Drill bits to drill main spar attach holes.

[Gaynier]

I used the Sonex method and recommended drill bit (McMaster-Carr) to do my main spar bolt/pin holes. I was actually quite successful, or so I thought. The holes look clean and round, and my bolts were nice and snug.... with everything assembled. But when I pulled the wings off and tested each hole with the AN6 bolt individually I was surprised how much play there was between the bolt and each hole. Keep in mind that a bolt is a couple of thousandths smaller than even a perfectly drilled 3/8" hole and, as someone else mentioned earlier, when the wings are on it can seem as if everything is snug due to any misalignment of the individual parts or normal forces on the bolts due to the weight of the wings. You have to under drill the hole and ream to the actual size of the bolt (0.372-0.373) for a true snug fit.
So, what did I do to fix my problem and get a snug fit on each individual hole? I purchased some 3/8" drill rod (drill rod is 0.375 plus or minus 0.0005), I ground a slight taper on one end, and found it had to be tapped lightly with a rubber mallet to get it to slip through each hole. Viola! A perfect snug fit on each individual hole.
BTW, if you've really screwed up and need to go to 7/16" (or just want larger spar bolts) I have two drills that I purchased on Ebay that are stepped (25/64 - 13/32) and "double margin". Google "double margin drill bit" if you've never heard of them. I also have two reams from Brown Tool that are 0.4219 x 0.4375 for getting a really good 7/16" hole. I'm willing to lend them to a poor soul in need (since it's probably bad etiquette to offer anything for sale on this forum). Keep in mind even a close tolerance bolt (AN177-XX for a 7/16" bolt) is still going to be a thousandth or so less than the hole but I figure if you're within a thousandth of an inch you're probably doing better than 90% of the aircraft that are built and flying.

[rizzz]

Hello Gaynier,

Just a little bit of caution here:

I had similar issues, see this thread: viewtopic.php?f=21&t=750&hilit=wing+attach+block, and initially I was investigating going up to 10mm bolts which are only slightly thicker than the AN6 bolts. Only very little reaming would have been required to make them fit perfectly.

I also found some 8.8 grade high tensile 10mm bolts that would do the job nicely.
I know Sonex uses hitch pins on at least some of their aircraft (you can actually see this in the wing install/removal instructions you can download from their site) so I did not think replacing the AN6 bolts with something else was a big deal.

However, when I contacted Kerry to ask if these would do the job he did admit they have hitch pins on some of their aircraft as did his own airplane, BUT, he was reluctant to comment any further on the use of anything else other than the bolts called out for in the plans (apart from going AN 1 size up) given the criticality of these and he stated I need to carefully investigate myself if these would carry the loads required.
Remember, the max aerobatic weight of the whole plane at 6G could be hanging from these 2 bolts.

As I’m not an aircraft engineer I did not feel comfortable making that call even though I’m pretty sure the bolts I found would have been suitable (they had a shear strength of over 11 metric tons if I remember correctly) so this was one of the many reasons in the end I decided to just rebuild the spar box and forward attach blocks.

So just make sure you do your research to make sure those drill rods can carry the load is all I’m saying here I guess.

BTW. If the fit is that close anyway, why not try and use close tolerance AN bolts?
They are slightly thicker than the equivalent AN bolts. For an AN6 bolt, the min/max diameter is .371/.374. The corresponding close tolerance bolt,has a min/max diameter of .3737/.3742.
AN176 are the close tolerance equivalent of AN6 bolts.
Spruce might not sell them at the correct length but you should be able to find them at more specialized military standard aviation suppliers.

Regards,
Michael

[Gaynier]

Hi Michael,

FYI, you can call me Ron.

You make a valid point and, in general, I make it my policy not to make substitutions or changes without consulting Sonex Tech Support.

When I asked Sonex Tech Support about machining my own wing pins and what steel alloys they'd consider acceptable their reply was, and I'm paraphrasing, "we use trailer hitch pins." Reading between the lines I interpreted that answer a certain way ;-) . That wasn't a recommendation from them to use hitch pins and I can understand Jeremy's reluctance to recommend them. An AN bolt is manufactured to specifications, what specifications are there for trailer hitch pins? Who knows what kind of stuff comes into the US from overseas and is sold through hardware and auto parts stores? For all they know someone might get a trailer hitch pin from their local hardware store made from pot metal. If they specify an AN bolt and you use an alternative you're on your own and it's out of their hands.

Having said that, O-1 drill rod is also manufactured to a specification so I can be confident of the quality of the steel. I've checked the spec for O-1, and the spec for SAE 2330 steel (one of the common steels used for AN bolts) and I'm confident it will work as a substitution. When I mentioned to Sonex Tech that I was planning to use drill rod they were silent in response.

