Last year I bought a complete 235 off craigs list claimed it was newly machined and assembled but would not turn had zero miles on it. I got the thing dirt cheap almost free because the owner was done with it and hated looking at it and reminded him of how much money he spent on it. It was bound up more that a 90 year old on morphine. Upon disassembly found that the crank was turned .020 under and they had .030 under bearings in it. Advise to start measuring but first would look at a poor installation on the rope seal.

Steve

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration