If you are breaking shearbolts when pushing it, it's either a seized slipclutch or a worn bushing in the flywheel. The shearbolt should only break with a shock load.

The slipclutch should get warm when running. It should slip just a little bit every plunger stroke. The slipclutch protects the driveline and plunger. The shearbolt allows the flywheel to freewheel in case the plunger jambs but doesn't actually transmit force other than the energy stored in the spinning flywheel.