VZ BCM Removal

[bushute]

I would like the logical mechanics and physics of a scrivet explained.
To get a scrivet out you unscrew them in the normal way, that is, by applying downward pressure into the head of the scrivet with a screwdriver..
By applying downward pressure, that defeats the unscrewing, as the threads are so fine and plastic, nothing happens.

The engineer who designed these, and the bean counter who insisted they be used, should have been sentenced to 'unscrewing' 1,000,000 of these each whilst blindfolded in 35* heat and 5* cold on alternate days..

Hey @figjam, I want to try and explain the mechanics as requested. I hope others find it useful too.

'By applying downward pressure' and 'screw them in the normal way' - that's usually the problem. People apply too much pressure and treat it like a regular screw. They strip it as soon as they force a screwdriver down on the head and try and reverse it out. You need only apply just enough pressure to engage the driver head in the seat. A scrivet is not a high torque fastener. No plastic fastener is. Don't treat it as a regular screw.

When we (you and me) unscrew a regular screw the usual approach is to push down (usually firmly) on the fastener and twist. We need to overcome the torque that's been applied to the screw on tightening by engaging the driver head hard enough - which can be approx 5-10Nm for a metal screw (hand tightened). A scrivet has a tightening torque (from my research) <= 1 Nm.

Applying a similar pressure to a scrivet, as you would a normal screw, deforms the threads, causing the screw to spin and not back out. That's when one has to lever it out.

I screw/unscrew scrivets using a hex driver bit (like used in a drill driver) using my fingers. Works great, gets into small spaces and doesn't strip the thread. It can apply enough torque to expand the fastener shaft and be removed without stripping. It does work. Need a bit more grip? Wrap some electrical tape around the driver bit shaft (which is what I do). Also, using a small stubby driver with light pressure works well too.

Hope that helps. If not, all good, go with an alternative - sometimes scrivets just screw us over (no pun intended), and we curse to the gods and find an alternative.

 

[Fu Manchu]

The threaded scrivets are a monster. Just lever them up with a nylon trim tool and they come out a bit. Then cut them off with side cutters. Lever out its remains.

Replace with a push fit.

 

[figjam]

Hey @figjam, I want to try and explain the mechanics as requested. I hope others find it useful too.

I must be too fumble fisted.
I have tried using a stubby screwdriver, turning the shaft with my fingers, and hoping the weight of the screwdriver will be enough.
Nope. All that does is turn the driver in and out of the head. Apply enough pressure, it turns the scrivet, but will not 'unscrew' due to that pressure.
Bastards.
Rage and bad language are the way to remove them.

 

[Fu Manchu]

The plastic threads are too soft and strip out more often than they don’t.

Always replace them with these.

*All Commodores use 7mm holes for them. So when looking, you want 7mm. Plenty of 8mm but they don’t fit. Falcon and BMW use 7mm as well so that makes it a bit easier.