What is a 'PK" screw?

mhorowit wrote:
I see mention of "PK" screws. Can someone point me to an illustration?

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Dear Mike,

The correct answer (and illustration) depends upon the application.
That's because 'PK' has both generic and specific meanings.

In the generic sense 'PK' means any sheet-metal screw, the 'PK'
reflecting 'Parker-Kalon,' the company that first made them. (But
didn't invent them.)

In aviation a 'PK' was not only a sheet-metal screw it was a temporary
fastener, the precursor of the cleco. Key differences was a fiber
washer affixed to the underside of the head so as not to mar the
aluminum, and a hex head, to allow it to be driven with less chance of
slipping (and marring the aluminum). Otherwise, it was your basic
coarse-pitch, blunt-tip (Type B ?) sheet metal screw, which was often
pressed into service when you ran out of 'real' PK's (and is still
commonly used as a sheet-fastener for repair work).

The funny bit here is seeing kit-builder's wings a'bristle with a
forest of clecos at two-bits a pop... when a real tin-bender would use
half a dozen clecos... and several hundred PK's at perhaps a penny each
:-)

I'm pretty sure I mentioned PK's in my 3-part epistle 'Riveting 101,'
which I've uploaded to my blog ( bobhooversblog.blogspot.com )

-R.S.Hoover