wow you really see your way open to tackle this job? first you need to get the old gear out and the easiest way is by drilling a hole at back off machine in line with screw that hold gear,iff you dont wanna drill the hole the only way is to remove all of top mechanism and that is a huge job.you can now pull shaft down after removing timing belt.insert new pulley,its a time consuming job,and fitt onto shaft.to adjust timing loosen gear so you can turn handwheel without moving hook,by turning handwheel towards you and observe when needle is in upwards motion you should adjust hook point as it must touch needle approx 2mm above needle eye.after all assembly is done you can adjust timing more precise as to ascertain cotton gets picked up when machine set to zig zag
johan, May 2014
