My 1980 RX7 has a loose steering column. I can grab the steering wheel and jiggle it around about 3-5mm.
How can I tighten it?

 

I would start with checking your steering components first, as the age the get worn out and will cause slop in the steering:
If any of the bushings are torn or crack you should replace them. Even if they don't look that bad, after 30 years of use, I'm sure that replacing them should help your situation. If there is still slop in the steering after that you can go to this link were they talk about adjusting the steering box. But you should check these other components first before adjusting the steering box.

https://www.rx7club.com/showthread.p...steering+grind

 

I have taken the steering wheel and indicator/light stalk housing off and can see that the top steering bush is worn. I cannot get a replacement part as they are no longer available. I am in Australia. Is there any ideas how I can get hold of a part, or is there any remedial work I can carry in lieu of getting a new part?.
Vehicle ID: SA22C578331.
Tassiejeff

 

check powerflex or superpro, they should suplly

 

I would starting by checking that the bolts holding the steering rack to the dash support crossbar somewhere behind/under the dash board are tight.