Just as important is the fit of the pin through the holes. Since this is a shear application it's important to get a nice snug fit. This is what Sonex Tech Support did emphasize, it has to be snug enough that it requires tapping with a rubber mallet to get the pin to go in. While the holes look good, less force than that was required to push them through. The drill rod fits perfectly in each hole.

I have ordered two AN176-51 bolts, yes I know they're 7/8 too long, from Skygeek at $22 each. I'll try them and if I like the fit I'll use them instead. BTW, this was the best price I could find for bolts where a manufacturer's certification was also offered (for an additional $10 each). One quote I received for AN176-44 bolts from another source was $150 (One Hundred, Fifty dollars) each. Cost shouldn't be a factor in making this decision, it's just FYI for anyone considering using close tolerance bolts.

Again, you make a valid point and I'm not encouraging substitutions. I'm only saying that in my case the drill rod was a good option for a snug fit.

Ron

[Bryan Cotton]

Hey guys,
I will soon be drilling the aft spar tunnel holes using my bushings. Like some have suggested, my floor and floor structure are out. Is there any chance the fuselage could shift without the floor being clecoed in place? I do have the tank cover and instrument panel clecoed in. Maybe I am just paranoid but would like to know if anybody had an issue from removing the floor.

[wlarson861]

My floor was riveted in place when I drilled the wing attach. The tail cone bottom skin was removed to gain access. I think the floor helped insure that nothing moved in the spar areas. I may have been able to do without the floor but I don't know. I think the added rigidity was a good thing. Your milage may vary.

[sonex892.]

Hey guys,
I will soon be drilling the aft spar tunnel holes using my bushings. Like some have suggested, my floor and floor structure are out. Is there any chance the fuselage could shift without the floor being clecoed in place? I do have the tank cover and instrument panel clecoed in. Maybe I am just paranoid but would like to know if anybody had an issue from removing the floor.

I really doubt you will have a problem Bryan. My floor panel was updrilled, de-burred and put aside well before rigging the wings. My glareshield wasnt even in place when I rigged the wings. The floor panel was actually the last thing to be rivetted to the plane just before the approved person ( what you call a DAR) arrived. All the rivets went in normally, nothing had shifted.

Steve
Sonex 892

[vernd]

Hi – I know this subject has been much discussed already – but I just want to get some input from those who have drilled these parts already before I move forward.
I’m using the piloted double margin drill bit from Fastenal and working on a practice piece right now using drill press.
The 3/8 drill diameter is .3745.
The AN6-44 bolt mics at .372 dia and the trailer hitch pin mics at .371.
So for those of you who have drilled out the attach blocks with 3/8 bit and used AN6 bolt or trailer hitch pin – have you been happy with the fit? I’ve heard comments about having to lightly “tap” the bolt or pin into place after drilling. However my experience with the practice pieces is it fits nicely but in no way requires a tap to fit together. Which makes sense with the fact that the bolt is 2 – 3 thou smaller in diameter than the drilled hole.
I’m probably over thinking this, but would appreciate learning from the experience from others before committing to drilling these parts.
Thanks....Vern

[Corby202]

Mine were a tap fit when putting it all together. The bolts slide through the individual parts easy enough, if that makes sense.

[ededoad]

I used an NAS close tolerance drill bit. It has a 1/4 inch nub on the end to center on a 1/4 inch hole. I built from scratch so I was very careful to drill all of the holes in the first spar web and match drill to the others. I used an old Portaline drill guide and a drill press to drill the final 1/4 inch holes. After the spars were completed I drilled the main bolt holes to 3/8 inch with the NAS drill bit and the Portaline drill guide. On the wing install I made a drill guide out of a piece of 3/8 thick walled chrome molly tube drilling the center out to 1/4 inch. The rest is by the Sonex on line instructions drilling front to back and so on with the wings aligned. I had a fairly tight fit when finished and drove An bolts in with moderate hammer blows.

I don't have the full kit anymore but I do have the NAS drill bit. These are expensive and I can loan you mine.

Ed

[vernd]

Thanks Phil – yes that does make sense. I’m still at the stage where the bolt slides through the test piece with a good fit but not ready for wing rigging to try it yet. So I can see when installing all the pieces together where it would require a tap to fit.

Ed thanks for your reply also. I have never heard of an NAS close tolerance drill bit but will do some research there too. Thanks for the loan offer – I’m quite a ways off from needing it yet. Still at the practice drilling stage while assembling the wing but will keep it in mind.

There is a lot of good information here and I appreciate all who share their learnings.