The Rail Bass (1985, white body color, black hardware) Tension knob replacement

The Tension knob that allows you to move the pickup module back and forth along the steel rails is no longer providing enough tension to be able to lock the module in place. The thread on the knob still looks sharp, but something else has clearly crumbled away inside and has clogged up the threads. Does anyone know how to fix this or where I can buy a replacement tension knob? 

Also there doesn't seem to be access to the knob area inside the moveable module (except for the small whole allowing the threaded pole to come thru)once you pull the pickup out, so i am assuming the whole tension knob unit is replaced thru the top "screw port". 

Any advice is appreciated!

Thanks!
Chris

Remove the strings

Remove the 4 screws on the back of the bridge section. The one's going into the rails.

Slide off, then slide off the pickup.

All will be revealed.

Hey Westbone- thanks! I have removed the pickup- the pocket only has a couple of holes drilled for wires. The pocket sits below the top rail and the tension knob is above the top rail, so no access to it from the pickup pocket or from the back either. It appears that the only access point is thru the hole that the knob sits in.

Either way, I need to source the tension knob assembly. The only part that hasn't crumbled to bits is the actual knob itself and the threaded rod extending from the knob. I assume there is a threaded receptacle and/or spring that sits in the "knob hole", but once again, everything inside has disintegrated into bits.

Put a new insert in.

What am I inserting and where can I get it?

Epoxying a nut into the housing?

http://www.modelfixings.co.uk/threaded_inserts.htm
cant remember which oone but pretty sure it has an internal 4mm thread