'Water pump O-ring carrier corroded into backplate. With a 1/2" ring spanner try to access from the bottom the two 5/16" bolts securing the jackshaft sprocket to the jackshaft - you'll need to knock back the tab washers of course. Undo these bolts and gently loosen the sprocket from from the jackshaft. From the same access point with a 7/16" ring spanner undo the single 1/4" bolt securing the backplate to the block'
Forgive me 2cams, that sounds like a lot of grief to me, trying to undo tabs, then bolts etc to get the sprocket off. I leave the backplate in place and with my specially picked bits of wood, CAREFULLY [not shouting, just highlighting
] lever both sides of the water pump at the same time, top and bottom, a little at a time, and it always gives it up in the end. Then I tackle the backplate, which I always warm up with a blow torch, as that softens wellseal, which backplates have usually been assembled with. At least in the UK. If the backplate has been assembled with some sort of RTV, then you could be in trouble, and will need to use liberal amounts of Loctite 7200 gasket remover. Brilliant stuff.
All that assumes that the block is secured in an engine cradle or stand, without wheels, as the force needed can be considerable, and if the block is not held firm, it will just move in the direction of force.
Both plates can be bent, so as I said, care is needed. I have found that it is not corrosion that usually prevents easy removal of the plates, but RTV or whatever has been last used to seal the covers, particularly around the water pump.
Interested to hear how others remove. Everyone has their own methods of doing such and such a job.
Leslie