hole saw for waste pipe?

[Fred]

Hello,

I understand that 15mm, 22mm, 28mm, etc. refers to the external diameter of copper pipe, but does 32mm and 40mm still refer to the internal diameter of waste pipe?

I have some "32 mm" waste pipe but it is 35 mm in diameter. I would like to drill a hole for this pipe to pass through. What hole saw should I use? I suppose 35mm would be a tight fit. is 38 mm the next size up? is that what plumbers use?

For future reference, what size would I use for 40mm pipe?

Thanks.

[Bob Minchin]

It is generally better to have some clearance for waste pipes rather than trying to achieve a honeymoon fit. Expansion and contraction as hot waste passes through can lead to creaks and clicks; leaving clearance helps reduce this noise.

Bob

[The Medway Handyman]

A typical small size specific 'plumbers holesaw set' contains 19, 22, 29,

38, 44, 57mm holesaws, so in your case 38 & 44.

[John Stumbles]

I use whichever size hole saw in my box looks big enough - and a bit over if I can't guarantee that the hole will be in exactly the right place wrt the pipe.

[Fred]

I'm guessing that the 19mm is for 15mm pipe with 2mm either side but what is the 22mm saw for?

29mm for 22mm pipe would leave 3.5mm either side which is a gap almost twice as big as that around the 15mm pipe.

Yet 38mm for a 35mm pipe leaves only 1.5mm either side of a 35mm pipe.

Why isn't there any consistency in gap size? I would have expected them to follow a pattern such as pipe diameter + 4mm.

Or am I just thinking about this too much?

Thanks.

[The Medway Handyman]

Microbore?

Maybe its the possible expansion with hot water rather than the clearance.

Sounds likely to me :-)