The best tensioner (imo) is using a mech, with the spring around the bolt that holds it to he hanger removed, and the mech locked in place with the end of a brake or shifter cable. Slip the end of the cable (with the metal end bit) through where the wire passes in the mech normally, push the mech until it guides the chain straight on the cog, and tighten up the pinch bolt that holds the cable...
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