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Technical Coarse vs Fine threads

Discussion in 'The Hokey Ass Message Board' started by Boones, Nov 26, 2020.

  1. Boones
    Joined: Mar 4, 2001
    Posts: 9,691

    Boones
    Member
    from Kent, Wa
    1. Northwest HAMBers

    I am fabbing up a new battery box to go on my firewall and got thinking about hardware. I did not know the answer so I thought I would ask. In general when building chassis or brackets or something else you want to bolt on, when do you use Coarse Thread bolts vs Fine thread. is there a rule of thumb.
     
  2. My belief is that a fine thread withstands vibration better, due to more surface contact in relation to bolt stretch. But I'm far from an engineer
     
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  3. pitman
    Joined: May 14, 2006
    Posts: 5,148

    pitman

    I'z injuneer, does depend upon thickness/depth of matl, # of threads engaged, coarse cuts a deeper profile, if matl is adequate. Tighter desired? Fine thread, But also asks if you've got quality 'steel'. In four frame-chassis builds, I always used grade 5 coarse, FWIW.
     
    Last edited: Nov 27, 2020
  4. goldmountain
    Joined: Jun 12, 2016
    Posts: 4,442

    goldmountain

    Coarse thread because I have more of them lying around the garage.
     
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  5. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 4,217

    ekimneirbo

    Generally coarse threaded bolts are used for most things and work well. An exception is when you don't have much thickness in the material you want to put a thread into. A fine thread will give you an extra thread or two. With aluminum, often a coarse thread seems to work better and is less susceptable to tearing threads out. Often the problem is the bolt quality that you select. A cheap low grade bolt will often ruin a good threaded hole. Use Grade 5 as a minimum and Grade 8 as the bolt of choice.
    I have found that going on Ebay and buying bulk is the best way to have quality bolts available when you need them. Buy longer lengths and cut them off to suit yor needs. Run a nut on the bolt and then cut it to length. Then belt sand the cut off end with a small chamfer. Then back the nut off and it insures the thread is straight.
    You can search Ebay for "serrated" or flange head bolts and you have some nice looking bolts that look more professional when assembled. Here is one example of a vendor that I have dealt with, but there are lots of others. You can get serrated nuts too for through holes. Even tack them in place for areas that may be hard to access later.

    https://www.ebay.com/itm/200-1-4-20...612886?hash=item3a7f959616:g:QcAAAOxyoA1RSbu3
     
    Last edited: Nov 26, 2020
  6. TrailerTrashToo
    Joined: Jun 20, 2018
    Posts: 1,289

    TrailerTrashToo
    Member

    I go to Tractor Supply and buy Grade 8 hardware by the pound. Sure it is overkill, but buying unknown quality hardware in baggies (Gimme a Dime bag of 1/4"-20) fromthe big box store is more expensive.
     
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  7. squirrel
    Joined: Sep 23, 2004
    Posts: 55,941

    squirrel
    Member

    I would think it would have more to do with the angles involved...but I'm an engineer, so I look at things a bit differently.

    There's a lot of fine thread aircraft hardware....and a lot of coarse thread automotive hardware....hmmm....
     
  8. tubman
    Joined: May 16, 2007
    Posts: 6,915

    tubman
    ALLIANCE MEMBER

    I like fine thread because I see a lot of them on old Fords. If cheap old Henry used 'em, they're good enough for me!
     
  9. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

    I use fine thread bolts for most suspension parts. Things like spring bolts, shackles, shocks, etc. And always grade 8 when I use fine threads.
     
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  10. 51504bat
    Joined: May 22, 2010
    Posts: 4,755

    51504bat
    ALLIANCE MEMBER

    Its been my experience that fine threads have a tendency to bind up more than course threads. IH used a lot of fine threaded bolts on it's trucks and they were a PITA to remove years down the road.
     
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  11. 73RR
    Joined: Jan 29, 2007
    Posts: 7,175

    73RR
    Member

    Some good observations have been made.
    As an old Engineer I'll offer a word of caution regarding the use of GR8 for 'everything'. GR8 is, of course, generally stronger than 2-3-or 5 but, the hardening process also makes them a bit more brittle.
    As an example, MaMopar, back in the 50-60-70's, used GR5 for alternator and PS pump mounts because the vibration would cause a GR8 to break (yes, personal experience). GR5 would simply bend and not leave a neatly plugged hole in the head. MaMopar used GR8 for attaching engine brackets to the blocks; they were used for high clamping force to keep the brackets from getting loose. Same logic for con-rod bolts.
    GR8 is superior in tension and the reason for most GR8 having fine threads. If you think that you need more 'shear' then use a larger diameter.

    .
     
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  12. pitman
    Joined: May 14, 2006
    Posts: 5,148

    pitman

    Sound comparing of GR5 vs GR8.
     
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  13. jaw22w
    Joined: Mar 2, 2013
    Posts: 1,671

    jaw22w
    Member
    from Indiana

    I had exactly the same experience with a power steering mount. Broke 2 grade 8 bolts off in the block before switching to grade 5. No more problems.
     
  14. 73RR is correct
    As an old Engineer I'll offer a word of caution regarding the use of GR8 for 'everything'. GR8 is, of course, generally stronger than 2-3-or 5 but, the hardening process also makes them a bit more brittle.
    As an example, MoMopar, back in the 50-60-70's, used GR5 for alternator and PS pump mounts because the vibration would cause a GR8 to break (yes, personal experience). GR5 would simply bend and not leave a neatly plugged hole in the head. MoMopar used GR8 for attaching engine brackets to the blocks; they were used for high clamping force to keep the brackets from getting loose. Same logic for con-rod bolts.
    GR8 is superior in tension and the reason for most GR8 having fine threads. If you think that you need more 'shear' then use a larger diameter.

