Round boot or octagonal?

[-JD-]

So I decided to get an octagonal boot to switch with my round one. The existing round boot is 7.25". The HP100 tank spec is 7.25". I measured the tank as 7.25". So I ordered a 7.25" octagonal boot. It came, and it took some time to get the round boot off, as it's quite snug. Then I tried to get the octagonal boot on. It doesn't want to go. I put a piece of wood across the bottom to hammer on, and the tank vibrates and sort of rings. I'm thinking that might not be good for the valve. Is it safe to keep hammering, or am I doing something wrong? This seems complicated for something that should be simple.

I haven't had cause to do this with SCUBA cylinders, particularly, but ...

As a general practice to deal with an interference fit: Put the "top" of the boot in warm water to soften it a bit and make it easier to stretch (keep the bottom of the boot cool so it remains stiff) and apply a lubricant to the cylinder (non corrosive and compatible with the plastic - dish soap and water mix should do nicely) before tapping the boot on to start it.