<VV> 'Hour-glass' head screws?

Hugo Miller hugo at aruncoaches.co.uk
Thu Jan 2 16:38:43 EST 2020

I'm wondering whether they mean 'clutch' as in grab hold of, rather 
than the 'one way clutch' security screws. I'm sure these things don't 
exist in England - I've never seen one in my life till I changed the 
throttle cable on my Corvan. There is a shield around the front end of 
the cable where it attaches to the pedal and the inner cable is exposed. 
But that is not somethng that people should be discouraged from 
touching, nor are there any painted surfaces nearby which ned 
protecting. The reverse is true in each case. They coud just as easily 
used hex-head bolts to hold the shield on - especially in a location 
exposed to the elements and posible corrosion, and on an item which is 
bound to need replacing at least once during the life of the vehicle!

On 2020-01-02 20:49, Jim Becker wrote:
> One sentence in the Wikipedia article gets to the heart of at least
> half of this discussion.
> "They are sometimes called one-way clutch screws, but should not be
> confused with true "clutch" screws."
> The screws GM used are "true 'clutch' screws" and have nothing to do
> with the one-way security screws.  Back when GM started using clutch
> head screws, the main choices were the standard slot head and the
> Phillips head.  The clutch head tool stays in place in the head like 
> a
> Phillips, avoiding damage to nearby surfaces.  However it resists cam
> out that Phillips is prone to and the tool can hold the fastener.  In
> essence, the features are a lot like a Torx although probably not as
> good as Torx.
> It always looked to me like GM used clutch fasteners a lot more on
> trucks than cars.  Thus I was never surprised by their heavy usage on
> FCs.  Maybe it had to do with the acceptability of exposed fasteners
> on trucks on surfaces they didn't want marred otherwise by errant
> drivers.
> Jim Becker
> -----Original Message----- From: William Hubbell via VirtualVairs
> Sent: Thursday, January 02, 2020 11:37 AM
> To: hugo at aruncoaches.co.uk
> Cc: virtualvairs at corvair.org ; William Hubbell
> Subject: Re:  'Hour-glass' head screws?
> The “hour-glass” or “bow tie” screw head was the first to be called a
> “clutch head” screw - long before the more modern one-way security
> version was developed and took on that name.
> As for WHY GM used them, who knows?  Maybe they got a good deal from
> a supplier.   GM used a lot of different types of small hardware, not
> all of which makes sense.   However, the odd-shaped head might’ve 
> just
> been chosen to discourage disassembly, not necessarily to prevent it.
> Bill
>> On Jan 2, 2020, at 9:56 AM, Hugo Miller via VirtualVairs 
>> <virtualvairs at corvair.org> wrote:
> I am trying to differentiate between the two! But it seems that the
> 'hour-glass' slot is also called a clutch-head, even though it isn't 
> a
> clutch drive like the 'one-way' security screws.
> I don't think I've ever seen one until I changed the throttle cable
> on my van. I wonder why GM would have used them? They could just as
> easily have used a hex-head or a slotted screw. I can't see any
> practical advantages to the 'hour-glass' slot. And I have certainly
> never seen a tool for undoing them, so I had to improvise.
>> On 2020-01-02 14:46, William Hubbell wrote:
>> Hugo,
>> I think you are confusing the more modern “one-way” screw with
>> the older, now largely obsolete “bow tie” screw. The “bow tie”
>> was indeed called a clutch head screw and that designation can still
>> be found on many sites.
>> This Wikipedia article clearly distinguishes between the two.
>> https://en.m.wikipedia.org/wiki/List_of_screw_drives [1]
>> Bill
>>> On Jan 2, 2020, at 8:49 AM, Hugo Miller via VirtualVairs
>>> <virtualvairs at corvair.org> wrote:
>> That's a rather confused article - not least because it keeps
>> referring to screws as 'rivets'. But a 'clutch-head', as the name
>> implies, has a drive that works one way but not the other, for
>> security purposes. The 'hour-glass' slotted screws that hold on the
>> shield over the throttle cable, for example, do not have any sort of
>> clutch, nor are they security fasteners. After all, who is going to
>> steal a throttle cable? You can, if you're lucky, undo them with a
>> screwdriver, but I had to grind down an allen key to undo some of
>> mine.
>> So two questions - why on Earth would GM use such a fastener in
>> routine positions on Corvair vans, and what is their proper name?
>>> On 2020-01-02 12:54, R wrote:
>> http://blog.mutualscrew.com/2015/07/07/salient-features-of-clutch-head-screws/
>>> ockquote>
>> Links:
>> ------
>> [1] https://en.m.wikipedia.org/wiki/List_of_screw_drives

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