This is an upgrade to the Orca 0.43 x-axis. I bought a raceway 360mm long, anything up to 373/374mm long should fit fine. If in the UK, http://www.marchantdice.com are great, and extremely local to me. They'll cut it to the length you specify, just make sure to email them for a quote/paypal payment, as their website doesn't have selectable prices for lengths. As the raceway is completely rigid and straight, it stops any bowing of the axis due to tension between the original two rails, which was causing a rise in the nozzle in the middle of the bed, making it difficult to level the bed. It also frees up the front of the carriage for a fan, whilst still using the original top aluminium plate. The only issue with this now, is that the original mounting bolts for the extruder can no longer be used as the carriage is in the way. I've used M4 threaded rod and penny washers to rectify this.Remove all the extruder, belt holder, and one rear bearing holder from the carriage. Use the drilling jig to drill four new holes for the HGH15CA carriage, then countersink them. Use M4x8mm CSK SKT bolts to attach the carriage to the x-axis plate, and then fit the carriage to the rail. Fit nylock nut and normal nut to the mounting bracket, then push the mounting brackets onto the rail, making sure to get the holes in the bracket on the right side of the printer, as these are mirrored. Fix to the frame using M4x20mm bolts, and clamp the raceway using M4x25mm bolts. Use the new belt-clamp (the old one doesn't fit anymore), and push two M3 nuts into the recesses. Push the clamp down onto the belt, and tighten using M3x10mm bolts to clamp. Fit to x-axis plate using M4x16mm bolts. Assemble extruder as normal, but using M4 threaded bar, penny washers and nylocks instead of the original bolts. The penny washers should pull up on the underside of the HGH15CA carriage. You can also use M4 threaded rod to fix the x-axis opto if you like. Use nuts to tighten to the frame, then nuts to lock the opto holder in place.