Comment by buildsjets | Hacker News Reader

Comment by buildsjets | Hacker News Reader

If you specify all your bolt heads as 19mm and 13mm, then a 3/4" wrench or a 1/2" wrench is usually good enough to get the job done.

Similarly, a 1/4-20 bolt will fit in a M6 tapped hole if you use a large enough hammer.

A hazardous aspect of US threadforms is that #10-32 machine screws and #8-32 machine screws have the same pitch. So you can fit #8 bolts in a #10 hole and sometimes, they FEEL like they made good torque, because they engaged one side of the tapped hole, when they really have no tension capability whatsoever.

Reference the British Airways flight 5390 accident where the pilot got sucked 3/4 the way out the cockpit window and slammed against the side of the fuselage while a flight attendant clung to his feet and the co-pilot safely landed.

Also note that a British designed and built product was using US threadforms...

https://admiralcloudberg.medium.com/the-near-crash-of-britis...

https://en.wikipedia.org/wiki/British_Airways_Flight_5390

> Reference the British Airways flight 5390 accident where the pilot got sucked 3/4 the way out the cockpit window and slammed against the side of the fuselage while a flight attendant clung to his feet and the co-pilot safely landed.

That incident led to a major change in how aircraft windows were designed. Instead of being fitted from the outside they were changed to being fitted from the inside.

When fitted from the outside the job of the fasteners is to keep the cabin pressure from blowing the window out when the plane is high up and the outside pressure is low. When the plane is on the ground gravity will keep the window in place and the fastener isn't doing much.

Botch installing the fasteners and you don't find out about until the window blows out at high altitude, like on that flight.

When fitted from the inside the job of the fasteners is to keep the window from falling in due to gravity when the plane is on the ground or at low altitude. At higher altitudes the cabin pressure pushes the window firmly into the frame and the fasteners aren't doing much.

Botch installing the fasteners and the window falls in on the ground or at low altitude which is a lot easier to deal with then a window blowing out at high altitude.

The diabolical case is M5 versus 10-32. About a 4% difference in diameter and about 0.8% difference in thread pitch. Basically indistinguishable with the naked eye and even the "nest the threads in each other" check doesn't help without a pretty long length of threads.

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