<VV> 'Hour-glass' head screws?

William Hubbell whubbell at umich.edu
Thu Jan 2 12:37:16 EST 2020

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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