Another "36mm" pipe query

[bordonbert]

Here we go again I'm afraid. I have to fit a new washroom basin and the modern style chrome bottle trap comes with it's own section of chromed metal pipe to take it into the wall. Standard stuff! Luckily I can fit it straight inline with the existing soil pipe fitting, but it isn't quite long enough to fit in one hit. So I will have to fit a short length of plastic 32mm pipe into the soil pipe connection with a compression connector hard up against it between the 2.

Everything is ok so far except, measuring the metal pipe supplied, it is that mystical 36mm OD. Oh yes it is, look at the photo!

The picture isn't brilliant but it is clear enough, that looks very like 35-36mm to me! I have seen many posts which deny the existence of anything close to 36mm sized pipe, and I take the word of professional plumbers who say it should be 32mm ID sized, except it is 36mm OD!!! It would seem sensible to assume that it is an equivalent to 32mm ID polypipe, does that seem sensible? So the compressiion connector I need will presumably be 1 1/4" or 32mm?

Has anyone else been down this route and is able to confirm that the sizes will match up here?

 

[Charnwood]

11/4" McAlpine Multifit should do the job.

 

[bordonbert]

I have now had an opportunity to break down the existing setup and it's going to be a pain if you can't help with sizing advice.

I have a joint welded into the soil pipe which offers a pushfit to the plumbing. That seems to be 36mm exactly. This has a grey 90deg bend pushed into it with a compression joint on the outer end. The 90deg bend is clearly labelled 1 1/4" moulded into it. It's OD is a mm or so over the chrome pipe I am trying to mate to and it fits perfectly with no clearance at all into the mouth of the pushfit.

But, despite being so close in size, the chrome pipe will not fit the pushfit joint in the soil pipe, it is too slack! That 1mm makes all the difference apparently. I have a length of white 32mm ID polypipe, (40mm OD), and this is far too big to even go into the mouth. That existing soil pipe joint seems to be matched to the 36mm OD polybend exactly.

I wonder if, over the years, the rubber compression ring inside the soil pipe joint has expanded to the point where it is permanently stretched to its upper limit and no longer seals a slightly smaller pipe as it should? The difference in pipe sizes is minute but one fits the entry perfectly and the other rattles from side to side by about 1mm, no more. Is it possible to fit a new ring into the joint to allow it to seal such a close range of sizes again?

 

[bordonbert]

Thanks Charnwood. I would have thought that 11/4" would be the standard but as you can see things are not entirely straightforward. As the differences are so slight but make it impossible to just fit as is, this is probably just due to the age of the system with no maintenance for years. If I can make sure that I can swap out the sealing ring in the soil stack pushfit joint then I should be ok. Otherwise, if it can't be changed, it may need a replacement of the pushfit joint and I have never broken down a welded joint before. Opinions about doing this seem to vary.

The outer ring of the pushfit which holds the flexible sealing ring in place is tight but can be rotated by hand. Maybe this can be popped off to replace the ring? I have seen references to rubber connecting boots. I presume these are slipped over the existing joint and tightened in place using a sort of jubilee clip to get a good seal.

 

[Charnwood]

I have a length of white 32mm ID polypipe, (40mm OD)

4mm wall thickness, what sort of pipe is that? Not waste, that's for sure.

McAlpine MULTIFIT is called that for a reason - it FITs MULTI pipe sizes.
It might be the case that you need 11/2" Multifit for your 36mm pipe. I can't remember the size ranges.

 

[bordonbert]

You're absolutely right Charrnwood, my apologies. The ID of that white pipe is 36mm not 32mm. The chrome pipe to the basin trap actually fits into that absolutely snug. I had thought of using it with a jubilee clip to tighten as a short sleeve once it is inside the stack box. Unfortunately as I said it is too large to fit into the pushfit joint.

The ideal part seems to be a McAlpine S18M in the range you suggested which I will get. This offers me the compression joint I need for the new pipe but, as I posted, if the straight coupling end is just a tad on the small size the old pushfit in the stack may not seal properly. I can't stress how close the existing 90deg fitting and the metal pipe are in OD but one seals and the other rattles about. The S18M is offered in 2 versions, one to BS5254 and the other to BS5255 which needs to be added to the part number.

I can't help feeling that the sealing ring does need replacing if it is possible to make everything else easy.

 

[Ric2013]

FOR FUTURE REFERENCE

Inch and a quarter solvent weld pipe (PVC or ABS) tends to have a 36mm O/D. PP push fit pipe is more like 34mm OD but we still call it inch and a quarter, and I note OSMA still stamps its pipe in imperial measurements. This is handy, because PP will not solvent weld!

This used to confuse me a lot before I started studying plumbing as a professional.

Plumbers don't tend to measure diameters as they can usually tell by type of pipe and connexion what the diameter will be, though I do have a vernier in the van because I always prefer to check... and I can confirm that 36mm OD pipe exists.

Probably the final pushfit connection to the stack is designed for 36mm because PP pushfit pipe (34mm OD) is rarely used by plumbing professionals, and so the final connection, though a pushfit joint, is sized for solvent weld pipe.