Top stich loose - Singer 8280 Mechanical Sewing Machine

Download manual here - http://www.singerco.com/accessories/manuals.html

Refer to the maintenance section for cleaning the machine.

Have the needle at it's highest position when removing and replacing the bobbin case.

Refer to the manual for the tension setting, stitch length and correct needle type and size for the fabric you are sewing.

Thread the needle with the presser foot up - always.

If the stitching remains loose, the machine will need to go to the shop for service as the timing may need to be adjusted.

There is a drawing and guide for doing this at the website sewusa (repair).

However, it would be best to have this checked out by a professional as attempting yourself may lead to more problems. And that could cost more than the service fee.