You should never scale your drawing in model space. There is absolutely no reason to do this. Go to your Layout and double click inside of the viewport to activate model space or type MS at the command prompt. Now position your detail inside the viewport border and zoom in until the detail fills the viewport. Now double click outside the viewport to get back to paper space or type PS at the command prompt. To set the viewport scale, single click on the viewport border and then right click and select properties. The Properties palette will now open. In the "Misc" section, go to "Standard Scale". You can set the scale to 1:10 or 1:20 or whatever you need. Once you get the scale set, move to the next option up, "Display Locked" and change that to yes.