The GSR uses a different timing belt. The GSR water pump has a different gear than the LS, thus the timing belt won't fit. Just go to a dealership and get a timing belt from them, and bring it back to the shop.
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2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket
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