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Thread: Head studs

  1. #1
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    Default Head studs

    I am thinking of making new head studs for my vintage side valve Jeep engine, using 4140 stock . The standard size is 7/16" with UNC threads at the block end and UNF threads at the other end . I bought some new studs from a Jeep dealer but they are a loose fit in the UNC block female threads. I measured the original studs I removed from the block , these are a nice firm fit in the block , and these are slightly larger in OD than the new replacement studs

    All of the advice I have read suggests that modern stud threads are rolled rather than cut and DO NOT use cut stud threads that are for use in a engine block . The Jeep engine is a low compression vintage engine. I previously have made new 4140 head studs for a Vintage Lister diesel engine. 3/4" diam. long studs. No problems so far.

    BTW the Jeep engine originally used bolts rather than studs in all but three of the head fixing locations. Later on, the factory changed over to all studs which is what most restorers do. Any ideas ? Thanks
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  2. #2
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    Just make new studs, better a good fit in the block than new studs that are a loose fit.
    I have rebuilt quite a few vintage engines over the years and in some cases necessity has meant breaking the "rules".
    Some of the engines I have rebuilt have covered many thousands of miles (over 100,000 in one case)
    without any catastrophic failure.
    With these older low compression engines the stress on most components is quite low compared to modern engines.
    Another thing to consider is the amount of use the engine will get, one engine I rebuilt for a friend of mine has only covered around 5000 miles in 20 years and is still going strong, we knew it wasn't going to get much use, so to keep the cost down a fair few shortcuts were taken during the rebuild.
    Regards
    Bradford

  3. #3
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    Hi Morrisman,

    Just a rush of poo to the brain

    Outside of buying the dies if you don't happen to already have them, the preference would be to cut them on a lathe to permit customising the thread OD.

    I can't recall if you said you had a lathe, but as usual when you really need to own such a machine, you are likely not to have it.

    Thats what happens to me at times. I sure one of guys will step up to help you.

    Or alternately ,Check the OD on some off the shelf hi tensile bolts -usually 4140- from a fastener shop.- Again that's assuming you can buy the lengths you need.

    If Unc is selected then you only have to lop the heads off and cut the Unf ends and only need to buy one button die ( which you certainly will not have because you need it now )

    Grahame

  4. #4
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    The od of the thread doesn’t really affect the fit of the thread all that much, and the tolerance on the od of a thread is pretty big.
    The pitch diameter is what will mainly affect your thread fit. Can be measured with 3 wires. Will lookup wire sizes and dimensions at work tomorrow.

  5. #5
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    Saskatoon, Saskatchewan, Canada
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    Default

    The radial aircraft engines I worked on had a range of oversize stud sizes to allow for repairs and overhaul. Any chance the same exists with vehicle engines?

    Pete

  6. #6
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    A friend is making new head studs for his vintage OHV engine.
    He said two interesting things.
    The shank length between the threaded end will be reduced to slightly less than the root diameter of the threads, so any stretch will mostly happen there, not so much at the threads.
    He is considering following advice in a book by Carroll Smith, which is to not tightly install the studs into the block, but rather leave them loose so that thread engagement is better under tension and less likely to break the stud.

  7. #7
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    Default Thanks

    Quote Originally Posted by nadroj View Post
    A friend is making new head studs for his vintage OHV engine.
    He said two interesting things.
    The shank length between the threaded end will be reduced to slightly less than the root diameter of the threads, so any stretch will mostly happen there, not so much at the threads.
    He is considering following advice in a book by Carroll Smith, which is to not tightly install the studs into the block, but rather leave them loose so that thread engagement is better under tension and less likely to break the stud.
    Yes, I've see the reduced diameter between the threaded ends on some block studs.

    Interesting theory re: the loose thread engagement. In my case , I think the cast iron block threads are slightly distressed or distorted due to the crud/rust on the ends of the studs, as you unscrew the old studs , the crud can mess things up. Some people reckon it is not a good idea to run a clean up tap through suspect old cast iron threads , doing this can weaken the already damaged threads and lead to a bad situation where the thread strips under load. Another tip is: Don't over tighten up the studs in the block, a firm screw down is all that is needed, some just finger tighten the studs and leave it at that. From memory the recommended torque on the Jeep head studs is only 60-65 ft-lbs .

  8. #8
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    7/16" UNF 2A:
    Major dia. .4362-.4281
    pitch dia. .4037-.3995
    measurement over 3 wires: .4470-.4428*
    wire dia. .02884"

    7/16" UNC 2A:
    Major dia. .4361-.4258
    pitch dia. .3897-.3850
    measurement over 3 wires: .4515-.4468*
    wire dia. .04116

    min-max wire diameter .03608-.07217**

    *i can recalculate this to what size wire you have

    **minimum and maximum wire diameters that will fit correctly in the thread if you have something within this range you can compare the PD of your old and new studs. If you have steel tig filler wire i have used that before.

  9. #9
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    I wonder if some previous owner ran a 7/16W tap into the block holes? This would account for the looseness. All sorts of unexpected things can happen to a vehicle in 70 something years. Do you have a 7/16W bolt to try in the holes? In this case a solution might be to make the new studs with a Whitworth thread on one end and UNC on the other.

    Frank.

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