I presume you mean the front struts, and that you can't find a supplier for
86 struts.

It's easy enough to tell by comparing to the original.  Check the position
of the two bolts that secure the strut to the knuckle. These are M12 and are
probably the same. The strut diameter is probably the same or possibly
slightly larger: it's not really important.

More important are:
- the length of the tube.
- the position and angle of the spring cup on the tube.
- the diameter of the spring coil.
- the stroke of the insert

You'll likey find that the coil spring diameter and the cup position are
different. If all but the spring diameter are the same or very similar, you
would have to use the 2004 springs or get some made by a local supplier eg
Kings. These are designed for the 2004, and the ride height when fitted to
an 86 could be unsuitable ie high because the 04 is heavier than the 86.

You have the option of reusing the 86 strut tubes (after getting them sand
blasted and painted...) and putting in some Koni or Tokico or KYB inserts.
All that's required is that the insert stroke is suitable.