will be an allen bolt....................
It is an allen bolt on these but i've got 4 that i bought years ago with schrader valves in the top

For those who have asked here's some pics of the acc's and how they fit..........

The block is and electric lock valve and the two elbows sticking straight up are where the hoses connect.


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With the power off the valve is closed and the fluid passes straight through the block, ignoring the acc.
With the power on, the valve is open and acts like a tee so the fluid still passes through but the acc is also switched into the line to give a dampening effect.
If you were using a manual valve, a standard tee would be needed to connect the valve to the line then the acc would be screwed into the valve.
Either type can be fitted into the line anywhere between the dump and the ram
