For hand sewing leather, use blunt saddlers’ harness needles and thread that pass through the prepared holes with slight resistance. John James 001 and 002 share a 54mm × 1.02mm body; 004 is finer at 48mm × 0.86mm. Choose the thread diameter for the leather and stitch appearance, then test needle, thread and holes together.
Match leather sewing thread to the prepared hole, leather and stitch appearance. Our round waxed polyester sizes are 0.45mm for a lighter stitch line, 0.55mm for fine small-goods work and 0.65mm for a bolder line on mid-weight work. Use blunt harness needles that carry the thread through with slight resistance, and test the combination on an offcut.
I use linen thread for repairs and deliberately traditional pieces, and polyester where historical material is unimportant or the item needs to be hard-wearing. Both are available waxed or unwaxed. I prefer waxed thread and may still pass it across a wax block on a longer stitch run to make it easier to pull. I have only used Fil au Chinois linen, so this guide does not make a general quality claim across linen brands.