Help required on boxford tcl 160

Hi guys n gals

I'm new to this forum and machining so please be gentle. I've recently purchased a boxford tcl 160 and I am learning fast. I was doing some manual cuts on it last week when I noticed that there was lag in the z axis, upon inspection there was a shim/spacer shattered next to the drive belt pulley. Now I've got the new shim from boxford and I've removed the 2 grub screws in the drive cog but it doesn't seem to want to come of. It looks like the cog should just unscrew to allow the new shim to be put on. So is my thinking correct and need to use more bfi or is there a more subtle solution.

Thanks in advance
Snatch