You didn't say what make or model of engine you have, but most engines in that horsepower range use a spark plug gap of 0.030 inch. If you know the engine model number and manufacturer there is a listing of many engine specifications at http://outdoorpowerinfo.com/engine_specs/ .
.03
The gap is 0.030 in. The plug is Champion RJ19LM. The torque is 7-15 ft.lb
.030
The correct plug gap for my Toro GTS 6.5 horse mower is 0.035
torch GL4RC or Champion J19LM
.030
.030"
0.030-0.035
0.028-0.030"
J19lm
.030 Refer to your owners manual of remove the old plug and just replace it with an identical plug.
More info is needed. Sears uses many different engines. We will need to know which engine is in the mower. There should be a tag or sticker on the top of the engine with the brand and HP.