Acorn vs. Hex Nuts

Discussion in 'Motorized Bicycle General Discussion' started by 2door, Mar 21, 2009.

  1. 2door

    2door Moderator
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    Somewhere in my early research about the Chinese engine kits I remember reading some disscussion about using hex nuts in place of the kit supplied acorn nuts on the cylinder head. There were some who maintained that the chrome acorn nuts would bottom out on the studs and therefore not allow proper torquing of the head. I've never removed the head on my first engine but neither have I had any issues with leaks at the head gasket or evidence that things have not remained tight after nearly 800 miles. Nevertheless, yesterday I removed the head on a new engine that I have yet to run. I was going to paint the head and polish the fins. Two acorn nuts came off easily, they had not been torqued from the factory, and two stayed tight to the stud and unscrewed them out of the block. When I held the studs in the vice to get the acorn nuts off I found them very tight and unscrewed hard all the way off. Examination revealed that they had bottomed out in the acorn nuts, damaging the top threads and consequently the threads inside the nut. I was able to run a die over the threads and restore them on the stud but I replaced all fours acorn nuts with shouldered hex nuts when I reassembled the head to the cylinder.
    Now, after some thought I'd like opinions on this: Even if the acorn nuts bottom out, you'd still be able to torque properly IF, and here's the rub, if the studs are not bottomed out in the block.
    IF, however they are, you'd stand the chance of doing real damage to the block, and, have a head gasket leak. Based on what I discovered I'd be inclined to suggest that this item be investigated on any new engine and/or those experiencing head gasket/compression issues. Just my observation.
    Tom
     
    #1 2door, Mar 21, 2009
    Last edited: Oct 30, 2009
  2. 2door

    2door Moderator
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    I'm bumping this very old thread because I just finished building a new motor and I encountered the same problem on it as I've found on several others previously. That being that the factory supplied acorn nuts were bottomed out on the studs. That in itself might not cause a problem but something else I've discovered is an inconsistency in stud length. I've compared them in several engines and found a variance of over a quarter inch in length. A longer stud could bottom out in the engine case and if the acorn nut also bottoms out before proper contact with the cylinder head then that stud/nut is not doing its job. I have to wonder if this is not the reason for some blown head gaskets and compression loss experienced by some. Just a thought. I always replace the kit supplied acorn nuts with hex nuts on the cylinder head/studs and the studs and nuts at the intake and exhaust with Allen head cap screws because if this problem. Any comments?
    Tom
     
  3. taddthewadd

    taddthewadd New Member

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    I have learned to check the torque on all hardware in a new kit. On my last kit half of the screws on the engine were loose. I also replace the acorn nuts with regular nuts right away that way I can sleep at night :)
     
  4. Kevron99

    Kevron99 New Member

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    I bought 2 chinas already and all with loose acorns. I will always replace the acorn.
     
  5. Cabinfever1977

    Cabinfever1977 New Member

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    I have acorn nuts on my head,(lol that does sound funny) but any way when i first was breaking in my engine i noticed a little oil residue near the head gasket and tighten the acorn nuts using very little pressure, and they stayed tight and leak free for 1,300miles now.
     
  6. mountain80

    mountain80 New Member

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    I replaced the acorn nuts with hardened 8mm nuts and washers along with the studs which are grade 12.8 . Never had a single problem since.
     
  7. Pablo

    Pablo Master Bike Builder & Forum Sponsor

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    Acorn nuts are junk. Use them in your next concrete pour.
     
  8. Creative Engineering

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    Good grief! laff

    Acorn, or cap, nuts have been around for a long time. I certainly hope they aren't junk! Motorcycles, autos, aircraft, all use acorn nuts. Acorn nuts aren't different...they simply have a closed crown that covers the exposed end of the threads.

    They are used to:

    1) To protect the threads from damage/corrosion/dirt.

    2) To protect skin from the sharp exposed threads.

    3) To create a clean, finished, appearance.

    The problem comes from the length of the threaded portion that the nut engages. Obviously if the stud length is not correct, the Acorn nut will bottom out before it clamps the mating parts.

    I'll agree that, (for most MB'ers), it's easier to replace the acorn nuts with standard hex nuts as opposed to correcting the length of the stud.

    Jim
     
  9. Pablo

    Pablo Master Bike Builder & Forum Sponsor

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    OK I wasn't specific. THE CHINESE SUPPLIED BIKE KIT acorn nuts USED for the HEAD studs are a JUNK IDEA - Use them in your next concrete pour.

    In and of themselves acorn nuts are not bad. They are just not what to use for any head stud, unless you leave some exposed stud thread and use them for decorative caps.

