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Thread: Question on strut fork bolts

  1. #1

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    Question on strut fork bolts

    Hello members, another question from a first time carbon cub builder.

    the strut forks are attached to the fuse with 3/8 bolts.
    I reamed the holes on the fuselage with a 3/8 reamer but for some reason (i think reamer misalignment) one hole came out slightly larger diameter than i would expect.
    I can feel some slack when I insert a 3/8 bolt into the hole. I think it is about 0.006 -0.008 inch or so.

    So my question is - should the fork bolts have tight fit or some slack is ok?


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    Thanks!

  2. #2

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    how does the actual bolt that goes in there fit? its a special bolt that goes in there and i dont remember if its exactly 3/8s?

  3. #3
    Mauleguy's Avatar
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    .006-.008 is a lot oversize. Ream .374-.376
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  4. #4
    Crash, Jr.'s Avatar
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    The Univair strut bolt U13241 measures .3700" by my calipers. That would be .005 clearance in a nominally sized hole so that would be the max clearance I'd shoot for.

  5. #5
    mike mcs repair's Avatar
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    Question on strut fork bolts

    Those holes get HUGE(not uncommon to see 1/16” slop in fork movement) on high time working Cubs. Not an issue. They beat around as you taxi. If it bothers you yo can throw new bolts in every decade or so. Not scary at all to me.


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  6. #6

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    Quote Originally Posted by mike mcs repair View Post
    Those holes get HUGE(not uncommon to see 1/16” slop in fork movement) on high time working Cubs. Not an issue. They beat around as you taxi. If it bothers you yo can throw new bolts in every decade or so. Not scary at all to me.


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    Thanks for sharing your experience - as I said before - I simply don't know if this is an issue or not an issue. Every slop scares me.
    So what do people do if the slop becomes excessive? Do they ream holes to 0.45'' and insert tight fit bushings?

  7. #7
    mike mcs repair's Avatar
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    Quote Originally Posted by Cooley View Post
    Thanks for sharing your experience - as I said before - I simply don't know if this is an issue or not an issue. Every slop scares me.
    So what do people do if the slop becomes excessive? Do they ream holes to 0.45'' and insert tight fit bushings?
    you drill it out larger and weld in a bushing... or i should say others do.... me i ignore it, it's not an issue...

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    Quote Originally Posted by tempdoug View Post
    how does the actual bolt that goes in there fit? its a special bolt that goes in there and i dont remember if its exactly 3/8s?
    yeah, the bolts are the same diameter, I found interesting that it is tight fit between the bolt and fork - it takes serious pressure to stick the bolt in, but very loose fit between the bolt and the eye in fuselage..

  9. #9
    cubdriver2's Avatar
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    For the last 4000hrs I have used just AN bolts in strut forks. My Vagabond had some play so my IA recommend zero tolerance bolts, all good

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"
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  10. #10

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    Quote Originally Posted by cubdriver2 View Post
    For the last 4000hrs I have used just AN bolts in strut forks. My Vagabond had some play so my IA recommend zero tolerance bolts, all good

    Glenn
    Would knurling the bolt be an idea to create an a interference Fit?




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  11. #11

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    people chrome plate bolts for this very purpose. chrome plated aluminum is extremely hard to scratch - it looks like steel if you dont know it is aluminum.

  12. #12
    BC12D-4-85's Avatar
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    I flew an old leased Frontier Flying Service Cub that clunked every time it hit turbulence. Lifting the wing tips up and down found the source - fuselage fittings worn. Their mechanic 'fixed' it by tightening the nuts on the fork bolts. Even IFR it never bothered me much.

    Gary
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  13. #13
    Gordon Misch's Avatar
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    Would knurling the bolt be an idea to create an a interference Fit?
    In my opinion, that would be a resounding "no". Stress risers.
    Gordon

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  14. #14
    aktango58's Avatar
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    Quote Originally Posted by JeffP View Post
    Would knurling the bolt be an idea to create an a interference Fit?




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    To expand on the above comment, knurling does not add metal, it only indents and forces the displaced metal to the side and pushes up the surrounding metal. This leaves less of the outside expanded metal bonded to the internal metal, and as said above- areas for stress fractures are created.

    Bolts are a funny thing in structural environments. Sometimes you want grade 8, sometimes you want grade 5. Sometimes you just swear at the mechanic that put that worn out bolt back in!

    As far as slop goes, you have responses from very reputable folks. Good luck, hope the project gets into the air soon!
    I don't know where you've been me lad, but I see you won first Prize!
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  15. #15
    Steve Pierce's Avatar
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    Does chrome introduces hydrogen embrittlement? I think you have an issue operating the reamer. Not a fun job reaming something like that by hand. Had to ream all four of those fittings on a couple of Carbon Cubs while they hung from the ceiling because you can't get to the holes with a reamer with the gear installed. There was a fixture issue at Cub Crafters on the strut attach/gear attach fitting that caused them to need to be reamed as well as the forks and an oversized bolt installed. Standard bolts need to be installed upside down because they are longer than the special strut bolts and can contact the landing gear tube if the prop length bolt is used with no threads in the shear area of the fitting. I do not want any slop in the strut attach fittings.



