You will need 3 pieces of wood of the following sizes:
325mm long x 225mm wide x 18mm thick
325mm long x 80mm wide x 18mm thick.
325mm long x the gap in your lathe runners wide x 18mm thick.
1. Take the 325mm x 80mm piece and mark a centre line along the length.(40mm)
2. Now find the centre of this line and mark it.
3. From this centre point make four further marks (two each side) at 63mm centres.
4. At each of these marks drill a 40-45mm hole. You should end up with something like diagram A.
Diagram A

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