QUESTION ABOUT WHEEL BOLTS ON PAWNEE TOW PLANES

[miltonpilot JLH]

At Air Sailing we have two Pawnee tow planes. One of them has recently broken one of the three bolts that hold the wheel halves together. The shank failed where it meets the threads. Examination of the failed bolt appears to show crystallization at the failure point. Once is bad enough but I have found out that this has happened to this one plane four or five times over the past year. It is not always the bolt in the same spot on the wheel and it is not always the same wheel. These are standard AN bolts that are properly torqued. Has anyone else experienced this on their Pawnees? Unfortunately I don't have any photos of the failed bolt. I will have to ask one of our tow pilots if he still has the failed one that we can get a photo of.

[Ian Molesworth]

Not my area of expertise but I recently saw an article on setting up the tracking ( wheel alignment ) on another tail dragger. Could that be affecting the loads on the bolts? 

I can imagine that a few extra degrees of toe-in or toe-out could impose quite a bit of load on the hub bolts. How does the particular aircraft handle on the ground? Different to others?

On Sun, 7 Jun 2026, 03:18 miltonpilot JLH, <milton...@gmail.com> wrote:

At Air Sailing we have two Pawnee tow planes. One of them has recently broken one of the three bolts that hold the wheel halves together. The shank failed where it meets the threads. Examination of the failed bolt appears to show crystallization at the failure point. Once is bad enough but I have found out that this has happened to this one plane four or five times over the past year. It is not always the bolt in the same spot on the wheel and it is not always the same wheel. These are standard AN bolts that are properly torqued. Has anyone else experienced this on their Pawnees? Unfortunately I don't have any photos of the failed bolt. I will have to ask one of our tow pilots if he still has the failed one that we can get a photo of.

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[miltonpilot JLH]

I should have mentioned that I am not a tow pilot. The broken wheel bolt problem was related to me by our primary tow pilot. He has flown this particular Pawnee for years and has never indicated to me that there is a tracking or alignment problem with the plane but I will ask.

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[Charles Mampe]

OK, I'm curious, would love to see the broken end of the bolts. I am sorta the "wheel and brake guy" for our club and some private ships. This includes a Pawnee with disc brakes and a split rim, a Supercub with disc brakes and a split rim, a number of Schleicher sailplanes with disc brakes and split rims along with a couple brands of drum brakes.

I have no clue how many times I have had rims apart for tube or tire swaps and other work.

I don't remember ever replacing the bolts.

I don't remember having a bolt break.

I don't see side loads (bad tracking, touchdown while side loading, etc.) would break the bolts without doing other obvious damage.

Possible reasons you have an issue in no particular order:

Bad batch of bolts

Over torqued in the past, now they're failing

Used hardware from the typical "misc hardware box" where all old stuff ends up in the hanger "just in case" but no history known.

Really bad luck

Maybe get a complete new set of hardware (from a new source) and replace everything (bolts, nuts and washers). Keep tabs on them.

I use a Unibit/step drill (by hand) to break the edges of each bolt bore so the bolt drops through and make sure the surface where the nut and head sit are "flat", no burrs. I also check the surface where each rim half contacts the other half is flat, no edge burrs, etc.

[Jeff Stetson]

Claiming no experience with Pawnees, but many split wheel setups like mine with Cleveland style brakes secure the brake disk to the wheel with the same bolts. The bolts must remain in tension as they are not sized to deal with shear forces. Hence torque from braking is meant to be handled by metal to metal friction between the disk and the wheel. If the mating surfaces are not clean, smooth, rust and grease free, this doesn't happen, resulting in the twisting force being applied to the bolts instead. An excessive force in the lateral direction can also result if the calipers are not "floating" properly. If the sliding pins are stuck, the piston only shoves the pad from one side, possible exceeding the tensile strength limits of the bolts. Repeated pull cycles from stuck calipers on the bolts could also induce fatigue cracks. 

[miltonpilot JLH]

FYI: the wheel halves on this Pawnee are new this year. I'm told when the one bolt broke, all three were replaced with new and properly torqued. I'm told this is the procedure anytime a bolt breaks.

I finally got photos (see attached) of the broken bolt with the nut and of a new bolt I'm told is the type specified for our Pawnee. I have not yet been able to find out the source of the new bolts.

I am not an A&P, and I am doing this research outside the chain of command of our club just because I like to know why things work and why they don't. JLH/H4

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[Tango Eight]

That looks like low cycle fatigue failure with at least a significant component in flexure.  See here for instance https://www.scribd.com/doc/262921674/Fatigue-fracture-surface-atlas

That's... a little suprising... suggests something might be amiss with the assembly.  Bolts are not typically loaded in flexure by intent.  

Oh, and I'm pretty sure those bolts crystallized... before they were bolts, lol.  

-Evan (materials engineering dweeb).

[Charles Mampe]

Nice link Evan.

Yes, a "clean break" within the threads of the nut (no signs of twisting or rust) threw me for a loop as well. Sorta rules some failures out, makes you think.

I have no new/better thoughts than I posted before.

[Tom Higgins]

What model # wheel is being used that is having this issue?

Reason I ask is that the Pawnee has two different wheel models. 

The 40-61 is used on the PA25-235 only and the 40-84A can be used on the PA25-260 or -235.

The main difference, as it applies to this discussion, is that the 40-61 wheel uses the AN5-36A bolt and the 40-84A uses the AN5-35A.

So if you happen to have the 40-61 wheel then your bolts are too short.

Tom

[miltonpilot JLH]

I don't know which model wheel is on our Pawnee. I will try to find out.

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