I don't kow if you have the instructions for your machine but maybe this will help.
http://www.sil.si.edu/DigitalCollections/Trade-Literature/Sewing-Machines/NMAHTEX/2672/imagepages/image31.htm
The biggest thread I can use on my 29K58 is B69 with a 29x3 23 needle.
I am going to put my machine up for sale and buy a new 3200 Artisan