My Driver side seat belt is badly frayed. Seems the passenger side has the same part number C7K9A-AGB so I am thinking of replacing it with a used passenger side belt as it will be a lot less worn. But are both font belt/mechanisms interchangeable? Or will it only work when I am reversing!!

Before anyone panics I am not putting the frayed belt on the passenger side. It is my wifes car and I am usually the passenger ha ha.

A scrap dealer suggested the mechanism is weighted differently on either side. Hmmm!

A ford parts dealer said the retractor springs work the opposite way. This sounds like rubbish to me.

One issue is that I am reading the part number off the seat belt not the whole belt mechanism. So there may well be some difference.

They should both be the same, which the part number confirms (same on both)

Finally. The driver and passenger front seat belts on the Fiesta Mk. 4 (and 5) are different. The belt, mounts and most of the mechanisms are the same. However because the (coiled belt) mechanism is not mounted vertically the ns and os belts are not interchangeable. All of this because the door pillers lean backwards. Also the mechanism for the 2 door version has an extra top bracket added which gets in the way of the trim on my 4 door body.

Thanks to uninformed ebayers I have ended up with quite a collection. The mechanism on the Passenger side is marked with a red L and the drivers side with a green R.

When I put the passenger side belt on the drivers side you cannot unroll the belt when the car is facing uphill (or when two fat people are in the back). So I removed the black side plates to find out why.

The coil spring is on one side and the cable jerk locking mechanism are the same. The delicate plastic mechanism engages a heavy metal lock to lock the belt. A 10 degree movement is required which translates to about 1cm of belt travel. Pretty neat. The handedness is achieved via the impact sensor which is a 1cm ball bearing in a plastic basket. Great fun to play with and see how it works. You do have to break off the side plate to see inside but it does fit back on. So to answer my tongue in cheek question - the belt will lock whichever direction one is hit from - nice to know if you roll the car frequently. The baskets just push out so you could change a ns to os type. However I am not qualified to even suggest this is safe to do so do not do it. It just looks as if you can and the baskets are interchangeable. I hope this helps someone.