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Cylinder head helicoil thread

Discussion in 'Norton Commando Motorcycles (Classic)' started by laurentdom, Sep 13, 2019.

  1. laurentdom

    laurentdom

    Joined:
    Nov 30, 2012
    Hi Ladies & Gents,

    One of the 2 studs corresponding to numbers 7 and 8 of the tightening sequence is loose in my Combat type head because the thread in the head has been damaged.

    I plan to fix that by installing an Helicoil insert in the head, but I don't know the size and thread of the side of the stud which is screwed in the head.

    Thanks for any info or suggestion of alternative solution (buying a brand new Fullauto one being unfortunately out of scope :) ).

    Laurent
     
  2. lazyeye6

    lazyeye6

    Joined:
    Feb 28, 2014
  3. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Joined:
    Nov 20, 2004
  4. laurentdom

    laurentdom

    Joined:
    Nov 30, 2012
    Many thanks for your clear and prompt answer.

    L.
     
  5. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Joined:
    Dec 3, 2012
    Why for probably 50 years plus does the stock 3/8" stud have around 11 mm of 20 TPI thread on it when the cylinder head has a good 17+ mm of thread in it or some 50% of thread engagement going to waste ?
    No wonder that little 20TPI thread gets a hard time, its fine and has minimal (Almost a diameter to engagement ratio more suited to cast iron or steel than alloy)

    Why did Andover Norton persist in following what might have been a flawed dimension since 1967 ?

    Andover Norton, why not make a revised 3/8" stud that has extra 20 TPI thread but retains the stock stick out dimension from the cylinder gasket face ?

    Is the only way at this point to get a longer thread a do it your self ?
     
  6. RoadScholar

    RoadScholar VIP MEMBER

    Joined:
    Dec 28, 2008
    You could take advantage of the opportunity to purchase CNW's head fastening kit and be done with this condition/worry for, virtually, all time???

    Best
     
    Fast Eddie likes this.
  7. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Joined:
    Dec 3, 2012
    Nicely timed post.

    That might be an option if I wanted to insert the FullAuto cylinder head which has a good 17 mm of usable 20 TPI thread as does the Jim Comstock 750 head that has 20 TPI Alu/Bronze inserts. (You would have to be out of your mind to remove those JC 20 TPI inserts and yes they will not fail even with the stock stud)

    Both the JS Motorsport and CNW studs need the thread in the head changed to suit a lower TPI thread. (16tpi/18tpi) and might even wonder how much thread engagement there is on their studs ( Around the same as stock I bet)

    I am not particularly worried, just saying there is a fault that goes back to the drawing board and most likely a ready market for a 3/8" 20 TPI stud that actually uses the available depth of 20TPI thread in the cylinder head.

    20 TPI is not an ideal thread but it is there and in the majority of Commando's out there, not every one wants to spend $100 for a few studs let alone inserting the head (even if you can do it yourself)

    How about a stud that threads to the bottom of the hole, not part way ?

    I will contact AN and ask them what they were thinking, did they even look down the hole in the head or just blindly follow the drawing for 51 years. :D
     
  8. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Joined:
    Dec 3, 2012
    The FA cylinder head has the same 20 TPI as stock, we will hope the new US based manufacturer of that head will change the TPI to 16/18 tpi and include three studs to suit.
     
  9. 850commando

    850commando

    Joined:
    Nov 1, 2014
    Timesert or a repair by the Nortonshop (Jim Comstock is the way to go) and i've had Jim do mine in 16tpi. but be sure of your available studs before making your decision...
     
    Nater_Potater likes this.
  10. swooshdave

    swooshdave

    Joined:
    Apr 15, 2009
    AN doesn’t change the dimensions just because you think it might be a good idea. If you don’t like the stock solution there are aftermarket ones. For most people the stock stuff works just fine.
     
  11. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Joined:
    Dec 3, 2012
    Its not about me SD, I will just machine some longer thread studs based on there are no aftermarket 20 TPI studs available that I can find...

    What it is about is blindly following a drawing (hence this thread asking about repairing what could have been averted 50 years ago) when the problem could have a solution, and banging a 4 inch nail in 3 inches is not a solution.

