There is a locking tool that engagaes the ring gear on the flywheel. The torque for the M104 is high - I don't recall the exact figure but it is around 275 ft./lbs. I don't know of any way to lock the crank on those engines other than with the tool. Also, I had to use a 3/4 drive breaker bar and a pipe extension to loosen the bolt and a borrowed 3/4 drive torque wrench.