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As for how far apart of the existing skin fittings centre to centre distances I can measure them when next onboard. I’m not sure when that visit may occur, it might be more than a week away.
In the heads compartment, within the cupboard under the washbasin where I replaced my original bronze skin fittings and sea valves, the spacing between the upper two fittings for shower and wash basin drains is centre to centre 103mm, while for the lower two fittings of flushing water inlet and black water outlet the spacing centre to centre is 192mm.
As I mentioned in an earlier reply, fitting the bulky Tru-Design valves in place using the original holes through the hull was not an easy task due to the very limited space within that cupboard under the washbasin. Doing a dry practice run revealed that there would not be enough space to fit the valves and attach the parts in the logical sequence of skin fitting first, then valve and finally the hose tail without some sideways thinking. It actually resulted in having to fit the valves and hose tails together first, then screwing the skin fittings in to them from outside without forgetting the internal flange which was attached loosely and was the very last part to be tightened into position with a liberal amount of sealant acting as both lubricant and seal.
Also in regard to sealing the individual pieces together, namely skin fitting to valve and valve to hose tail, I found that using PTFE string (available through eBay or your local plumbing shop) rather than tape did a better job.