wget http://www.mcknight-instruments.com/linuxcnc/32_bit/kernel_4.1.7-rt-rt/linux-headers-4.1.7-rt-rt_4.1.7-rt.2_i386.deb
wget http://www.mcknight-instruments.com/linuxcnc/32_bit/kernel_4.1.7-rt-rt/linux-headers-4.1.7-rt_4.1.7-rt.2_all.deb
wget http://www.mcknight-instruments.com/linuxcnc/32_bit/kernel_4.1.7-rt-rt/linux-image-4.1.7-rt-rt_4.1.7-rt.2_i386.deb
sudo dpkg --install linux-headers-4.1.7-rt-rt_4.1.7-rt.2_i386.deb
sudo dpkg --install linux-headers-4.1.7-rt_4.1.7-rt.2_all.deb
sudo dpkg --install linux-image-4.1.7-rt-rt_4.1.7-rt.2_i386.deb
sudo add-apt-repository ppa:danielrichter2007/grub-customizer
sudo apt-get update
sudo apt-get install grub-customizer

Menu > Administration > Grub Customizer
General settings tab pick the rt-rt kernel and save then reboot

Menu > Administration > Synaptic Package Manager
Status > Installed
search for linux-headers
Right click and completely remove linux-headers except rt-rt
search for linux-image
remove linux-image except rt-rt
Apply then reboot