Hi all I am starting a k100 scrambler project and it started life as a 1986 RT
The tank is awful and the fork stanchions are shot . I brought a 1983 k100 naked for spares I don't gunk(?) the forks or the tank is a straight swap . I was hoping to keep CDI black boxes etc as spares . Did I mess up ? Please advise

The forks should be interchangeable but not the tank, they changed the fuel gauge and return line in Sept 85 the electrical parts are mostly the same especially the ICU (Ignition control unit) and ECU (Engine Control Unit or Fuel Injection Control unit)
realoem.com is a good starting place for parts identification

Thank you for getting back to me the tank I have is very dented but the bike runs lovely the 83 tank is in good condition , what a shame . I was planning on keeping the e throttlesthrottle bodiesrbodies starterb motor etc as a just incase for my 86 it's possible a lot of it is not compatible I guess

You could use the other tank but it will require some wiring changes and maybe the fuel gauge wont work as the early ones were part of the reason for the change The throttle bodies are the same. It is worth having for spares even if some parts don't interchange easily.

As Rick says - the fuel senders are completely different and you will see this as soon as you compare the undersides of the tanks. The senders aren't swappable.
If your planned bike isn't going to use the original instruments (which contain the circuitry to match the senders), then it won't matter a jot which tank/sender you use, but it will be quite a challenge to get a 83 fuel level sender to drive any sort low level warning light (e.g. extracting the driver circuit from the 83 instruments and adapting it) Even if you do this, the 83 fuel level sender only gives two outputs: 'Low' and 'Really Low' - it will not drive a gauge.
If your project is going to use the original instruments then you have a few few options - but these involve dismantling the two sets of instruments and swapping bits and pieces over. Fairly ambitious, definitely tedious, all good fun, very doable, and you will have assistance every step of the way from the fine guys on this forum who have all trodden these paths before...
What have you got in mind?

A good 83 tank complete is valuable!! See if you can engineer selling that one and getting a later one that you need.

I think if they were mine, I would use the '83 bike for the project and keep the '85 for parts.  The '83 has the tank and harness to support the fuel level sensor. 

The fact that the sender is obsolete and no longer available shouldn't be a big deal.  I doubt the conversion will be used long enough to outlive the sender.

An 83 K is considerably 'hotter' than an 85 K too........

But bear in mind there are a lot of minor detail changes even from 83 to 84.....brake calipers differ but interchangeable for one, the repair kits are NOT interchangeable.

How do you mean hotter

The intake cam opens 20° earlier than the later cams. They were changed to meet the Californication emission laws. It effectively lowers the compression ratio so that the burn isn't as hot. The later K100s are slightly down on low end torque and Horsepower

I never heard about this. If you have documentation about, I'd be glad to learn more.