    Steve Pierce

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  16. #16

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    and when you finally tighten that bolt, finger tight and 3/8 to 1/2 turn more, DONT crank it down. snug. i should say those 4 bolts.
    Last edited by tempdoug; 11-06-2020 at 08:23 AM.
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  17. #17

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    Steve - thanks for posting this. A priceless video. I agree that I do have issue with operating reamer but after thinking about it more and watching your video i have to conclude that 70% of issue comes directly from CubCrafters.
    if you closely look at the yellow fitting - you see that the hole was not really reamed thru, you can see a horizontal line inside the hole (the fitting made out of two plates) and untouched metal around it.
    I have similar story on my left side fitting where I promise you - I did not screw up with the reamer and still have a play similar to what you have in your clips. When I reamed left hand side holes - the reamer barely touched the surface - it almost fell thru..

    So my version is this:
    they have 2 plates that constitute the fitting, 1/8 inch each. They drill 0.37 holes in each plate first and weld two plates together after. When they weld them together - they suppose to exactly line up these holes as 0.005 which is left for the reamer is not enough.
    But they don't - the holes on two plates don't line up when they weld them together to the frame. So when you get your fuselage - you have upper plate misaligned with the lower plate, you ream it but since misalignment between upper and lower hole greater than 0.005 you get the slop we are talking about.
    They (CC) either need to align the plated better or use 0.36 drill bit when making those holes. See pic below:
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  18. #18
    Steve Pierce's Avatar
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    The issue was the misalignment of the two parts in the welding fixture and I believe someone on the shop floor used a drill to get the bolt to go in. That issue was addressed and fixed years ago. If you haven't reamed your other holes check them and verify. I have not heard of this issue and believe it would be a hot topic. What does Dave Emory say about it?
    Steve Pierce

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    Unfortunately I have reamed all 4 holes already. I have not checked with Dave yet. Will post my findings to their forum today... I am confident that misalignment caused my problems - lower plate if off and when you ream from the top - you ream upper hole and only part of lower hole...This is a damn shame (c)

    But Steve - you tell me this - how did you fix what you filmed? Oversized AN bolts? Did you reream 0.385 both fork and the fitting and use 0.385 bolt - something like this?

  20. #20
    RaisedByWolves's Avatar
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    Quote Originally Posted by Cooley View Post
    Unfortunately I have reamed all 4 holes already. I have not checked with Dave yet. Will post my findings to their forum today... I am confident that misalignment caused my problems - lower plate if off and when you ream from the top - you ream upper hole and only part of lower hole...This is a damn shame (c)

    But Steve - you tell me this - how did you fix what you filmed? Oversized AN bolts? Did you reream 0.385 both fork and the fitting and use 0.385 bolt - something like this?
    Over size forks and bolts that go in upside down and you have to wait awhile for CubCrafters to manufacture them


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    Quote Originally Posted by Cooley View Post
    Steve - thanks for posting this. A priceless video. I agree that I do have issue with operating reamer but after thinking about it more and watching your video i have to conclude that 70% of issue comes directly from CubCrafters.
    if you closely look at the yellow fitting - you see that the hole was not really reamed thru, you can see a horizontal line inside the hole (the fitting made out of two plates) and untouched metal around it.
    I have similar story on my left side fitting where I promise you - I did not screw up with the reamer and still have a play similar to what you have in your clips. When I reamed left hand side holes - the reamer barely touched the surface - it almost fell thru..

    So my version is this:
    they have 2 plates that constitute the fitting, 1/8 inch each. They drill 0.37 holes in each plate first and weld two plates together after. When they weld them together - they suppose to exactly line up these holes as 0.005 which is left for the reamer is not enough.
    But they don't - the holes on two plates don't line up when they weld them together to the frame. So when you get your fuselage - you have upper plate misaligned with the lower plate, you ream it but since misalignment between upper and lower hole greater than 0.005 you get the slop we are talking about.
    They (CC) either need to align the plated better or use 0.36 drill bit when making those holes. See pic below:

    if you go to pipers print that hole should be 1/4 inch then after welded made to size, i would think thats how cubcrafters does it also, but i dont know.

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  23. #23

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    Quote Originally Posted by RaisedByWolves View Post
    Over size forks and bolts that go in upside down and you have to wait awhile for CubCrafters to manufacture them


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    ahh, thanks.... it makes me thinking about joining Mike's camp ...

    I now understand what people mean when they say "it takes 3 planes to build to learn how to build a good one"

    on a different note: anyone has opinion on a plug Gage set, which one to buy? (Steve used in his video)
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    and i hope you dont mind this comment!! but start with a wire brush in the holes to get paint out and not reamers and have the bolt that goes in the hole from the very beginning. and again i am NOT saying yours wasnt bad to begin with!

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    Quote Originally Posted by tempdoug View Post
    if you go to pipers print that hole should be 1/4 inch then after welded made to size, i would think thats how cubcrafters does it also, but i dont know.
    no, they did not touch the holes after they welded two plates together. see blue metal on the pic below.

