had this same problem working on this rust bucket the other day

first the plastic screw in dust cover wouldn't screw off, spent about an hour digging that out
..to discover the crank tention nut was a bit on the round side, ended up using a smaller socket..belted that in..lucky it had enough grip to u-do {then had to belt the nut out of the socket**
all this to try and save the crank, but *sigh* that had a stripped thread, ended up cutting that off with angle grinder