Using block instance on a screw explode then unable to join to name as original block

Hi,
V5
I see the I beam tutorial on using blocks and previous advice (Margaeret) says simply altering a block then renaming it as original and getting refusal from rhino was due to not exploding first.
I explode the screw, scale 1D its length then go ctrl J and it wont join, says select objects to join, I click it it flashes yellow but thats all.
So if I wanted to alter screw shape using blocks what am I doing wrong ?
attached.
block fail.3dm (691.6 KB)
screw came from www.mcmaster.com step file as was suggested for source of bolts etc for rhino.

Steve