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I used PVC reducers and filed away the inside so the smaller pipe slides all the way through. A coupling is used to form the outer sleeve.
I hope both pipes converted this way will handle my flow, though I might close off the air vents of one or the other to get faster flow.
1" drainpipe:
1" cap
1" : 1.5" reducer
1.5" coupling
1.5" drainpipe:
1.5" cap
1.5" : 2" reducer
2" coupling
I am installing them this morning, and one concern I have is that there is only 1/4" of room between the 1.5" pipe and the 2" coupling, I notice in the other example a 3" sleeve was used.
I will try with what I have built and hope it works, I don't think there are 1.5" : 3" reducers, though the double endcap design might solve that problem.