After some chiding by Jetguy, here’s a cylinder head from the MBI radial engine:

Radial engine cylinder head – top – plug oblique

The side fins came out nicely, but the top fins had a few misplaced threads (far side to the left of the valve):

Radial engine cylinder head – intake

The view from the other port:

Radial engine cylinder head – exhaust

Seen directly from the spark plug side, you can barely make out the impossibly thin fin section arching over the plug hole:

Radial engine cylinder head – plug side

The cylinder side looks OK:

Radial engine cylinder head – bottom

I built it standing on one of the ports with the fins vertical, as shown above, which is probably the only way to do it without soluble support material. If I were doing it for real with non-soluble support, I’d be tempted build it flat on the cylinder side with support under the piston head and thin support blocks inside the side fins. It’d look about the same, but with better finish on the top fins.

All in all, I’d say it looks pretty good.

The Slic3r header:

; generated by Slic3r 0.9.10-dev on 2013-04-20 at 20:24:18

; layer_height = 0.20
; perimeters = 1
; top_solid_layers = 3
; bottom_solid_layers = 3
; fill_density = 0.1
; perimeter_speed = 60
; infill_speed = 80
; travel_speed = 200
; nozzle_diameter = 0.35
; filament_diameter = 1.73
; extrusion_multiplier = 0.9
; perimeters extrusion width = 0.52mm
; infill extrusion width = 0.52mm
; solid infill extrusion width = 0.52mm
; top infill extrusion width = 0.52mm

The STL file came direct from Thingiverse, riddled with the reversed normals and holes common to solid models generated by Sketchup, but a pass through NetFabb’s cleanup made it printable. The original STL positioned it far, far out on the X axis, so if you don’t see it right away, rummage around a bit.