    At the end of the day it is no skin off my nose but it might be to the majority of Commando owners out there with stock 20 TPI heads some time down the track.
    Replacing a stud (about $3 each @ AN) with a OEM superseded version is a whole different thing to taking the head to a machine shop for 16/18 tpi inserts if that was the case and $100 for three fancy 16/18 tpi studs.
    That is all I am suggesting, I am willing to bet most folk just screw those stock studs in with little thought, as I did until I looked down the hole.

    A bit late for the OP and one reason a helicoil might fail in this repair application, lack of thread engagement.

    065 - Copy.JPG

    The JC 20 TPI inserted 750 head I have will be fine (The inserts are near bulletproof) with that stock stud but on principle I do not like it.
     
    Last edited: Sep 13, 2019
  12. comnoz

    comnoz VIP MEMBER

    Joined:
    Dec 10, 2008
    Early model engines had longer studs with more engagement.

    The later engines had the shorter studs and they all fail sooner or later.

    Installing a 3/8-20 helicoil is a guaranteed failure. The fine thread with the large diameter will not hold for long.

    The cheap fix is to install 3/8th-16 helicoils.

    Timeserts work ok if you can find them with a course thread on the OD. Again -fine threads will not hold for long in aluminum.

    Then you can use early 750 cylinder base studs which are 3/8th-16 on one end and 3/8th-26 on the other.
    They a barely long enough to work.

    Better is to find some studs the right length with 3/8th-16 on one end and 3/8th-24 on the other end. They are normally available at auto parts stores. Then you can install a 3/8th-24 helicoil in the long nut. That makes a repair that will last.
     
    swooshdave and Time Warp like this.
  13. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Joined:
    Dec 3, 2012
    Thanks for yet another informative post, Jim.
    Why they would reduce the thread engagement is beyond me and not use a Whitworth pitch.

    Maybe I will just have to bite the bullet and change the thread pitch on my FA head straight out of the box.
     
  14. cliffa

    cliffa

    Joined:
    May 26, 2013
    New manufacturer ? who, where, when ?
     
  15. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Joined:
    Dec 3, 2012
    I'm not sure of the US company name but they are also developing a 5 speed.
    Someone will post the company name i'm sure.

    #170
    thumbnail_Image-9.jpg
     
    Last edited: Sep 14, 2019
  16. holtcorseaux

    holtcorseaux VIP MEMBER

    Joined:
    Jan 18, 2016
    IT is STS, who manufactures the CNW e start.
     
    Time Warp likes this.
  17. RoadScholar

    RoadScholar VIP MEMBER

    Joined:
    Dec 28, 2008
    Seems like the majority of the options involve changing thread pitch via installing helicoils or similar paradigm technology. One option appears to be using a longer stud with 3/8-20 that takes advantage of the depth of the existing threaded bore(s). Jim said: "Installing a 3/8-20 helicoil is a guaranteed failure". The forum's brain-trust supports installing some flavor of insert, why fight it, especially if the head is off?

    Using either 3/8-16 BSW or a 3/8-16 UNC thread pitch for head origination would seem to favor the UNC threads from an economic view point. Choosing a stud with 3/8-16 & 3/8-24 with the appropriate hardness/length, and commonly available helicoils/time-serts, etc. makes the most sense, to me, at least. Purchase a grade 8, 3/8-16 x 3" (more or less), cutting it to length and threading the other end 3/8-24 would be more productive than rebuilding a light bulb or replacing the side plates/rollers on a old Reynolds chain, no?

    Best



     
  18. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Joined:
    Nov 20, 2004
    Only, the stud lower end thread is 3/8"- 26 CEI/BSC so 3/8-24 would require new (UNF) sleeve nuts.
     
    Last edited: Sep 14, 2019
  19. comnoz

    comnoz VIP MEMBER

    Joined:
    Dec 10, 2008
    Or you just install a 3/8-24 helicoil insert in the stock sleeve nut. It works like a champ.
     
    MexicoMike likes this.
  20. rvich

    rvich VIP MEMBER

    Joined:
    Jul 25, 2009
    Or you take old head bolts with 26 CEI and cut off the head, then thread the end with 3/8-16 and you are ready to go. Oh - and make them a little longer too! OK, flame suit on!
     

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