If you are building an early RV (the RV-4 was the progenitor of the F1 Rocket that we are finishing up), you are probably familiar with the main spars that go halfway through the fuselage and are mated to the cockpit structure with steel splice plates and scads of bolts. Newer designs have heftier center sections and the stub spars don’t go as far into the fuselage—so there are a lot fewer bolts to install when stabbing the wings.
Cantilevered wings must fit tight—no bracing to complete a truss triangle is allowed. That means that the holes that attach the spars together with their splice plates must be precision-drilled and use close-tolerance bolts. The F1 uses 20 close-tolerance bolts for the assembly—16 AN6 bolts and four AN4s—and then a bunch of regular AN3s outboard of the splice plates whose holes are a little less critical. Close-tolerance bolts can be identified by the “dish” in the head, and if that’s what is called for, that is what you must use. The spars and splice plates are drilled “in assembly” using a precision reamer, and you aren’t allowed to “chase” the holes at assembly time to make the bolts fit. Taper-tipped pins (made on a lathe from long bolts) are generally used to install the wings initially, then they are replaced with the final bolts. Close-tolerance bolts can be hard to install, since there is—by definition—no slop. In the past, I have used dry ice to chill bolts down overnight, shrinking them an almost infinitesimal amount. Oddly enough, no dry ice was available in our nearby city—the state capital of Nevada—so I tried the first bolt after giving it a coat of Fast Dry lubricant spray. Tapping the bolt in with a small hammer and using the bolt to drive out the taper pin (think about the final bolts’ direction when installing the pins) worked so well that I stopped thinking about cold bolts and got all the main fasteners installed in an hour.
If you’re familiar with the old guideline that aircraft bolts should be installed with “heads up and forward,” you’ll look at our picture and wonder why I didn’t follow that pattern. The answer is that first—it is a guideline, not a hard-and-fast rule if there are other considerations. And second—the floor that goes aft of the spar has to be removable for inspections, and bolt heads are shorter than bolt tails and nuts, so this gives us better clearance for maintenance. “Other considerations …”
Last tip: I like the lubricant spray for the body of the bolt but didn’t want it on the threads because it can affect the torque. So I drilled holes in a chunk of handy wood (all builders have a scrap pile … right?), inserted the bolts, and sprayed them down with the threads hidden. It’s quicker than tape.
My RANS S-19 has only six high tolerance bolts to attach each wing to the spar carry-thru. I have confidence in the RANS design, but I would “feel better” seeing more bolts!
That’s OK – the original RV-1 had two taper pins per wing on the main spar attach points, and it was routinely used for Aerobatics!
Hey Paul! The WinCo Foods in my local area (southern California) carries dry ice, and it looks like they have a few branches in Reno. Something to keep in mind for your next build!
Paul,
It seems like there are a variety of dry lubricant sprays out there (silcone, graphite, etc.) Which did you use?
CRC Dry Moly-Lube is what I had on hand. We also have a dry graphics spray that we use on hinge pins, but I am hesitant to put graphite, steel, and aluminum together if I have another option.
I’m about to replaced a close tolerance bolt (an AN175-16) in my certified plane’s nose gear and the replacement purchased from well known aviation supplier was literally a normal AN bolt that had been machined down removing metal and much of the plating from the bolt. Now I am wondering whether the bolt is proper.
I am hardly what one would consider and expert in aircraft hardware, but even to me, these sounds pretty dodgy! The plating is part of the corrosion-resistance, and therefore part of the Mil Spec. That sounds like a scrap bolt to me unless the aircraft manufacturer told me it was OK.
Thanks!