ForcesR

If you have a hydra lock during the starting procedure and force the prop through the compression stroke without clearing the hydra lock, the cylinder head is usually stressed beyond what it is engineered to handle. Over stressing a cylinder head usually results in hair line cracking. This hair line cracking will not be noticed or immediately affect the engines performance after it starts. The crack will slowly spread over time as the engine is operated due to normal combustion pressures. The crack will continue spreading throughout the cylinder head to the point where cylinder head has become so weakened due to the spreading of the crack that the cylinder head will fracture and fail, which is usually in flight. This is actually what happens in full size radial engines when a hydra lock condition is ignored and not properly cleared.
I have seen this happen many times in the 37 years I have been working in the aerospace industry. These RC radial engines are really no different than their big brother full size radials when it comes to hydra locking and cylinder head fracturing.