According to the Daihatsu Rocky (aka Daihatsu Feroza / Daihatsu Sportrak) Service Manual you should replace your spark plugs every 30,000 mi or every 36 months which ever come first.
(1) Remove the air chamber.
Unlock the clamps.
Remove the bolts.
(2) Disconnect the spark plug wire at the spark plug side. (3) Remove the spark plug, using the following SST. SST: 09268-87703-000
WARNING: Since the spark plugs are hot care must be exercised to avoid getting scalded.
(4) Using a plug gap gauge, check the air gap on the new spark plugs. Specified Air Gap: 1.0 - 1.1 mm (0.039 - 0.043 inch)
NOTE: When adjusting the gap of a new plug, bend only the base of the ground electrode. Do not touch the tip. Never attempt to adjust the gap on a used spark plug. Make sure that all four spark plugs are of the same manufacturer and have the same heating range.
(5) Install the spark plug, using the following SST. SST: 09268-87703-00 Specified Tightening Torque: 1.5 - 2.2 kg-m (10.8 - 15.9 ft-lb, 14.7 - 21.6 N.m)
NOTE: Since the insulator strength of a small spark plug is comparatively smaller than that of regular spark plugs, when tightening be sure to sue the tool exclusively used for this application. Also, when tightening, never use the wrench in a crooked way.
(6) Connect the spark plug wire to the spark plug.
