diff --git a/python/libneo/eqdsk_base.py b/python/libneo/eqdsk_base.py
index d2987c7c..cf1a45ca 100644
--- a/python/libneo/eqdsk_base.py
+++ b/python/libneo/eqdsk_base.py
@@ -116,9 +116,9 @@ def write_array(fobj, arr):
             fobj.write("\n")
 
     with open(filename, 'w') as f:
-        f.write(eqdata['header'][:48])
+        f.write(f"{eqdata['header'][:48]:<48s}")
 
-        f.write(f"   0  {eqdata['nrgr']} {eqdata['nzgr']}\n")
+        f.write(f"{0:4d}{eqdata['nrgr']:4d}{eqdata['nzgr']:4d}\n")
 
         write_field(f, eqdata['rboxlength'])
         write_field(f, eqdata['zboxlength'])
diff --git a/python/tests/test_eqdsk_base.py b/python/tests/test_eqdsk_base.py
new file mode 100644
index 00000000..d7922581
--- /dev/null
+++ b/python/tests/test_eqdsk_base.py
@@ -0,0 +1,71 @@
+"""Tests for the plain EQDSK reader/writer in libneo.eqdsk_base."""
+
+import numpy as np
+import pytest
+
+from libneo.eqdsk_base import read_eqdsk, write_eqdsk
+
+
+def _synthetic_eqdata(nrgr: int, nzgr: int) -> dict:
+    boundary = np.column_stack(
+        [2.0 + 0.5*np.cos(np.linspace(0.0, 2.0*np.pi, 33)),
+         0.5*np.sin(np.linspace(0.0, 2.0*np.pi, 33))]
+    )
+    return {
+        "header": "synthetic eqdsk for header format test          ",
+        "nrgr": nrgr,
+        "nzgr": nzgr,
+        "rboxlength": 1.6,
+        "zboxlength": 1.6,
+        "R0": 2.0,
+        "rboxleft": 1.2,
+        "zboxmid": 0.0,
+        "Rpsi0": 2.0,
+        "Zpsi0": 0.0,
+        "PsiaxisVs": 0.0,
+        "PsiedgeVs": 0.15,
+        "Btor_at_R0": 2.0,
+        "Ip": 5.0e5,
+        "fprof": np.full(nrgr, 4.0),
+        "ptotprof": np.linspace(5.0e3, 0.0, nrgr),
+        "fdfdpsiprof": np.zeros(nrgr),
+        "dpressdpsiprof": np.full(nrgr, -3.0e4),
+        "PsiVs": np.linspace(0.0, 0.15, nrgr*nzgr).reshape(nzgr, nrgr),
+        "qprof": np.linspace(1.0, 4.0, nrgr),
+        "npbound": boundary.shape[0],
+        "nplimiter": boundary.shape[0],
+        "Lcfs": boundary,
+        "Limiter": 1.1*boundary,
+    }
+
+
+@pytest.mark.parametrize("nrgr,nzgr", [(65, 65), (129, 129), (257, 129)])
+def test_write_eqdsk_header_matches_3i4_columns(tmp_path, nrgr, nzgr):
+    """The g-file header ends in (3i4) integer fields at columns 49-60.
+
+    Fortran readers (libneo read_eqfile1/read_eqfile2 and EFIT-compatible
+    codes) parse the grid dimensions with format (6a8,3i4), so the writer
+    must place idum, nw, nh in fixed four-character columns. Free-form
+    spacing breaks every grid dimension with three or more digits.
+    """
+    path = tmp_path / "test.eqdsk"
+    write_eqdsk(path, _synthetic_eqdata(nrgr, nzgr))
+
+    header = path.read_text().splitlines()[0]
+    assert int(header[48:52]) == 0
+    assert int(header[52:56]) == nrgr
+    assert int(header[56:60]) == nzgr
+
+
+@pytest.mark.parametrize("nrgr,nzgr", [(65, 65), (129, 129)])
+def test_write_eqdsk_read_eqdsk_roundtrip(tmp_path, nrgr, nzgr):
+    path = tmp_path / "test.eqdsk"
+    eqdata = _synthetic_eqdata(nrgr, nzgr)
+    write_eqdsk(path, eqdata)
+    back = read_eqdsk(path)
+
+    assert back["nrgr"] == nrgr
+    assert back["nzgr"] == nzgr
+    np.testing.assert_allclose(back["PsiVs"], eqdata["PsiVs"],
+                               atol=1e-6*eqdata["PsiedgeVs"])
+    np.testing.assert_allclose(back["qprof"], eqdata["qprof"], atol=1e-6)