Enlarge hole in soil pipe for a soil pipe boss

Adam · 9 May 2020 at 08:26

This wasn't the finest piece of workmanship from my builders who did my loft conversion a couple of years ago so I'm looking to try and put this right myself. We have an extenal soil pipe from the loft to the drain and the builders connected the kitchen sink waste pipe to this via a strap boss. I think this boss wasn't the best way to go having looked in to it and it appears to be a bit of an after thought.

The boss has since failed and the waste pipes have come away. I tried to fix it yesterday and I think I cut the pipes ever so slightly short so there is too much pressure on the boss. I'll try and fix that as a first step today, but I believe I'll need a new boss to do this properly. As you can see in the second image, the bottom of the boss does not sit flush with the pipe. The builders didn't do this properly as there was loads of clear silicone everywhere which I have since scraped away. I've used PVC cement on the rest of the joins.

Most of the bosses require a 57mm hole but the hole I have is around 49mm. One of the best solutions appears to be something like this: https://mcalpineplumbing.com/plasti...lac-black-mechanical-soil-pipe-boss-connector

I just wondered if anyone has any practical ideas to enlarge the hole from 49mm to 57mm in the plastic pipe? I thought I could buy the 57mm hole saw and try to get the blade to bite in to the PVC by gently hammering it to get it started, then finish it with a drill?

Danm54 · 9 May 2020 at 08:40

Fix your old strap on boss to the soil pipe and use it to guide the 57mm hole saw. It probably won't fit snug in there but it'll save the hole saw snagging and going too far off line.

ANDARIAL · 9 May 2020 at 08:46

First thought is that it should be glued in place and not with silicone mastic

Danm54 · 9 May 2020 at 08:57

ANDARIAL said:
First thought is that it should be glued in place and not with silicone mastic

The OP is using PVC cement so all good there.

Don't know why anyone would use silicone for that though.

FlyingFish · 9 May 2020 at 09:09

Should be pretty straight forward, just use the right size cutter for the boss and deburr the edges before glueing.
Soil plastic is pretty soft, just stack some bricks or wood up to support your drill and grab it very sturdily while you enlarge the hole. Secure the bit as far as it will go into the chuck, to minimise the chance of snagging, and take it really steady.

If you can't support the drill, then use a cordless and kneel right up to the stack keeping the drill held close to your body while you drill. And use a nice controlled drill speed to cut cleanly.

ANDARIAL · 9 May 2020 at 12:31

Danm54 said:
The OP is using PVC cement so all good there.

Don't know why anyone would use silicone for that though.

Ahhhh right.Looked like silicone to me
Surprised the Boss came off if it was glued on with PVC glue though
Edit just reread op.was siliconed on originally
`Some`Bosses have a top and bottom(slight slope)Should be an arrow or writing if so

FlyingFish · 9 May 2020 at 12:54

If the hole wasn't big enough to start with, then the boss wouldn't of sat right for the solvent weld to adhere. Probably why silicone was used to fill the gaps.

jonzun · 9 May 2020 at 17:19

If you have hole saw with an Arbor, you can fit the larger one onto the Arbor first then put the smaller one on the inside to act as a guide, like this https://youtu.be/9-pBlZa1RBI. Unfortunately you need both holesaws though hth

Adam · 9 May 2020 at 17:19

Thanks to @Danm54 for their suggestion, the old boss was the perfect size for the 57mm holesaw I bought

Turns out this was 10x easier than I was expecting. The holesaw was nice and sharp and went straight through, none of the plastic fell in to the soil pipe either which was a result. The McAlpine mechanical boss seems to be a great product compared to the strap boss that was there before. It was 3-4x the price of the strap boss but it's a nice tight fit and having tested it there are no leaks coming from either the soil pipe or the kitchen waste. A nice video explaining how it works here: https://www.youtube.com/watch?v=CCSl3S3ebgQ

Here's the finished article once I had used solvent weld on the other joints. Will be much more permanent than the bodge using silicone before. I'm surprised it lasted that long.

norm · 10 May 2020 at 10:24

Good job. Goes to show with a little patience and understanding you can achieve a result better than some trades people.

JRJ · 10 May 2020 at 12:28

To be honest cleaning all the silicone off the reducer and boss and refitting them with cement in a bigger hole would have done just as good a job, those bosses when fitted correctly are pretty difficult to break.

Adam · 11 May 2020 at 09:19

JRJ said:
To be honest cleaning all the silicone off the reducer and boss and refitting them with cement in a bigger hole would have done just as good a job, those bosses when fitted correctly are pretty difficult to break.

You are probably right and that would be the cheaper option but it also seems quite permanent and I know how strong the cement is on PVC so I wouldn't get a second chance. This was I can use the tool supplied with the mechanical boss to reposition/tighten it if I ever needed to.

Thanks for your help to all those that contributed