Voids in concentric elements of a print

I'm printing a replacement part for a lawnmower:
difference()
{
union()
{
cube([75,18,8]);
translate([0,9,0])
cylinder(h=13,r=11);
translate([6,5,5])
rotate([0,30,0])
cube(8.25);
}
translate([0,9,4])
cylinder(h=11,r=6.25);
translate([65,9,-1])
cylinder(h=12,r=4.25);
translate([60.5,4.75,4.75])
cube([8.5,8.5,8.5]);
};

and after slicing the printed part has voids at the ends where the cubes and cylinders are subtracted from the surrounding area.

(updated code, commentary and photographs on the Shapeoko wiki: [www.shapeoko.com] )

I'm guessing that the problem is the perimeter wall thicknesses when added together leave behind an area which is too narrow to be filled, so it's skipped --- are there any warnings about that which I missed (I have to slice in Windows and am currently printing in Linux, so can't repeat the slicing for the nonce)?

I'm fairly certain that I'm (still) extruding too much filament (some areas of layers are slightly bulged up and/or out --- still calibrating, but wanted to make something useful), so it's not for a want of that.

If not, what would be the solution?

- reduce perimeter thickness? What are the tradeoffs of doing that?
- fiddle w/ layer height to get a slightly differen width of extrusion? Would that work / affect this?
- redesign the part, slightly reducing the perimeter by slightly more than the thickness of the void so that the two perimeter walls will line up?

I tried searching, but didn't find any especially helpful / topical links for this specific issue --- if I missed something, please point it out.

Edited 1 time(s). Last edit at 08/03/2014 02:56PM by WillAdams.

I would imagine you designed the part so that
Either
- Go to [gcode.ws] and examine your gcode file, layer by layer, so that you can better understand were the gap is coming from
Or
- Post the gcode and/or some relevant Slic3r settings

I would try to provide a tip, based on the following assumptions:
- you use a 0.5 mm nozzle
- you have a 0.5 mm value in Slic3r for the parameter above (in The Printer settings tab, Extruder)
- you use the default 0 setting for all extrusion widths (in the Printing settings, Advanced)
Do the following:
- set nozzle diameter to 0.49
- set all extrusion widths to 0.5
Slice again and compare the results (use the link above, is more accurate than many other gcode visualizers).