zoom lens for spotmatic

I have a Pentax spotmatic from the 70's, screwtype. curently own a 28, 50, and 135 lens, but want to get a zoom. Is a lens for a K type going to fit? or is that bayonet? What size zoom do you recommend?

There are some zooms available for 42mm screw thread, but older ones may not be technically as good as modern lenses. There are some very cheap and nasty ones around on the second hand market.

Try the Tamron Adaptall 2 range which may well include the appropriate adapter.

There is much better choice if you use the K mount camera bodies and the KM and MX operate in the same way that your Spotmatic does. If you do replace the camera body, using the K mount adapter would enable you to continue using your screw thread lenses whilst opening up all the modern high quality zooms to you.

Which zoom? It depends on the sort of photography yiou intend to pursue. What sort of pictures do you take?