Helical in userdat2

[Koff33]

Dear Nek Community,

I am trying to generate helical pipe mesh using the helix example with a straight pipe mesh (mesh can be obtained from KTH-Nek5000 Github). Things work fine when setting with the default amp and pitch value. 

I would like to simulate a helical pipe with large amp and smaller pitch values but using amp=3 with pitch=1, the pipe become a flat helical (see the attached figure).

Looking forward for some insights. 

Thank in advance.

Kind regards.

Code:
     !Helix
c     Geometry modification for helical pipe

      diam  = 1.0
      amp   = 3.0 !coil distance
      pitch = 1.0 !ZLENGTH


      z0 = 0.
      z1 = pitch
      call rescale_x (zm1,z0,z1)

      one  = 1.
      pi   = 4.*atan(one)
      dtdz = 2.*pi/pitch

      rad  = diam/2.
      radm = -rad
      call rescale_x (xm1,radm,rad)
      call rescale_x (ym1,radm,rad)

      n = nx1*ny1*nz1*nelv
      call copy(ox,xm1,n)
      call copy(oy,ym1,n)
      call copy(oz,zm1,n)
         
c     account for elliptic shape, and induce offset

      a2pi = 2*pi*amp
      gamm = atan2(pitch,a2pi)
      dpd  = 1./cos(gamm)      ! D' / D (eccentricity of ellipse)

      do i=1,n                  ! preset offset twist
         x  = xm1(i,1,1,1)
         y  = ym1(i,1,1,1)
         z  = zm1(i,1,1,1)
         th = -dtdz*z
         c  = cos(th)
         s  = sin(th)
         xm1(i,1,1,1) = c*x - s*y
         ym1(i,1,1,1) = s*x + c*y
      enddo

      call cadd ( xm1, amp, n ) ! offset center
c      call cmult( ym1, dpd, n )


      do i=1,n                  ! make helix
         x  = xm1(i,1,1,1)
         y  = ym1(i,1,1,1)
         z  = zm1(i,1,1,1)
         th = dtdz*z
         c  = cos(th)
         s  = sin(th)
         xm1(i,1,1,1) = c*x - s*y
         ym1(i,1,1,1) = s*x + c*y
      enddo

      param(59) = 1.   !  All elements deformed
      ifxyo     = .true.
     
      return
      end