Originally Posted by
manicmike
Quite right Miriam, the FW threads right to left, the 99 is the opposite (as is the way the needle is inserted). The 99K uses class 66 bobbins and they're horizontal.
Completely different machines. The 99K is a model 66 only smaller, and mechanically they're the same. You can DL a 99K (or 66) manual from Singer for nothing so just do that and you'll save yourself the guesswork.
With those two machines you can look at your needle bar. The flat part of the needle goes in that groove toward the needle bar - then you will feel a small groove in the needle. The thread goes through that side. It isn't totally a rule of thumb but it will work for those two machines. My feeling on it is if you are going to have just two machines get two with the same bobbin and that thread the same. It is a lot easier to keep track.