My 90 degree head of the holding tank coupler in PVC has cracked and this the second time a problem with this. This found upon inspection recently, but in 2015, the coupler broke under use, with awful consequences. I suspect that the connector pipe, surrounded by a metal helix is simply too rigid and stiff, with movement of the boat, under way, or at anchor with a chop, this exerts forces on this coupler. I have sourced new couplers, one for spare, and am now trying to source a new 2m length of 40 mm diameter pipe. This is proving very difficult. It seems as if everyone is saying that 38mm (1 1/2 " ) is the standard and I should use hot water and detergent to slip this on. I have sourced a sample of 40mm diameter steel helix surrounded pipe, a non marine, lighter weight PVC helix surrounded 40mm diameter pipe and these fit. The boat yard fitter has said that this non marine PVC helix surrounded pipe would end up creating smells. He would help to fit it, but my call re smell risk. It has the advantage to my mind, it should exert less stress on the coupling. Having been told by many marine suppliers, not only in the UK, but Denmark also, that 38mm diameter is the standard for sanitation pipe, using boiling water and detergent is the way to go to ease it on. I obtained a sample of 38mm diameter PVC helix surrounded pipe from a local chandlery, and tried to fit this over a 40mm diameter male end from the coupling. This proved impossible, as well as risky in puttng a too tiight pipe onto a thicker male end. Any thoughts from anyone on this welcomed, particularly of known suppliers who would supply sanitation pipe, PVC helix, smell retarding, with 40mm diameter internal. Particularly in Europe, but if not worldwide. Someone in Italy has suggested he could get it for me, from an Italian chanlder, but the minimum order is 50m. I clearly do not need this much. I am trying a German supplier, but await an answer. All input appreciated.