| Check the connections on the start solinoid aren't touching against the cylinder head.
If all OK there get a set of jump leads run one from the -Ve of the battery to s good engine earth, I find exhaust studs work well.
Then with the other jump lead connected to the battery +Ve prod the other end on to the starter motor connection, if you still only get half a turn I would suspect the starter motor but could also be a problem with the engine | 
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