    If you are interested in properly torquing your head nuts, throw away (or set aside!) the acorn nuts and use a proper torque wrench and good hex nuts.

    So in closing, throw away the BIKE KIT acorn nuts. Use them in your next concrete pour. Following this is very sound advice and will prevent stripped studs. No need to get all high and mighty about it.
     
  10. jeffhalfrack

    jeffhalfrack New Member

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    Hey Pablo what's that avatar???????????? scorpa trials bicycle??? I have a scorpa easy .250 I'm looking to sell. I'd love to see a bigger pic of what ever it is JW
     
  11. Creative Engineering

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    I wasn't trying to be high and mighty. :D

    Most of the air cooled motorcycle engines that you will see, use acorn nuts to secure the cylinder head.

    Jim
     
  12. wildemere

    wildemere New Member

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    The top of the acorn on the HT nuts is only a pressed on thin metal stamping. They are not one piece items like you might find on a real, properly engineered engine.

    It wouldn't have enough strength to cause a bottoming out situation. The top will come off or be deformed first. Any mechanic would notice this. Unless its rusted together or you substitute real acorn nuts and/or you are ham-fisted.

    The Chinese don't care much about these finer points of assembly, so its an item that needs checking on every motor.
     
    #12 wildemere, May 2, 2010
    Last edited: May 2, 2010
  13. mapbike

    mapbike Active Member

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    Nothing but just good ole common horse sense here....Amen Bro., the acorn nut is not the issue it's the length of the stud that is the problem, and something as simple as a dremel tool or a bench grinder will do wonders for this problem, and then you can still have them purdy little chrome nuts on the top of that purdy little china smoker.......LOL

    reddd
     
  14. 2door

    2door Moderator
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    Yeah, whatever.
    Tom
     
  15. mapbike

    mapbike Active Member

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    I have 2 engines with acorn nuts and the nuts are solid steel nuts and not 2 piece " pressed on thin metal stamping" or anything of the sort, but I say if someone wants to change theirs more power to them and if not then just run-um like most of us do and dont worry about it. make sure they are tight so the head is torqued the way it should be.
     
  16. wildemere

    wildemere New Member

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    Sorry Tom, its not whatever, its just what happens with this crap.

    The headbolts are one of many things that need tweaking on a HT.

    I'm not saying you are ham fisted or otherwise, in fact your builds look really nice.

    Its just the initial asembly that causes these stupid problems.

    I'm just trying to help others with their HT crap while passing on my experience with it.

    It can be and is good fun when setup right.

    A perfect HT is lots of little things done right...
     
  17. wildemere

    wildemere New Member

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    You are wrong Mr.Mapbike, knock the top of the acorn sideways with a hammer with the hex in a vise and get back to me.

    They "look" convincing.
     
  18. Pablo

    Pablo Master Bike Builder & Forum Sponsor

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    Glad we aren't arguing about our nuts. We are having a discussion about our nuts.

    I personally would rather have the new guys be aware of not automatically torquing the acorn nuts. Instead replace them (easiest), or shorten the studs a hair. The stock acorn nuts I've fondled didn't have many good threads in the first place.

    Carry on.
     
  19. mapbike

    mapbike Active Member

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    Ya know maybe your right and maybe they are 2 piece and just look 1 piece,"look Convincing" but there are thousands of engines out there running them with no problems and if I were going to give advice on which fasteners to replace on these engines the acorn nuts would be the last I would mention, this is the only point I'm making here.

    Replace all the studs, exhaust, carb., mounting studs and these motors work pretty darn good for hundreds if not thousands of applications so far.
     
  20. BarelyAWake

    BarelyAWake New Member

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    I developed a hatred for the bedanged things workin' in a brake & alignment shop as even the ones used as lug nuts on a car are prone to problems in the long term - obv stud length wasn't the issue in this case, the nut would corrode under the chrome "cap" and distort the "acorn" enough to either get stuck in the impact gun's socket or not fit in a socket at all.

    I remember (with a deep hate in my soul) having to split the chrome caps with a chisel to get them off (then ofc no socket quite fits) and/or sticking the socket in a vice & beating the bejabbers outa it to get the deformed thing outa there.

    Even the lil ones on the HT's head gave me a lil bit of a problem, I had minor head & base gasket leaks despite having replaced them both & (seemed) torqued properly (new engine, had jus' ported), but being familiar with the acorn's nefarious shenanigans - I just added thicker lock washers and that did the trick.


    Thing is, acorn nuts can obv be used if you fully understand the potential shortcomings, that you may be sacrificing jus' a lil reliability for style alone. That's fine ofc, such sacrifices are made alla time - but they aren't ideal.
     

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