May 13, 2007
    There should be internal tooth lock washers under the heads of those bolts. I would never use thread locker on them, since I want to be able to turn them with my fingers once they've been cracked loose with the Allen key.
    batrider likes this.
  8. dynodave


    May 28, 2003
    since 1988
    SPRING WASHER 5/16" 22528 (5/16 reduced OD/ thin profile )- on the IPL as the carb lock washers, but I use them on the manifold allen bolts.
    On the carbs, I use a extremely thin SS plain washer as to not gouge and crush the zinc carb body/flange.
    swooshdave likes this.
  9. SteveBorland


    Nov 15, 2010
    Hmm, lots of suggestions here. I'm using plated steel allen bolts from RGM, with spring washers. I've tried using gasket sealent on the threads, but I'm not really happy.
    By the way, I've found that a 5.5mm spanner is a good fit on the allen key for these bolts, allowing easier tightning & loosning.

    I still think that I'll go to lockwire though. Just need to think the details through of how to hold the bolts while drilling.
  10. oldmikew


    Jul 25, 2015
    I am assuming its an issue with track use... Back in the day I modified the manifolds and used radiator hose and jubilee clips to hold it all together.. The hose had small indents cut to snug up against the allen bolts which were then restrained from undoing. Might look unsightly but it worked . You will have to machine the external ribbing of the manifolds to take the hose .Gave a useful heat break and never any problems.. Less radical would be to take a segment of hose and cut it so it will slip over close to the flange and then secure with just one jubilee.
  11. Chris

    Chris VIP MEMBER

    Jan 21, 2008
    Hi Steve

    Marked mine with a felt tip pen when they were good & tight & the engine had been run. Drilled & locked. I only do this on the race bikes, however I have never had a problem with them coming undone.
  12. CanukNortonNut


    Aug 8, 2005
    I've always had success with 242 blue loctite at the Allen head cap screws on the manifold. If you want to try using nord-lock washers there then give these a try. These are special designed for holding fastener under extreme vibration. Expensive but could solve your problem. NL8SS
    M8 Nord- Lock washer Stainless Steel. YouTube Nord-lock as they have lots of information you can look at.
  13. seattle##gs


    Oct 28, 2014
    Are the threads in the head OK? I've never had a problem with these.
  14. joe czech

    joe czech

    Feb 21, 2019
    similar - https://www.mcmaster.com/90895a030
    drilled cap screws - just throwing this out as food for thought - https://www.mcmaster.com/90117a583 . originals are 7/8", so you may have to get creative on the length. also available in stainless - https://www.mcmaster.com/90152a337
  15. johnm

    johnm VIP MEMBER

    Feb 26, 2008
    This is on a race bike. How are the rest of the carbs mounted. On my race bike I had extended length manifolds and joined the extension tubes using the expensive Mikuni bayonet rubber connectors. The amal ones are useless and last 10 mins in race use especially with ethanol or methanol. This allowed me to use loctite gasket cement on the face and threads of the head fittings because they became a non movable fitting. Done this way the screws never moved. They were lockwired as well but never showed any sign of moving.
    Last edited: Aug 7, 2019
  16. SteveBorland


    Nov 15, 2010
    Think I will simply go for lockwire and be done. At Spa this year, the loss of one screw has so far cost me 2 rather expensive pistons and 1/3 of the event, so a reoccurrance is very unwelcome.
  17. ninja


    Oct 10, 2017
    Yep I use shakeproof washers which I think are the same.
  18. SteveBorland


    Nov 15, 2010
    Update - Just drilled 8 holes, 1.5mm dia in the 4 bolts. Not nearly as difficult as I had thought, used a center drill ('cos it's short and does not flex as much as a normal drill). Easy!
    Craig likes this.
  19. desmo


    Aug 16, 2019
    an easy way to lockwire is cut a small groove down the allen srew head
    then get a welder to tig the top of the slot a tiny weld is all thats needed
    you can do it with hex nuts too a hacksaw cut aross the apex then seal the edge to leave a bridged slot for the wire
    no more fiddling with small drills and jigs

    the harder stuff can be done with a 1mm cutting disc
  20. joe czech

    joe czech

    Feb 21, 2019
    this may be a dumb question, but wouldn't this be an ideal situation for blue thread locker? just askin'....

    will be mounting the carb manifolds shortly, and was planning on (new) correct internal tooth lock washers, blue thread locker for good measure, and of course, torque to spec. :)
    Last edited: Sep 12, 2019

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