What is causing the lugnuts to be swollen?
I imagine it's from the heat caused around the wheel just wondered if there's a more scientific answer.
I've already switched to aftermarket ones after the tire shop had trouble getting the factory ones off - glad I wasn't stranded on the side of the road!

 

Why would you believe them to be swollen rather than thinking some nutcase tightened them way beyond their torque specification?

Guess that answer wasn't very objective considering they're two piece and therein lies the problem.

 

they are a two piece design and unfortunately multiple manufacturers used this design, there's basically an outer shell on them and thats what swells. Corrosion eventually gets them and I'm sure the heat they absorb speeds that along.

 

That, and aggressive use of an impact wrench hammers the outer stainless steel shell.

FWIW (For What It's Worth)... Have used shelled lug nuts for SEVERAL years on various vehicles with absolutely no issues. HOWEVER, the lug nuts are/were always taken-off and put-on using a ½ drive, long handle ratchet using a 6-point 13/16" deep-well, impact socket.
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Impact wrench has nothing to do with it but if it makes you happy continue to believe you can protect your capped lug nuts with a hand wrench until they swell and you have to believe me.

A pretty sheet metal cap is formed over your steel lug nut. Eventually the steel lug nut rusts, in part because the sheet metal cap retains water. There is nothing you can do about ti other than throw the OE lug nuts away in favor of solid stainless steel. Or chromed steel, which is subject to damage from impact wrenches.

Amazon (and I'm sure other sources) sell 21.5mm sockets to remove swollen lug nuts. Heck, if you buy the socket perhaps you might as well continue to use the swollen lug nuts.

 

Oh but it does. Didn't say it was the primary cause for the failure of the shells, however, once there's corrosion infiltration the hammering will accentuate and expedite the issue. *Well that and, worn or non-6-point sockets.

Me: Use the capped stainless steel for the winter shoes and I be happy for the last 20-years. But then too...have been a loyal Gorilla / McGard customer since 1986. Prior to that solid, chrome lugs nuts were the order-of-the-day.
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Cheap lugnuts. The factory ones are going to fail sooner or later. Not just Ford, but anyone who uses the ones with sheet metal stamped over the real lugnuts. Using an impact wrench and overtightening them just speeds up the process. Best bet is to just buy better lugnuts ASAP.

 

Where is the corrosion that caused this failure? I have a whole box of them if you want to cut them apart. That nut was 10 years in service.

 

If you look closely, you can see pitting, that's the corrosion.