[Mr Teddy Bear]

I suffered a catastrophic malfunction of this critical interface while driving through a thunderstorm yesterday afternoon.

Both arms have just worked as they should until I removed the passenger arm because the wiper blade wouldn't stay located in the shepherds crook.

My initial thought was that the [what appeared to be] splines were designed to locate rather like a splined gear shaft and a pinion.

On further reflection and after the application of a contact adhesive between the cone shaped drive face on the spigot and the wiper arm hole, I'm wondering if the serrations are merely designed to allow the arm to grip the cone, due to the clamping force of the coned nut?

Looking at one or two other salvage wiper arms most are chewed up to varying degrees.

Does anybody else have any thoughts on this

[Johnny 216GSi]

On further reflection and after the application of a contact adhesive between the cone shaped drive face on the spigot and the wiper arm hole, I'm wondering if the serrations are merely designed to allow the arm to grip the cone, due to the clamping force of the coned nut?

That's my conclusion, too. I'm not sure what the answer is, though. Use some kind of "hard" filler in the wiper arm hole, so it cures around the motor spindle splines? Something like Araldite repair putty, which can replicate threads, etc., so would mould to the splines in the drive shaft and actually make the wiper arm hole properly splined?

The whole issue of the windscreen wipers needs further investigation, particularly WRT the problem of the securing nut / arm body hitting the bonnet edge when it is opened. Maybe you've hit the nail on the head with the inner hole wear you've noticed.

Just FYI, I had the "slack arm" problem appear when I first bought my GSi back in 1995 (a 2 year old car). I even remember where it happened - at Stafford train station! Actually, outside on the main road, but I pulled into the public car park to fix it. Luckily, I had an adjustable spanner in my battery charger box in the boot to reposition the wiper arm and nip the nut up tight and get home!

The issue of the wiper arm "random slackness" has stayed in my head ever since.

[crepello]

The whole issue of the windscreen wipers needs further investigation, particularly WRT the problem of the securing nut / arm body hitting the bonnet edge when it is opened. Maybe you've hit the nail on the head with the inner hole wear you've noticed.

My take is that, over time and under continuous pressure, the die-cast part of the arm relaxes away from the glass, so the knuckle
can contact the rear edge of the bonnet as described. Wear in the spindle bearings and any slop in the spindle/arm interface will
add to the problem. If you lay the rear of the arm on a straightedge a gentle curve becomes evident. I hardly think Rover or Trico (?)
would have tooled a sexy curve when straight would do, and be more in line with the car's design cues anyway!