The Singer 7-27 is a beast of a double needle machine (don't get your hand in there . The bobbins are smaller than the ones in the single needle models (7-31) because there are, of course, two bobbins instead of a single bobbin. Other than that; the thing is a horse man; should sew 3/4".
If it has a hi speed motor on it look it over good, to my knowledge, these machines were not made to run at high RPM's.
Sim