Milman Spar Bolt Grip Lengths

[Ctort1]

This very specific question is for anyone that has installed the Milman conversion. Did you find most of your bolts were too short?
Milman specifies that the compression struts should use the supplied AN4-13A bolts instead of the -12A the parts book calls for.
Ok, but the aileron brackets, sector bracket, large ribs, etc also seem to be not showing enough threads. Do all bolts need to step up a dash size????

Here are some photos. Most of these are just a TIGHT snug, but I won’t get much more turn when it torque.

 

[Thedude]

They look short to me. Standard practice is 1-3 threads beyond the nut, partially to ensure you have full engagement of the locking surface on the bolt threads.

 

[Ctort1]

They look short to me. Standard practice is 1-3 threads beyond the nut, partially to ensure you have full engagement of the locking surface on the bolt threads.

Yes, well, and they look short to me, too, which is why I made this post to see what others have experienced.

 

[Bartman]

no nuts under the heads of the bolts, right? As I understand it, the FAA changed guidance on this from requiring three threads to requiring the end of the bolt be better than flush with the nut. Do the Milman drawings require the full thickness washer?

 

[Bartman]

I found a few pics from my project that show the nuts/bolts were not much better than yours. not everything was fully tightened yet in some of these pics

Post in thread ''68 Citabria 7ECA Milman Spar Conversion'

Apr 21, 2018

A few examples of the spacers used to mount the old ribs & fittings to the new aluminum spars. The wood spars had a rectangular cross section, the aluminum spars are an I-beam so the spacers make the thickness of the I-beam match the wood. There are both wood and aluminum spacers. The wood ones come pre-sealed and the aluminum spacers are primed.

Ribs (nuts are not tightened yet on this one)


Rear wing attach fitting


Front strut attach point


Compression strut

• Bartman

 

[Ctort1]

no nuts under the heads of the bolts, right? As I understand it, the FAA changed guidance on this from requiring three threads to requiring the end of the bolt be better than flush with the nut. Do the Milman drawings require the full thickness washer?

Appreciate the response.
I’m not familiar with “better than flush” terminology.
I have only found “1-3 threads exposed”.

If you mean “no WASHER under head of bolt” there are places where you absolutely need a wide area washer under the head as specified in the parts book. Perhaps there are just a few expceptions where this is not required such as the aileron bracket because the bolt head used to be against wood but is against a solid metal spacer now.

Regardless, the Milman guidance is “use original hardware” (specified hardware), except one note where they say use a dash longer on the compression tubes because the combination becomes thicker.
Well, it seems the combination is thicker just about everywhere.
My solution is a combination of either longer bolts (argh!) or thinner small area washer (ie. AN970-416L).
In some cases adding a longer bolt risks bottoming out the nut (can feel it tightening). Just an odd situation all around.

I definitely looked at your photos in the build thread last night and couldn’t find anything conclusive. Thanks.

In the meantime this is the response from Mike at Milman:
“In our spar kits we supply 24 pcs. of AN4-13A to replace AN4-12A bolts at the compression members due to this condition. Over the years we have noticed that the hardware manufacturers tend to favor the -1/32" end of the length tolerance. This makes it more likely that the stock bolt length will not meet the minimum 1 thread exposed beyond the nut.”

Frustrating after ordering all the “correct” hardware per the parts manual.

 

[Tangogawd]

We had the same issue when I was doing my conversion. I used thin washers where I could, and up sized just a few bolts.

 

[Bartman]

I also don't think you need a washer and a fender washer on the wood spacers unless there are other issues with bolt/grip length. A fender washer counts as a washer and yes, fender washers wherever hardware bears on wood.

 

[Sully]

To add to the problem. Bolt length is + 1/32 - 1/64. total of .0468".
32 thread pitch is .0313" per thread.
I've been finding a good deal of hardware manufactured closer to minimum spec.

Agree one washer is all that is needed.

 

[Ctort1]

I also don't think you need a washer and a fender washer on the wood spacers unless there are other issues with bolt/grip length. A fender washer counts as a washer and yes, fender washers wherever hardware bears on wood.

To add to the problem. Bolt length is + 1/32 - 1/64. total of .0468".
32 thread pitch is .0313" per thread.
I've been finding a good deal of hardware manufactured closer to minimum spec.

Agree one washer is all that is needed.

I agree with you also, and this is NOT the problem as you can see with the 20 other fasteners.
To address this bolt, and draw attention away from it, it is the one funky bolt on the aileron sector bracket that faces the “other way” and is held in by a large cotter pin. I have never found the small -416 washer under the head, yet the book calls out an AN4-14A bolt, wide area washer, and -416 small washer. In order to keep the nut from bottoming out on the threads the -416 washer seems needed.
So either I ignore the book, step down to a -13A bolt and omit the small washer, or I roll with it like is. Ps. There’s no need for a small washer under the head either against the metal bracket.