    I am not a engineer but sometimes a self proclaimed one , I have been around the block several times and spoke to a lot of smart individuals and self experience in this , the above statement is true in the fastener world.
    Vic
     
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  15. TERPU
    Joined: Jan 2, 2004
    Posts: 2,372

    TERPU
    Member

    Old Guy once told me "fine for forged, coarse for cast" So far it's worked out. That is when fastening into a piece. through fastening though it really doesn't matter. In tight spots I like coarse because it doesn't take so long to tighten.

    - Tim
     
  16. Los_Control
    Joined: Oct 7, 2016
    Posts: 1,125

    Los_Control
    Member
    from TX

    while bolting my old dodge back together, I find myself making many trips to ace hardware for fine thread bolts. To replace the original 70 year old bolts.
     
  17. DDDenny
    Joined: Feb 6, 2015
    Posts: 19,143

    DDDenny
    Member
    from oregon

    Lockwashers and whatever you have on hand that match.
     
  18. BJR
    Joined: Mar 11, 2005
    Posts: 9,817

    BJR
    Member

    All the pre 50s cars I have worked on used fine threads. Now I use coarse threads for most stuff as it is readily available.
     
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  19. I rode on a train once squirrel :D
     
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  20. 51504bat
    Joined: May 22, 2010
    Posts: 4,755

    51504bat
    ALLIANCE MEMBER

    Dodge motor home with a 440 kept breaking the bolt that held the alternator on until I used a grade 5 instead of a grade 8.
     
    stanlow69 likes this.
  21. Needed some flex ;)
     
  22. The off-road guys prefer grade 5 over 8 because it'll bend instead of breaking...
     
  23. OT north star engines used fine thread for the motor mounts. When they pulled out, they had a repair that also used fine thread bolts. Not too smart. My son, Dual quad 55, here on The H.A.M.B. was a mechanic, not a technician, that had to do this repair frequently.
     
  24. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 4,217

    ekimneirbo

    You can always find instances where something failed, but using a high quality higher grade bolt will rarely prove to be the wrong choice. Grade 8 bolts are not "brittle". Thats completely the wrong term to discribe them. If you look at General Aviation requirements you will seldom (if ever) find a Grade 5 or 3 bolt spec'd anywhere. Try looking at any helicopter and find somewhere that uses any spec less than Grade 8. ARP makes plenty of bolts that are spec'd higher than Grade 8 for use in engines. In past decades there was a problem with bogus Chinese Grade 8 bolts being dumped on the market and some of them made their way into the wrong places. Lots of sh*t hit the fan over them. I'm talking about them finding their way into government stuff before it was rectified. Probably still some of them sitting in someones garage.
    ARP Chart 1 001.jpg
    ARP doesn't even use Grade 8 but has even higher yield/tensile strength materials. I'm sure they are not more "brittle" than Grade 8. The thing about the lower grade bolts is they are somewhat softer and will bend some before breaking. Sometimes thats a good thing, but you don't want to use Grade 5 bolts in your suspension system because they will bend before breaking......you want the bolt that has the strongest tensile strength.
     
  25. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,264

    gimpyshotrods
    ALLIANCE MEMBER

    Correct, on all counts.

    Don't even get me started on split-lock washers, and the myth that they actually work.
     
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  26. Boneyard51
    Joined: Dec 10, 2017
    Posts: 6,442

    Boneyard51
    Member

    Ole, Henry believed in fine threads! That’s good enough for me!






    Bones
     
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  27. David Gersic
    Joined: Feb 15, 2015
    Posts: 2,730

    David Gersic
    Member
    from DeKalb, IL

    If you’re dealing with properly engineered fasteners, absolutely right.

    At the hardware store, they have zinc plated (“un-hardened”), “grade 5” (“half hardened”), and “grade 8” (“yellow, hardened”) fasteners. I buy the supposedly grade 8 because they’re less likely to shear off when tightened with an actual wrench. I don’t believe or expect that they meet any actual engineering specs, and wouldn’t trust them for any critical application.



    Sent from my iPad using The H.A.M.B. mobile app
     
  28. Fordors
    Joined: Sep 22, 2016
    Posts: 5,373

    Fordors
    Member

    I’ve seen plenty of supposed G-8 fasteners in my time but if I’m not sure of the manufacturer or origin I give them a simple test. If you take a three cornered file and try to nick the fastener and the file glances off it then it is case hardened. On the other hand if you can nick it and sacrifice one to a hack saw test it will feel “gummy”when sawing. I take that to mean it is a good alloy steel and hardened properly throughout.
    I’m looking for opinions, is my backyard metallurgical inspection valid or not?
     
  29. DDDenny
    Joined: Feb 6, 2015
    Posts: 19,143

    DDDenny
    Member
    from oregon

    When you are feeling rich look up Supertanium bolts.


    upload_2020-11-26_12-32-11.png

    Before ARP fasteners, back in the late '60s and early '70s Trans-Am era, the top of the line fasteners to use were Premier "Supertanium".
    "These originals are no longer available but you might get lucky scrounging around in military surplus stores or on ebay every once in a
    while."


    http://www.camaros.org/forum/index.php?topic=8134.0
     
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  30. Fordors
    Joined: Sep 22, 2016
    Posts: 5,373

    Fordors
    Member

    A7993AA5-F7B2-433C-B116-734DA225E2B8.jpeg D20B34CD-5066-4BA9-91FD-049BADB45F29.jpeg

    Another fastener to look for is Lamson & Sessions L9. Top quality alloy, rolled threads, and a slightly thicker head. Many years ago they were G-8, but today the L9 they call them refers to a better alloy material used.
    They are also known as Lamson & Sessions Lamalloy.
     
    Last edited: Nov 26, 2020

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