diff --git a/tar.py b/tar.py
index 5e2b640..0394bcc 100644
--- a/tar.py
+++ b/tar.py
@@ -37,11 +37,11 @@ def header_chunk(filename: str, filepath: str) -> bytes:
     buffer += '0001000'.encode('ascii') + b'\x00'
 
     # Field 5: file size in bytes in ascii
-    buffer += str(stat.st_size).rjust(11, '0').encode('ascii') + b'\x00'
+    buffer += oct(stat.st_size)[2:].rjust(11, '0').encode('ascii') + b'\x00'
 
     # Field 6: last modified, zeros for now
     # buffer += '00000000000'.encode('ascii') + b'\x00'
-    buffer += '1721728914'.rjust(11, '0').encode('ascii') + b'\x00'
+    buffer += oct(1721728914)[2:].rjust(11, '0').encode('ascii') + b'\x00'
 
     # Field 7: checksum, we put spaces and we will edit it at the end
     buffer += '        '.encode('ascii')
@@ -56,7 +56,7 @@ def header_chunk(filename: str, filepath: str) -> bytes:
     buffer += b'\x00' * 255
 
     # Compute the checksum
-    checksum = str(functools.reduce(lambda x, y: x + y, buffer)).rjust(6, '0').encode('ascii') + b'\x00 '
+    checksum = oct(functools.reduce(lambda x, y: x + y, buffer))[2:].rjust(6, '0').encode('ascii') + b'\x00 '
     buffer[148:156] = checksum
 
     return bytes(buffer)