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Got it - you gotta have the tech specs on the exact parts you're using. The catalog of my fittings' manufacurer lists centerline-to-seat dimensions. Measure centerline-to-centerline, subtract the centerline-to-seat dimension for the fitting at each end, should give a perfect fit IF you seat the pipe perfectly.
For example, while the centerline-to-seat dimension for a 3" elbow is 3", and you can see the logic behind that, the centerline-to-seat dimension for a sanitee is 3 1/16". So my pipe length will be x - 6 1/16" for a perfect fit.
Of course I suppose that only works on short runs where the triangle made by the 1/4"-per-foot slope is insignificant. On a 20-foot run, let's see, that'd be 240-inch horizontal, 5"-vertical, makes the hypoteneuse = sqrt(240^2 + 5^2) = 240.05", a difference of 0.05", or somewhere between 1/16" and 1/32" - ok, so never mind, we're good for pretty much any run.
Very nice.
Last edited by bob_cntrctr; 01-20-2009 at 01:17 PM.
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