I have a feeling the motor was stuck at TDC when youve been trying to start the motor. The motor must be un-stuck before trying to crank it any method or the weak point will break and the crank and rod arent going anywhere anytime soon.

I have to do the same with my STS motors on my starter box or the rubber wheel will wear quickly. Get it un-stuck then start it , otherwise you will break stuff.

On my old cv-r long time ago. I had to sometimes take the motor completely apart and tap down the piston with a wooden dowell on a paper towell. I would never try to unstick a motor with a pull-start or especially rotostart.

You cant even get them to barely move via the flywheel and a wrench. So I can only imagine what a small roto gear or pull-start is taking.

STS motor is tight , heat it up with hairdryer and loosen glow-plug for the first start of the day and make sure its completely not stuck. It should turn over fairly easy then without broken pull-start cords and rotostarts.

Its a new motor which is the hardest time on starting components. Afterwards it will be fine if you use the precautions.