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  26. #26

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    Quote Originally Posted by Cooley View Post
    no, they did not touch the holes after they welded two plates together. see blue metal on the pic below.

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    i dont know, there building there airplanes and not pipers, so if you visit with them let us know how they do it.

  27. #27
    fobjob's Avatar
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    Quote Originally Posted by tempdoug View Post
    and when you finally tighten that bolt, finger tight and 3/8 to 1/2 turn more, DONT crank it down. snug. i should say those 4 bolts.
    Recently I found myself reattaching my wings, (long story) and my IA sent away to Univair for replacement bolts. When using the new 3/8 bolts on the strut forks, I discovered that the short castellated nut could not be tightened enough to pass through the proper sized cotter pin unless really tightening down so much that the forks were being pre-loaded way too much for my self-survival instincts...even with a thin washer...
    Investigating, the new (close tolerance) bolts had a grip length about 1/16 inch shorter than the old bolts. Apparently, the grip length spec on close tolerance bolts has been changed. (Or something)...
    The requirement of the Clevis forks is 5/8 inch grip length, the old bolts are 5/8 inch, and the new bolts only come in grip lengths of 9/16 and 11/16 inches....and the bolts I got were 9/16.
    I think this is dangerous, as it tempts someone to really preload the Clevis forks so as to get a proper cotter pin through. Of course, we all think that no one would do that....does anyone have any info on this?

  28. #28

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    Interesting thread.
    Is there anything that would prevent using elastic stop nuts (shear type) on those bolts, except for the obvious hole in the drilled bolt?
    I see a lot of Cubs these days with stop nuts on all eight landing gear bolts, and have not seen any of them come loose. And those bolts can rotate in service, unlike strut bolts,
    My impression is that those lower strut bolts are simply cut-off AN bolts.
    I am disappointed that oversize AN bolts are no longer available. We saved a 180 empennage with them.

  29. #29
    fobjob's Avatar
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    I expect that if they decided to use castellated nuts on the strut fork bottom ends, they had a good reason. A more certain containment of the nut, for example.
    Apparently the old bolts were a NAS1103 series, which is obsolete... replaced by NAS 6203 ...?

  30. #30
    Steve Pierce's Avatar
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    Not supposed to be a washer there.
    Steve Pierce

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    Will Rogers

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    Oh, yeah - I missed that in Fobjob's text. No washers!

  32. #32
    fobjob's Avatar
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    Yikes! The old bolts/ nuts had washers...so, I thought it was standard practice to use a washer under every nut. Are they just for spacing, and not to isolate the nut from torsional forces?

  33. #33
    Steve Pierce's Avatar
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    All I know is that they are in shear and no need to be very tight. I have spent to much time looking at the drawings I guess. Also rebuilt 5 Super Cubs that had never been apart.
    Last edited by Steve Pierce; 09-07-2021 at 12:19 PM. Reason: correction
    Steve Pierce

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  34. #34
    acroeric's Avatar
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    Several years ago I repaired a ground looped Super Cub. When we removed the wings I found the strut attach fittings elongated. I contacted Piper and got a manufacturer approved repair procedure to ream the holes oversize and install a bushing. The bushings took forever to arrive so I ordered a few extra sets just in case. This particular PA-18 was one of the last ones built and was very low time and I doubt the groundloop had anything to do with the holes. I was surprised to find them like that.
    Anyway- my point is that I can get the N# (for the 337) and engineering data and have a few of the bushings if it helps anyone. It was an easy repair but correctly reaming the holes required some thought.

    EDIT- Found the engineering order and just added it for future reference.
    Attached Files Attached Files
    Last edited by acroeric; 09-07-2021 at 07:28 AM. Reason: Adding Piper Engineering Repair
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  35. #35
    Steve Pierce's Avatar
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    Quote Originally Posted by acroeric View Post
    Several years ago I repaired a ground looped Super Cub. When we removed the wings I found the strut attach fittings elongated. I contacted Piper and got a manufacturer approved repair procedure to ream the holes oversize and install a bushing. The bushings took forever to arrive so I ordered a few extra sets just in case. This particular PA-18 was one of the last ones built and was very low time and I doubt the groundloop had anything to do with the holes. I was surprised to find them like that.
    Anyway- my point is that I can get the N# (for the 337) and engineering data and have a few of the bushings if it helps anyone. It was an easy repair but correctly reaming the holes required some thought.
    I would be very interested in that data and a set of those bushings.
    Steve Pierce

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    Will Rogers

  36. #36
    acroeric's Avatar
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    Steve- I just added it. I have several of the bushings. Happy to share.
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  37. #37
    fobjob's Avatar
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    Thanks again to tempdoug for sending me the service bulletin that makes clear the strut bolt configuration!
    http://www.shortwingpipers.org/photo...er_sb1172a.pdf



    too impatient to be a craftsman...

  38. #38
    skywagon8a's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    All I know is that they are in Shear and no need to be very tight.
    Shear not tension.
    N1PA

  39. #39
    Steve Pierce's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Shear not tension.
    LOL, yes, hadn't had my coffee yet.
    Steve Pierce

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  40. #40
    fobjob's Avatar
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    9/16 grip bolt with no washer fits perfect....

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