can't go back to Sparkys grub

[jnwright]

I created another partition and installed a different os,
and of course, it installed its own grub.

But I want to go back to Sparkys grub, so I
booted back into Sparky from the other os grub.

Then I did a sudo grub-install /dev/sda
and then I did a sudo update-grub

Rebooted and the grub was still the other os grub

Then I tried the same thing, only this time
I did a sudo grub-install /dev/sda1
followed by sudo update-grub

Still the computer boots into the other os grub.

How do I get back to Sparkys grub ?