The aircraft departed Page at about 1150. At about 1238, the aircraft struck the south slope of a 6,280-ft canyon wall at about 6,000 ft MSL. The canyon is one of a series of about five 'finger' canyons forming the Vermilion Cliffs group. SIGMET Lima 15 in effect called for occasional severe turbulence below 16,000 ft, especially within 50 miles of rough terrain. The area forecast called for occasional moderate turbulence surface to 15,000 ft due to strong westerly winds. A pilot who landed at Page at 1345 stated his flight was marked by light to occasionally moderate turbulence with frequent sustained up and down drafts. He also stated that during his arrival the winds were 160 degrees at 28 mph gusting to 45 mph. Report of engine teardown revealed that none of the sparkplugs were torqued, and the sparkplugs of cylinder #4 were only finger-tight. In addition, cylinder #1's exhaust valve leaked, cylinder #3's rings leaked, and cylinder #4's intake valve and rings leaked.