<VV> 'Hour-glass' head screws?
hugo at aruncoaches.co.uk
Thu Jan 2 17:38:25 EST 2020
That's interesting. But it's unfortunate that 'clutch-head' also refers
to the completely different 'one-way' security screws. The cargo area in
my Corvan has normal hex-head bolts, so presumably they have been
changed at some point. And I don't think I have ever seen one on my '64
Monza, but please don't write in with a long list of where they are to
As always, you live and learn. The fact that this is a US patent
suggests that they don't exist in the UK.
Another piece of trivia - UNF threads are widely used in the UK whereas
GM and other American manufacturers prefer UNC. Most of the clips that
secure the clutch cable to the underside of the Corvan have UNC threads,
but some are UNF. The UNV fixings go into capitve nuts, but where the
threads are just tapped into the sheet metal (crossmembers etc) they are
UNF, presumably because there is not enough thickness of metal to cut a
UNC thread. It does mean you have to remember which bolts go where
On 2020-01-02 22:19, Jim Becker wrote:
> For all I knew, it could have been invented by somebody named
> "Clutch". Turns out the inventor was JOSEPH F. FIEG and his US
> granted in 1933, is number 1,894,034. You can look it up to read the
> whole thing. There is an explanation of the advantages. However,
> part is where "clutch" came from:
> "In accordance with this invention I provide a screw head having a
> recess or socket rather than a slot adapted to receive and retain a
> screw driver blade therein, the recess being provided with tooth-like
> diametrically opposite projections which closely resemble clutch
> whereby the screw driver is in a sense connected to the screw head by
> a clutchlike connection."
> So it does relate to grad hold and has nothing to do with one-way.
> There are multiples of this style head under the dash board of
> early-early Corvairs and FCs. They are also scattered around other
> places in the interior. I think they are on some of the door
> They are also all over in the cargo areas of FCs.
> Jim Becker
> -----Original Message----- From: Hugo Miller via VirtualVairs
> Sent: Thursday, January 02, 2020 3:38 PM
> To: virtualvairs at corvair.org
> Subject: Re: 'Hour-glass' head screws?
> 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
> 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
> the cable where it attaches to the pedal and the inner cable is
> 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
> 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
> 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
>> Phillips, avoiding damage to nearby surfaces. However it resists
>> out that Phillips is prone to and the tool can hold the fastener.
>> 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
>> FCs. Maybe it had to do with the acceptability of exposed fasteners
>> on trucks on surfaces they didn't want marred otherwise by errant
>> 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
>> “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,
>> all of which makes sense. However, the odd-shaped head might’ve
>> been chosen to discourage disassembly, not necessarily to prevent
>>> 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
>> 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:
>>> 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
>>> be found on many sites.
>>> This Wikipedia article clearly distinguishes between the two.
>>> https://en.m.wikipedia.org/wiki/List_of_screw_drives 
>>>> 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
>>> 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
>>> 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:
>>>  https://en.m.wikipedia.org/wiki/List_of_screw_drives
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