Also the splines could be worn and shifted and locked . try to hold the driveshaft and rotate the crank backwards to realign the splines again if that is what is going on.
Cool! Be sure to clean those splines thoroughly before reinstallation…With the powerhead upside down, spray WD40 down inside the crank splines and work the driveshaft in and out of the crank several times to free up the rust….Please be sure to lubricate the driveshaft splines with OMC moly lube and replace the oring before final assembly…
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