sptt, the exterior appearance of a used engine doesn't matter, but the internal condition is critical. That relates to regular oil changes, use of proper coolant, routine spark plug & ignition component replacement, use of premium gasoline, and a guarantee the engine was never overheated so as to warp the head. While a vendor may offer to replace a non-performing used engine it's likely that same vendor will not pay labor costs. Best to deal with an established, reputable vendor even if their price is slightly higher.

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Fred Hoelzle