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Compression joints


The idea of this is so you get an idea of how to make a compression joint correctly, so you know if your engineer is doing his job correctly, if you attempt a repair yourself,be careful

There are several common sizes of pipe, both copper and plastic.
  • 10mm
  • 15mm (1/2 inch)
  • 22mm, (NOT 3/4 inch)
  • 3/4 inch
  • 28mm, (1 inch)
  • Larger - not a diy job!
  • To begin with we will look at 15mm copper, and how to make a joint, but lower down there are notes to cover plastic, 22mm and 3/4inch, but you will need to know this first.
    First thing is to cut the pipe. This can either be acheived by a hacksaw, or a roll cutter,

    A roll cutter produces a nice cut, with a tapered edge, that makes for easy joint insertion. A hacksaw does not. Up to you really, a roll cut joint is often lethaly sharp, and by rights you should actualy debur the inside of the pipe to remove the tapered edge. No one ever does, and if your cutter is sharp, it wont be a significant taper, but, you should.

    Here is a connection, this one is an isolator valve, but a joint, tee or elbow are all the same.

    Take the nut off, and remove the olive.

    Take a little bit of plumbers paste and rub on the inside and outside of the olive, not too much, just a smear all over.

    Not all pastes are the same, some are thin and watery, some are thick and sticky, any will do, so long as its appropriate for the water the joint carries. The job of the paste is more of a lubricat than a gap filler.

    The paste you use must be badged 'POTABLE' for drinking water, for central heating systems it doesnt matter, for hot water use 'Potable' as well, but you dont strictly have to..... (unless you drink the hot!)

    Now put the olive and nut back onto the fitting, align the olive so its straight, and, with a finger, wipe away excess paste from the inside.

    With some kind of pokey instrument - match, twig, bit of wire etc, work out how deep the pipe should go into the joint, if you look down the joint you will see the 'shoulder' the pipe butts up against.

    With a pencil, mark this depth on the pipe.

    Now insert the pipe into the joint, it should slip in with a little resistance.
    If you have trouble, 'wobble' the pipe as you put it in, the pipe should go all the way to the pencil mark.

    While gripping the main body of the joint, rotate the nut till you feel it butt against the olive, you should be able to do this by hand.

    Grip the body of the joint with a wrench, and with another, rotate the nut 1/2 - 3/4 of a turn. By this point, the nut should be pretty tight.

    A copper olive (copper coloured) will be softer and allow a little more tightning, a brass (gold) olive will allow less.

    To check, try to pull the pipe / joint appart by hand and also check for rotation - is it 'tight'? Dont strain the pipework doing this tho.

    To remove an old olive, pinch the nut / top of pipe as shown and squeeze gently, Do not deform the top of the pipe!

    If this doesnt work, slide the nut back from the olive and cut diagonaly through the olive, till a groove deep enough to get a screwdriver in is made. Do not cut all the way through. Now insert a flatblade screwdriver and twist. The olive should snap and you can then remove it. A new screwdriver is best, as it has nice square edges.

    To make a joint on plastic pipe, we need one more piece - an insert, this is designed to support the pipe wall from crushing, under the force of the olive.

    There are many types of insert, get one from the manufacturer of the pipe (its written on the pipe wall) - so a Hep2o insert with Hep2o pipe, a speedfit insert with speedfit pipe. When tightening a compression joint on plastic pipe, especially when using a copper olive, 3/4 of a 'tightening' turn with the wrench is needed.

    22mm joints - same deal as 15mm realy - but less 'tightening' turn needed - often 1/2 turn is enough, but at this size, 'overtightening' wont be easy, so screw it down tight.

    Overtightened olives - if you are replacing a joint, and find the olive has crushed the pipe, then a wrap or two of PTFE tape will bulk it back up nicely. This is not ideal, but sometimes it has to be done. When tightening down on the ptfe, the 'hand tight and 1/2 - 3/4 of a turn' rule can be ignored. Use best judgement, remembering that the ptfe will compress.

    3/4 inch pipe - this is the 'old' 22mm pipe. You will usualy find this by accident, and the first you will know is that the olive is a realy baggy fit over the pipe.

    As your not likey to be prepared for this, and often I'm not either - make sure the job is 'safe' - water fully off, no temporary tank bungs etc, and pop down to the plumbers merchant for a 3/4" olive. This olive is simply a slighty thicker version of a 22mm olive. Reassemble as per '22mm'. If desperate, you can wrap a bit of ptfe around the pipe, then put a 22mm olive over, then tighten. This is a VERY temporary solution, and will not take much pressure, so i do not advize it. (but - if for example your working on a pipe where the stopcock is letting by, and you need to go to the merchants for a 3/4" olive, then do the 'temporary' LEAVING THE TAPS OPEN, so the joint takes no pressure, it just seals in the flow of water to the open tap...., the let by is contained and sent down the sink)

    Leaking joints can sometimes be 'nipped' tight, alternatively, if your feeling brave, hold the joint with a wrench and loosen the nut, hold the joint and the pipe together, undo the nut, (some water will escape) and put a wrap or two of ptfe around the olive and retighten. This may produce a MAMMOTH flood.


    Frost bursts are often where a compression joint has been pushed appart by frost. often the original nut and olive can be re used, just disassemble, put the nut and olive on the pipe and re tighten.

    On this subject, frost damaged pipe is very hard to work with, the ice will have swollen and weakened the copper, which makes getting the nut and olive on a challenge, in the case of a burst joint, the pipe will not have swelled, as its held by the olive, its just pushed appart, but bare pipe is a different matter. Try gripping the nut with a wrench and rocking it down onto the pipe end, alternatively, cut the pipe further back to where its not frost swollen, sometimes a bit of insulation, a drainpipe or some earth will have stopped some of it swelling.

    Quality of fittings is crucial, the quality can be seen by the sheer mass of brass on the fitting, if its thick and chunky, and the pipe 'shoulder' is way down inside the fitting, its good. Dont be fooled that a plumbers merchant will always have a quality item, sometimes B&Q have better. shown here are a sample, going from worst to best, notice the squared off edges on the good one for wrench jaws.

    Things that can go wrong
    • Rotating the pipe while tightening the nut can loosen or strain other fittings, my personal favorite being a joint buried in a wall......, second favorite is flexi connectors getting kinked. Consider investing in a 'helper' with a 3rd wrench to hold such things.
    • Under tightened - joint will pop open
    • Overtightened - joint will leak
    • Crossed threads - making you think its tight when its not