Well... Jeff sez...
All bolts stretch. If they didn't they would be too brittle and would fracture. That is why we should NOT be using grade 8 bolts to mount our props. They are very hard, but also brittle. But, the stretch of an AN5 bolt on a wooden prop is so minimal that the fatigue is miniscule. They are more likely to fail from the prop being loose from not retorquing them regularly. I guess if one had their plane long enough, they could eventually fatigue from retorquing, but it would be decades. By then corrosion would likely be the dominating factor for replacement.
AN bolts are typically cad plated grade 5 bolts. Cad plated for corrosion resistance and grade 5 for fatigue resistance.
By comparison, the bolts used to hold your connecting rods together are torqued to spec one time, then disposed and replaced. They could probably be used more than that, but as a safety factor, we do recognize that they fatigue and we do not use them a second time. On some engines, there is no torque value for the rod bolts. Instead, the manual gives a spec for the stretch to be measured with a micrometer when torquing the rod bolts. The 200 hp angle valve Lycomings come to mind as one of the engines that require measuring the stretch of the bolts.
When I was still working for a living, part of my job was heat treating metals. Hardening the parts was easy. The more difficult part was to temper them back to a bit softer Rockwell or Brinel hardness to meet the specs specified by the engineers. But, without tempering, the parts would be so hard they were brittle and would fail.
-Jeff Scott
Arkansas Ozarks
Sent: Sunday, May 25, 2025 at 9:27 PM
From: "MS" <propbala...@att.net>
To: "KRnet" <krnet@list.krnet.org>
Subject: Re: KRnet> Propeller toque
From: "MS" <propbala...@att.net>
To: "KRnet" <krnet@list.krnet.org>
Subject: Re: KRnet> Propeller toque
I've worked with props a lot . . . balancing them primarily and in contact with Ed Sterba and others. I recall that for KR sized props 15 lb/ft is what you're needing. I've never heard of stretched bolts. Something new to learn I guess. See what Jeff Scott has to say about this.
Mike Stirewalt
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