B&S #13 chuck removal

[wildcatter]

I am close to being able to fire up my grinder but want to clean the headstock first. It has a 4 jaw chuck on it now and I am wondering what the proper procedure is for removing the chuck. I don't see a spindle lock so how do I unscrew the chuck? None of the manuals I have been able to find say anything about it. Any help would be greatly appreciated!