Create the symbol without strokes and then apply the stroke to the symbol instances. When scaling uncheck the "scale strokes and effects" setting in the preferences. This only works when all the strokes in the symbol are of the same weight. Only thing you could have is for instance a thicker outline aorund all.
Could you show the example?
I will describe what I am doing and possibly there might be a better method than using a symbol. I have created a plant layout. There will be 8 different "You are here" locations, each with an enlarged detail area showing an evacuation route to the nearest exit. I have the plant layout on a layer and have created a symbol for the plant layout to use for the detail area. The detail area size will vary. The strokes of the detail area become too heavy when scaled, and thus the reason for maintaining the original stroke weight.
So the new question would be: Is there another method instead of a symol, I could use for the duplicated plant layout art that will be enlarged and cropped for the detail area.
If I understand it in the right way, a rather simple way, applicable in any version, may be:
1) Create a copy of the relevant part(s) of the plan,
2) Create the path to crop it on top,
3) Select 1) and 2) and Object>Clipping Mask>Make,
4) With Scale Strokes & Effects unticked scale up from the size of 2) to the desired enlarged size,
5) Move the enlarged part to where you want it.
You may change the order of 3) - 5).
The important question is: are all the strokes concerned of the same weight?
Also important to the suggestion of adding a Stroke to a Symbol Instance is whether any of the stroked objects overlap. Given that you are trying to create details (in the sense of detail views in drafting), I doubt that would work for your purpose.
Your need is one I have argued for years: An option to disallow scaling of strokes in Symbol Instances. The same option is equally needed for the base artwork contained in Brushes. The usefullness of these features is seriously debilitated by this missing feature.
Illustrator also remains delinquent in providing Lens Fills, as has been provided in FreeHand and Corel Draw for many years--a much more elegant and flexible way to make detail views (and other things).
Obvious workarounds would be:
Capture the Symbol at original scale.
Place an Instance.
DoubleClick the Instance to view it in Isolation Mode.
Scale its content with Scale Strokes & Effects off.
Exit Isolation Mode.
Duplicate the Symbol whenever you need other Instances at different scales.
Capture the Symbol at original scale.
Place an Instance.
RightClick the Instance and Release From Symbol.
Scale with Scale Strokes & Effects off.
Yep. Good one.
The Pathfinder Effect (ex: Merge) would need to be applied to the unstroked contents of the Symbol. The Stroke would be added to the Instance via the Appearance Palette.
Still does not accommodate objects with various stroke weights, or Symbols with content having both stroked and unstroked objects. But does work even if the Symbol contents includes open unfilled paths (so long as they also have a stroke of none):
Does not meet the need of the thread subject, and does not obviate the need for the requested feature: A straightforward option to simply disallow scaling of Strokes contained in Symbols and Brushes.
Both Symbols and Brushes have huge applicability to any kind of recurring-object projects. For example, think of any kind of parts assembly drawings (anything from tricycle assembly instructions to Lego kit instructions) using Symbols and Brush libraries of frequently used fasteners, etc. which have to scaleable. A near universal convention in such drawings is the use of two Stroke weights; thick for object edges with "air behind them", and thin for edges without. In such a Library, one Symbol should suffice for a hex nut of any size. One Pattern Brush should suffice for a bolt of any diameter/length. Or, consider a Symbol Library intended for road maps. One Symbol should suffice for an overpass bridge or intersection drawn to any scale, even if the content involves multiple stroke weights. Such potentials are severely hindered by the omission of an option to disallow scaling of stroke weight.
But for other things, this is a useful trick, nonetheless.
Nice one with the open path. I couldn't get it to work without a fill though. With a little cheating even this works:
An outline around the symbol can be made like this:
or like like this:
Nice to think about this again.
Being able to just check "don't scale stroke inside symbol" would indeed be easier ;-)
For anyone who may not be following the argument for the need for an option to disallow scaling of stroke weights in Symbols and Brushes:
In both the Bolt Pattern Brush and the Nut Symbol, the Thick and Thin stroke weights were applied as Graphic Styles.
When the Appearance of an application of the Pattern Brush is Expanded, the connections to the Graphic Styles are lost. (Actually, they are lost as soon as you store the artwork in a Brush.)
When a Symbol Instance is Released From Symbol, the connection to the Graphic Styles still exist. In fact, editing the Graphic Style with, for example, a different color, will update the Instances. Yet Stroke Weights are nonetheless unavoidably scaled when an Instance is scaled, despite the Scale Strokes and Effects setting being turned off.
I actually found a way to make this work using nine slice scaling.
When making the symbol enamble nine slice scaling in the New Symbol dialog.
Then edit the the nine slice guides by moving them to the perimeter of the symbol art as in the video it works with closed paths as well.
Watch the video.
You are correct CS 3 it worked in CS 4 I believe it was broken and fixed in CS 5.
So the OP can reinstall CS 3 (if they have CS 3) perhaps if they do not already have it do the scaling there or upgrade to CS 5.