Tapping..

..not Coupé-related, hence this location.

The rain-water-pipe outside our kitchen flows into a collection point c.2 feet underground. Every so often I need to remove the trapdoor and gasket shown in the photo to release pressure after heavy rainfall combined with the outflow deriving from the three flats above.

The root of the problem is the sharp bend underground which was constructed just after 1900, when current usage levels were unimagined. The overwhelming accumulation of rainwater, soapy washing machine waste and other occasionally more solid objects contrive to block proper water egress, resulting in it trying to escape upwards...the reality is it feeds back up our bath and shower waste pipes. On one occasion several years ago we returned to find our bathroom underwater, the bath half-full with mucky water and the shower tray having overflowed, so my remedy has been to leave the pipe trapdoor slightly loose to relieve the pressutre.

The first time I attempted to remove it, the two bolts retaining the trapdoor were well stuck after 100 years but fortunately one did eventually oblige - the other, lower, one sheared a little way in.

My question is, how do I tap the lower hole for a new bolt as it's so close to the ground - I have a tap and die set but the cross bar has no room to manoeuvre.

Also, as the installation is cast-iron, does it need a special tool to tap it?

Thanks in advance.

In this case being cast iron you'll not find it easy to tap that material, and being so close to the floor so like a pulled thread on an engine have you thought about heli-coiling this?

If that doesn't work what about filling the hole with chemical metal and drilling a tapping a new thread with a slightly smaller bolt size.

Chemical metal - mm, will investigate that.

One of the holes is a bit shallow, is a special bit required to drill cast-iron?

Would a heli-coil (I'll have to look this up) procedure still allow me to access the bottom bolt hole given the lack of room?

You can drill and tap cast iron with conventional drills and taps used for mild steel, you'll need oil as a lubricate, heli-coils don't use anything special to fit them in the thread, google a how to and it'll all become quite clear.

What's the thread diameter and pitch? I'd think that'd be your biggest challenge - identifying what you need.

As above, you'll drill the old thread no issue, but you've got to be careful as you go.

You need a good quality cordless drill as you need to go slowly both with the drill speed and feed. You also want to a good set of new drill bits (general HSS bits will do just fine, but be prepared to blunt them).

Make sure you have good light and you are square to the cut (so far as possible so you don't drill at an angle). Clean the area up with a wire brush. Give it liberal coats of wd40. Centre punch the middle of the old screw. Start with a small drill bit (maybe 2mm). Remember to apply a decent amount of pressure and not to spin the drill too fast. What you're trying to achieve is the bit not skipping over, but cutting and also not over heating. And fingers crossed if it's as old as it looks then the metal might have gone soft.

Drill as deep as you need to clear the bottom of the screw. Then move up a drill size or 2 (make the call based on how tough the first cut was).

Repeat until your at clearance for the thread (for example M 8 standard would be in the region of 6.75 - but 6.5 would be okay for this task). At this point you should be able to pick the remains of the old screw thread out.

Best of luck

Ratchet tap wrench with sliding t-bar will get in there, something like:
https://www.ebay.co.uk/itm/232106574473

Is it not illegal to have rain water mixing with grey water in Scotland?

Thanks for all the tips.


@szkom - you're right I should dig out the remainder of the lower bolt, though I have done a bit of this in the past

@GrahamL - hadn't thought of that, bet you've got one

@magooagain - I had no idea Joe, don't tell anyone

Simon
Your photo shows an access trap. This really is at a junction of pipes to clear blockages.
The rest bend at the bottom of the pipe is a change of direction from vertical to horizontal.
The amount if users shouldn't really affect this unless it is 2" diameter, but probably 4".it is more than likely that something is affecting the flow from the rest bend downstream.
Is there a manhole or other access point.
If so lift it and investigate.
Drains generally flow well.
Blockages caused by, tree roots penetrative, physical displacement due to roots or highway traffic, something trapped, nappies, sanitary towels, biros, etc

Call me after 19th September if you want
Paul

Appreciate you answering from holiday Paul

You're absolutely right, here's a pic of the manhole cover which I'm all too familiar with...I've got a complete set of rods which screw together, with one I've set in a curve to get round an awkward bend. I used this to check for obstructions in the 3 feet between it and the right-angled bend under the trap, and I can see the rod-end poking through.

A ream downwards through the trap completes the process, and a flush through with the hose results in a normal flow all the way through to the manhole.

The only thing I can think of is a stone might have wedged itself within that 3 foot horizontal...

Mmm.

No signs of blockage in the manhole ?

Is there a local drainage CCTV company ?

A survey down and upstream would identify any displacement of the pipework. Ideally, each pipe should be concentric with the next. However, they can become displaced, or even may have been installed in a non concentric way to form a bend.

With the manhole being so close I wouldn't be going out of my way to mess about with cast iron.

Cctv man can jet up and downstream

Are there young women or children sharing the above groun
Pipework.

Paul

Yes there are three floors above and yes to your question. However I'm confident I've cleared anything that might be pertaining to them.

I'll have a go with one of those flexiscope things and see if I can spot a stone blockage.

Manhole I inspect every 2/3 years so am confident it's running clear.

It’s possible the pipe has a sharp edge as it turns / drops given its age / material. Modern pipe work uses slow radius connections so it maybe that to effect a permanent fix it would a dig and replace that last section of pipework from the down pipe.

Did you get the drain pipe situation resolved ?

The gasket worked fine and there's "just" enough grip for the two bolts to hold it and the cover in place.
Trouble is that at first it was too efficient and didn't allow plan B to work, that is any rush of water couldn't escape from it.

Unfortunately to do a proper job I fear would cause major disruption as well as incur a lot of expense.