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HP no clue. How thick would depend on material used to make the plate. I used aluminum at about 1" thick. This one is what amounts to two offset holes made with only using a drill press and burr. Others used a more wedged shape. As long as there is clearance for the valves to come down and popup for top of pistons you can get a lot of squish out of the adapter. I got the water outlet from Gene French years ago.

Just standard pistons. The 1928 head does have larger valves.