<VV> 'Hour-glass' head screws?

Jim Becker mr.jebecker at gmail.com
Thu Jan 2 15:49:47 EST 2020

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.


> 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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