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I fitted a new tap and waste outlet to a small basin, because the new outlet is shorter than the old, the plastic pipework no longer reaches (see photo) I can force the parts to mate but I don't like having the pipe under a lot of strain and it doesn't seal properly (I havent tried applying copious amounts of plumbers putty).

I am considering cutting off the white-painted down-pipe (at right of photo) and using a solvent-weld straight coupling to insert a new section of pipe to raise the pipework about 1 cm.

The existing pipe has an outside diameter of 36 mm.
The pipes available in catalogues are listed as 32 mm or 40 mm.

I understand that a nominal 32 mm plastic waste pipe has an outside diameter of 34.5 mm. Is this the right pipe?

plumbing drain drain-waste-vent
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    I am a little concerned about those 2 90 degree bends that are visible. Those look like vent 90s that should not be used to waste. IMO they should be smoothe bends instead of sharp corners. – auujay Sep 17 '12 at 13:12
  • @auujay: Thanks for taking the time to respond. I think this may reflect a difference in plumbing practices in the US and UK. The bend on the left is part of a standard "P trap" basin waste outlet (example) and the bend on the right is also a fairly common part marketed for basin waste pipes (example) – RedGrittyBrick Sep 17 '12 at 14:32

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Cut where marked, but adjust the location so you've got clearance to work later.

Seeing as you only need a couple of millimeters, you might be able to get away with buying only the coupling connector similar to this:

So a good straight cut where marked, insert the connect and solvent weld it back into place.

Take the cut piece in with you, and just make sure you've got a snug dry fit in the store. No need to actually figure out the exact diameter. Just buy the piece that fits.

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  • Thanks, I guess I'll have to try sanding off the gloss paint from the pipe before welding. – RedGrittyBrick Sep 17 '12 at 15:35
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So far I have discovered there are at least two incompatible types of "32mm" plastic waste pipe.

  • "solvent weld" PVC or ABS pipe with an inside diameter of 32mm and outside diameter of ~37mm.
  • "push fit" plastic pipe with an inside diameter of ~29mm and an outside diameter of ~35mm

Because of the difference in diameters, you can't use push-fit fittings on "solvent weld" pipe and you can't use solvent-weld fittings on push-fit pipe. I guess this is deliberate to stop people trying to use solvent to weld a pipe made of a material that the solvent won't melt.

Diameters above are approximate measurements made with a cheap vernier gauge.

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