Help with welding rotation?

The script works fine, however all I want it to set the Back (which is a union) to be rotated by 180 degrees. Because when I am in the game, the union is upside down...

I have played with w.C0 = CFrame.fromEulerAnglesXYZ(0,0,0) I kept changing the cordinates inside.. all it seemed to do is rotate the Back and the Front.

How can I set the Back union to change its angle when its welded?

attachment = "Torso"

function stick(x, y)

weld = Instance.new("Weld") 

    weld.Part0 = x
    weld.Part1 = y

    local HitPos = x.Position


    local CJ = CFrame.new(HitPos) 
    local C0 = x.CFrame:inverse() *CJ 
    local C1 = y.CFrame:inverse() * CJ 


    weld.C0 = C0 
    weld.C1 = C1 

    weld.Parent = x

end

stick(script.Parent.Back, script.Parent.Front)

function onOwnerChanged()

    local attach = script.Parent.Parent.Owner.Value:findFirstChild(attachment)
    if attach~=nil then

         script.Parent.Back.Anchored = false
         script.Parent.Front.Anchored = false

        script.Parent.Parent.Parent = attach.Parent

        local w = Instance.new("Weld")
        w.Parent = attach
        w.Part0 = script.Parent.Front
        w.Part1 = attach

        w.C0 = CFrame.new(0, -0.7, 0) * CFrame.fromAxisAngle(Vector3.new(0,1,0),math.pi/2) * CFrame.fromAxisAngle(Vector3.new(1,0,0),math.pi/2)

    end

end


script.Parent.Parent.Owner.Changed:connect(onOwnerChanged)