There Are Always Alternatives

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My recent Kneeboard Note on drilling bolts to create lighter weights for prop balancing generated a fair number of responses – all the way from “What a great idea, I need to make a few” to “you must be insane – you’re going to kill us all if those bolts break because you weakened them!!”  To be honest, I was worried enough about weakening the bolt that I drilled a couple of extras, spun a nut on them, put them in a vice, and went after them with a torque wrench to see where they’d break. In most cases, I spun the threads off before the bolt broke. In the one case where the bolt broke, it was because I had drilled it too deep and gotten all the way into the threaded portion, which obviously weakened the shaft too much. The moral is—measure carefully, and don’t go too close to where the threads start. Also, in defense of the “you’ll kill us all” comment, the writer thought originally that I was suggesting drilling the prop mounting bolts, not balance bolts. And I agree with him—do not TOUCH anything like the mounting bolts!

Standard Titanium aircraft-grade bolts and nuts give you a very lights alternative to steel bolts, and attach like any bolt and nut.

But there are alternatives to drilling your steel bolts, and few of these showed up in my mail after we published the article.

The real answer to lighter bolts is Titanium—a metal that is less than half the weight of steel. If you can find aluminum bolts, they will also work, but you have to be very careful not go tighten the nuts so much that you strip the threads. And… they are harder to find! Titanium bolts in 1/4″x28 size are more easily obtained, either new from aerospace-specific fastener houses, or on eBay, where they appear as surplus from aerospace companies. Pictured below are a couple of flavors of titanium fasteners that you might find. If you are going to be balancing a lot of props a bag of either type might be handy to have in your box of tools. You’re going to be spending close to a dollar apiece even at eBay pricing – but that’s a lot better than the six dollar starting prices for new fasteners!

Hi-Lok fasteners can be obtained in Titanium (left)—they feature a smooth flat head on one end, and provisions for an allen wrench on the end with the threads, so you put it through the flywheel, add weights, start a nut, then put an Allen wrench on the end to hold it from turning while tightening the nut. Very slick looking in the end. Right: a close-up of the HyLock fastener shows the end that gets the Allen wrench—another tool to add to the balancing kit.

 

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Paul Dye
Paul Dye, KITPLANES® Editor at Large, retired as a Lead Flight Director for NASA’s Human Space Flight program, with 40 years of aerospace experience on everything from Cubs to the Space Shuttle. An avid homebuilder, he began flying and working on airplanes as a teen, and has experience with a wide range of construction techniques and materials. He flies an RV-8 that he built, an RV-3 that he built with his pilot wife, as well as a Dream Tundra they completed. Currently, they are building a Xenos motorglider. A commercially licensed pilot, he has logged over 5000 hours in many different types of aircraft and is an A&P, EAA Tech Counselor and Flight Advisor, as well as a member of the Homebuilder’s Council. He consults and collaborates in aerospace operations and flight-testing projects across the country.

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