I have a good friend who has been a La Marzocco/Mazzer authorized tech for 30 years. He told me 15 years ago to never use grease on the threads, and that them being stiff was because there is coffee in them. Because coffee doesn't compress, even one little spec will make them bind up.

Clean the threads as best you can with a brush and vacuum and then use a microfiber cloth dipped in Puro cleaning solution and wrung out so it doesn't drip and wipe the threads clean. Wipe dry and then wait 10 minutes till totally dry before reassembling.

They will go on as smooth as butter.

I have always used a food grade grease that is high temp and designed for food processing machinery.

Use it in everything from espresso machine assembly to grinder assembly.

Never had any issues with this.

A little dab will do ya

That's the good stuff.