Thanks, all. I realized that I can anti-tamper the bolts simply by
double-nutting them on the farside. They can spin the heads all they
want but the nuts won't loosen.

On Thu, 2011-05-12 at 18:44 -0700, Richard Koerner wrote:

> Obviously, the square, rectangular, or oval shoulders under the head all serve the same primary purpose I would think....anti-rotation to torque.  So, not sure what your needs are, but there are other useful ways to add anti-rotation to a fastener--like pinning with a cross-holes (dowel pin) for example.  Uhh-h-h....maybe even Loctite....depends on what you are doing, materials involved, amount of anti-rotation desired.  Spot-weld?
>
> And I guess you could make your own rectangular shoulder with file and a vise.  But all depends on if you want to make the side of the square longer or shorter, right?!
>
> Also, there may be an option with a third component, like some sort of washer with....a square hole (ID) to fit your bolt but a rectangular feature (OD) to fit your situation.
>
> Need more specific information to solve.  Think outside the box?  Yeah....we do that here.
>
> Rich
> San Diego
>
>
> --- On Thu, 5/12/11, Alistair Bell <albell@SHAW.CA> wrote:
>
> From: Alistair Bell <albell@SHAW.CA>
> Subject: Re: Modified carriage bolts?
> To: vanagon@GERRY.VANAGON.COM
> Date: Thursday, May 12, 2011, 6:21 PM
>
> there is an oval shouldered one in this handy ref doc on my blog
>
> http://shufti.files.wordpress.com/2010/06/jg.pdf
>
>
> alistair
>
>
>
> On 12-May-11, at 6:08 PM, Rocket J Squirrel wrote:
>
> > Carriage bolts have a short square section of shank under the head.
> > They
> > are used in square holes.
> >
> > Is there such a thing as a carriage-type bolt that has a rectangular,
> > rather than square, section under the head?
> >
> > --
> > Rocky J Squirrel
> > '84 Westfalia: Mellow Yellow ("The Electrical Banana")
> > '74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.)
> > Bend, OR
> > KG6RCR