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I just changed out the cylinder head on a 150cc 157qmj engine due to the previous owner cross threading the spark plug. I replaced the head gasket and put it in dry, then torqued the head bolts after checking the valve clearance to 15 ft lb inches. Drove 20 miles and found an oil leak at the head gasket, so re torqued the head bolts to 20 ft lb checked the valves again and no more leak. Be careful not to tighten them more than 20 ft lbs, use a good torque wrench and use a cross tightening patten. Hope this helps.
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