Easiest way is w/ an impact wrench. Not in your tool box? Try to hold the pulley with the gripping the belt near the pulley of the mandrel you are trying to remove. That usually works. Or do you have an old belt around you could twist tightly around that pulley using a bar to hold it. If the rest of the mandrel is good, you can save yourself a bit of work and expense by replacing the shaft kit. If you go this route, leave the spindle mounted to the deck.

I just rebuilt a deck on a 4000

Not that bad, just a matter of dropping the deck on the ground, then sliding it out from under the unit.

All bolts sheered off on the old mandrels. But the new mandrels had new bolts!

I did replace the two idler pulley's on TOP of the deck,

Whole process was $50 bucks (Ebay bought) and 2 hours with some beer sipping in between/

New blades too

no impact wrench and no old belt. funny thing is i just replace the belt and the old one got tossed a couple weeks ago. I had tried holding the belt, squeezing the loop together but to no avail. I intend to keep the housing assembly attached to the deck and just replace the shaft, if I can get the nut off.

The guy at the local tire shop may just pop that off quick w/ his impact wrench for a thank you. You can reinstall it yourself by holding the blade.

I just done that same job on a 42" Craftsman deck (right mandrel). I made a tool which features two lengths of bar steel 5mm in diameter spaced fara apart to fit in the two wholes in the pulley itself. Used rathced to turn pulley nut whilst pushing with the tool the other way

Neat idea marinosp.

Here is an example of a "market available" tool that employs the concept used by marinosp. In the States, this is known as an "Adjustable pin, face spanner, wrench". I'm sure there are a number of other monickers for it as the term "spanner" evokes a very different tool in the U.K. and elsewhere.
{{gwi:353061}}

Mownie has all the cool stuff!