diff --git a/EXE_GdbCreateSurf.cpp b/EXE_GdbCreateSurf.cpp
index 595f7ca..208779e 100644
--- a/EXE_GdbCreateSurf.cpp
+++ b/EXE_GdbCreateSurf.cpp
@@ -507,7 +507,7 @@ MyCreateSurfTmConvexHullInBBox( int nParentId, int nId, const BBox3d& b3Box, int
    vTria[3] = MyCreateSidePlaneHull( vVert, bNXp, b3Box, -2) ;
   // invalido i triangoli con normale opposta alla direzione attesa
    for ( int i = 0 ; i < 4 ; ++ i) {
-      if ( ( vTria[i].GetN().x > 0) != bNXp)
+      if ( abs( vTria[i].GetN().x) < 0.1  ||  ( vTria[i].GetN().x > 0) != bNXp)
          vTria[i].Set( vTria[i].GetP( 0), vTria[i].GetP( 1), vTria[i].GetP( 2)) ;
    }
   // cerco uno o due piani validi