how do i set tappets on challenge lawnmower?

What engine is fitted? If it's a side valve of any sort, you can't (more accurately, you will never need to unless the valves have been replaced - in which case adjusting is accomplished by grinding the valve stem). If the engine is OHV, the rockers either have an adjustable pivot, or pin and locknut (like a car).

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Phil Saunders
July 2009