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Thread: Socket head cap screw - left hand thread

  1. #1
    Join Date
    Feb 2012
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    Missoula, Montana
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    Default Socket head cap screw - left hand thread

    I was given a compressor - 3.5 hp chinese junk - but the price was right. Only problem is - it doesn't actually work. It is a new unit the original owner was too lazy to take it back for warranty. It's an upright 3.5 hp 110v unit. I pulled the cover off of the crank housing and the problem is obvious - there is an 8 mm socket head cap screw gr 8.8 that bolts the crank/counterweight to the motor shaft. The cap screw is broken at about the third thread. I managed to get the stem of the screw out without damaging the internal threads in the shaft. I'm thinking I don't want to put in another gr 8.8 cap screw since the first one only lived for about a month. It could have been a bad bolt or overtorqued - but still. The internal threads don't look very good - I would like to clean them up a bit.

    I can't find a high strength metric left hand cap screw anywhere. I could turn one on the lathe - but I don't have a clue how nor the tools to broach the allen head socket. Plan B - I could drill the shaft to a 3/8-24 and tap for 3/8-24 left hand thread - except I can't find a 3/8-24 high strength fastener either. (3/8-24 should completely clean up the old metric threads - but not take too much meat off of the shaft.)

    Plan c - I can find 3/8 - 16 left hand high strength cap screws - I think I could drill the shaft out slightly more and go for the coarse thread. I can't get the shaft out without dissasembling the motor - so overdrilling and tapping will have to be done by hand held tools.

    I can't believe with all the tens of thousands of fasteners out there - I need one that sounds so simple but doesn't exist, aargh.

    Venting a little - but any suggestions? - sources?

  2. #2
    Dr Stan Guest

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    Quote Originally Posted by Johnh57 View Post
    I can't find a high strength metric left hand cap screw anywhere. I could turn one on the lathe - but I don't have a clue how nor the tools to broach the allen head socket. Plan B - I could drill the shaft to a 3/8-24 and tap for 3/8-24 left hand thread - except I can't find a 3/8-24 high strength fastener either. (3/8-24 should completely clean up the old metric threads - but not take too much meat off of the shaft.)

    Plan c - I can find 3/8 - 16 left hand high strength cap screws - I think I could drill the shaft out slightly more and go for the coarse thread. I can't get the shaft out without dissasembling the motor - so overdrilling and tapping will have to be done by hand held tools.

    I can't believe with all the tens of thousands of fasteners out there - I need one that sounds so simple but doesn't exist, aargh.

    Venting a little - but any suggestions? - sources?
    Try checking with the usual suspects, McMaster, MSC, Travers, Fastenall etc. Instead of just looking on their web sites call them. I've found many suppliers will have small supplies of, or can quickly acquire, unusual fasteners, etc. I'd also check with the local agricultural equipment dealer to see if they may have what you need.

    When you drill & tap the shaft I'd make a sleeve to act as a drill guide. The sleeve would have a slip fit over the shaft say 1 1/2 to 2 inches long and an additional length (another 1.5-2.0") with a hole the same size as the tap drill. This should keep you fairly well centered and straight.

  3. #3
    Join Date
    Jan 2003
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    413

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    Wondering if/why LH thread?
    Could you open it out to 3/8 RH & loctite it?

  4. #4
    Join Date
    Feb 2005
    Location
    Ohio
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    Quote Originally Posted by Johnh57 View Post
    Plan c - I can find 3/8 - 16 left hand high strength cap screws - I think I could drill the shaft out slightly more and go for the coarse thread.
    The coarse threads require a *smaller* hole. .3125" vs .332" for the fine threads.

  5. #5
    Join Date
    Feb 2005
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    1,015

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    I would bet 100% that you can purchase the item here:
    https://www.mastertoolrepair.com/

    Be aware that serveal companies sell the same compressor and paint it a differant color.
    If your specific brand is not listed try the husky brand. I just ordered some parts for one like yours and the husky brands have left hand threads like you describe.

  6. #6
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    First, a grade 8.8 IS a high strength cap screw. I have searched far and wide for stronger cap screws or any kind of screws and they are hard to find. I know of two possible sources and both are somewhat limited. One is the screws used by Greenlee for their chassis punches. Oddly, the smaller sized punches need to have the strongest drive screws and a grade 8 bolt can just simply break under the stress. They either make them themselves or have them made to their specs. They are significantly better than grade 8 but I can not tell you exactly how much because they do not publish any specs on them. I suspect 150 to 200%.

    The other source that I found was from the custom auto industry. There are companies that make replacement bolts that also are significantly better than grade 8. Here is one such source:
    http://www.arp-bolts.com/

    I do not think that either of these sources has any left hand bolts. I would drill the hole out and tap it for a larger, English thread, left hand, grade 8 cap screw and be done with it. McMaster has left hand cap screws in English (inch) sizes and you can buy them in quantity of one.

    Since you said this was an import compressor, I would strongly suspect that either the screw was defective or it was installed with too much torque. A replacement of the same size and grade would probably work just fine.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  7. #7
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    12.9 grade socket head cap screws are very available. 8.8 is roughly equivalent to SAE gr 5 and 12.9 is roughly equivalent to SAE gr 8 (I did not look this up to check, but can if I am way off base.....)

    Here is a list of major distributors, in order who I would try first. I highly doubt they would sell to you, but you might be able to get a name of one of thier approved vendors who could order it for you. Expect miniums

    Maryland Metric
    Metric and Multistandard
    Kanebridge
    Lindstrom Metric

    After that, I would consider a work around.
    Last edited by cuemaker; 09-26-2012 at 05:58 PM.

  8. #8
    Join Date
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    UK
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    A stab in the dark...

    I'm guessing this bolt just runs axially into the shaft to retain a keyed crank piece? - If so, have you checked the key is actually present?

    A bolt like that shouldn't need to be extraordinarily strong. It may be assembled with a fair torque to prevent fretting, but there should be little additional stress during running.

    If however the key was omitted at the factory, the crank may have repeatedly turned on the shaft over a few initial start-ups, and eventually over-torqued the bolt.

    Cheers


    .

  9. #9
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    Oct 2004
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    DFW Texas
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    I agree with Mr. Barrington. I suspect it was overtorqued at the factory. Or under-torqued, and it broke in operation.

    Here's what I'd do:

    Check the key as Barrington suggests.
    Cleanup the shaft and the flywheel bore.
    Replace the LH bolt with whatever you can find to fit. A stud and nut perhaps?

    Put it all together with red loctite on shaft and threads.
    Make sure the flywheel seats fully on the taper, and that the bolt (and washer?) pushes it on a little more.

  10. #10
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    My uncle gave me one of those "airless compressors" years ago. It was so loud I couldn't be within 100' of the thing while running. He went through two of them on warranty then on the third "airless" incident, he gave the compressor to me. It had a nice 30 gallon tank--Hardly even used. I removed the compressor, threw it in the trash and turned the tank into a pressure blaster. Works great now.

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