What are you using under the flywheel when you are hammering on the pins? I've done two engines, the first one I laid the flywheel on a block of wood, & beat on it til I was blue in the face - then I took it to work & laid it on an anvil - the pins drove out with just a couple of blows. The second one I took to work & used the anvil first off, & the pins drove out quite easily on that one as well. Apparently, the wood was cushioning the hammer blows, reducing their effect. Just a thought, hope it helps.