A press joint that was never fully seated does not leak on the test. It leaks in three weeks, behind drywall, and it looks exactly like a good joint from the outside.
Quick answer: the fitting has to bottom out on the tube before you press it, and the only way to know it still is at the moment the jaw closes is a mark you drew on the tube first. Viega’s own ProPress instructions make that the check: slide the fitting on “to the marked depth,” and then “visually check insertion depth using mark on tubing.” Mark it, seat it, press it, then look at the mark again. If the mark has moved out from under the fitting, cut the joint out.
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What actually goes wrong
A press fitting seals on an O-ring sitting in a machined groove near the mouth of the fitting. The jaw squeezes that section down onto the tube, and a second bead behind it grips. Both of those features are at fixed positions inside the fitting, which means they only land where they are supposed to land if the tube is pushed all the way in.
Come up short — even by a quarter inch — and you get one of two outcomes. Either the O-ring compresses against the very end of the tube instead of around its full circumference, or the jaw crimps the fitting onto a stretch of tube that has no sealing element behind it at all. In both cases you have made a mechanical connection that holds pressure on a test and has no reliable seal for the long term.
It is a quiet failure. There is no gap to see once the jaw is off, because the fitting has been swaged down onto the tube either way. That is exactly why the instruction sheet asks for a mark rather than a visual check of the joint itself.
Viega puts the mark in the procedure, not in the tips
This is not a habit somebody invented on a jobsite. Viega’s ProPress installation instructions lay out the sequence and the mark is a numbered step in it. Two lines are worth reading exactly as written:
“While turning slightly, slide fitting onto the tube to the marked depth. End of tube must contact stop.”
“Open the jaw and place at right angles on the fitting. Visually check insertion depth using mark on tubing.”
Read those together and the logic is plain. Step one gets the tube to the stop. Step two verifies it is still at the stop when the jaw goes on — because between those two moments you let go of the fitting, moved a heavy tool into position, and possibly rotated the assembly. The tube slips back. That is the whole reason the check exists.
The same document is blunt about two other things people improvise on: “Do not use oils or lubricants”, and “Copper tubing must be free of surface imperfections.” A smear of cutting oil to help a stiff fitting go on is not a shortcut, it is a different failure. You can read the full Viega ProPress Systems pocket guide yourself.
The habit, in the order it has to happen
- Cut square. A cut that leans puts one side of the tube against the stop while the other side is short. Your mark will look right on the long side and lie to you.
- Deburr, inside and out. An outside burr drags on the O-ring as the fitting goes on and can roll or nick it. An inside burr is a flow restriction you will never see again.
- Mark the tube. Measure the insertion depth for the system and size you are running, and put a line all the way around with a permanent marker. All the way around, not a tick — you will not always be looking at the same face when you check it.
- Seat to the stop, turning slightly. The slight rotation is in the instruction for a reason: it walks the O-ring on instead of shoving it.
- Set the jaw square to the fitting and look at the mark. Not at the fitting. At the mark. It should be right at the shoulder.
- Press, then look at the mark once more. Thirty seconds, and it is the only record you will ever have that the joint was right.
On a repair where you are pressing into an existing run, this matters more, not less — you are usually working with less room to rotate the assembly and more temptation to press whatever position you can reach. More on that in pressing a wet line.
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Where the depth number comes from — and why I am not giving you one
Insertion depth is not one number. It changes with the system and with the tube size, and the manufacturer publishes a chart for exactly that reason. To show how much it moves, here are the three largest ProPress sizes — the ones pressed with an XL-C ring — as Viega’s own minimum insertion depth chart prints them:
| Tube size | Viega minimum insertion depth |
|---|---|
| 2-1/2″ | 1-11/16″ (43 mm) |
| 3″ | 1-15/16″ (50 mm) |
| 4″ | 2-3/8″ (60 mm) |
Two things to take from that. First, the number moves with every size and it is not proportional to the tube — 2-1/2″ and 3″ sit only a quarter of an inch apart, then 4″ jumps nearly half an inch. Anyone extrapolating from a number they half-remember will be wrong. Second, that column is a minimum, not a target. The same chart runs from 1/2″ to 4″ in one table, and the instruction printed beside it is still to mark the proper depth and seat the tube until it contacts the stop.
So I am not going to print a depth for every size here, and you should be suspicious of any site that does without naming the system. Take it off the literature for the fittings in your hand, or off the depth markings molded into the fitting where the maker provides them. The number in a blog post is precisely the input that causes the failure this article is about.
The prep that makes the mark mean anything
A mark drawn on a crooked, burred tube end is decoration. Two tools do the real work here, and both are cheap next to the tool doing the pressing.
- Ridgid 151 quick-acting tubing cutter, 1/4″ to 1-5/8″ on Amazon → — a wheel cutter that tracks straight is what makes a square end repeatable. If yours is wandering, that is a fixable fault: see why a tubing cutter cuts crooked.
- Ridgid 227S inner/outer reamer, 1/2″ to 2″ on Amazon → — does both burrs in one pass, which is the only way it actually gets done on every joint.
Fittings and full kits are in the best copper press fittings guide.
If the mark moved
Cut it out. I know that is not what anyone wants to read standing on a ladder.
You cannot re-press a joint that was pressed short. The fitting is already deformed, the O-ring has already been squeezed somewhere it should not have been, and pushing the tube deeper into a swaged fitting is not possible. There is no inspection that tells you it is fine. The joint gets cut out and a new fitting goes on, and the second one gets marked before it goes anywhere near the tool.
The reason to build the habit is that this outcome costs four minutes when you catch it at the mark and costs a ceiling when you do not.
Frequently asked questions
Why mark the pipe when the fitting has a stop inside it? Because the stop only helps at the moment you push the tube in. Between seating the fitting and closing the jaw you set the assembly down, bring a heavy tool in and often rotate the joint, and the tube can slide back out. Viega’s instructions treat the mark as the verification step for exactly that gap, asking you to “visually check insertion depth using mark on tubing” as the jaw goes on.
What insertion depth should I mark for 1/2-inch copper? Take it from the literature for the system you are installing rather than from any website, including this one. Depth varies by product line and by size — Viega, for instance, publishes one minimum insertion depth chart covering 1/2 to 4 inch tube, on which a 1/2-inch fitting takes 3/4 inch and a 4-inch one takes 2-3/8 inch.
Can I re-press a joint I pressed short? No. The fitting is permanently deformed and the sealing element has already been compressed in the wrong place, so there is nothing to correct. Cut the joint out and start with a new fitting.
Does a short press leak straight away? Often not, and that is the danger. A partially seated joint can hold a pressure test and fail later, because the jaw has still swaged the fitting onto the tube even though the O-ring never sealed properly around it. Passing a test is not evidence the joint was made correctly.
Can I use lubricant to get a stiff fitting on? No. Viega’s installation instructions say plainly “do not use oils or lubricants.” If a fitting will not slide on, the cause is usually a burr, an out-of-round tube end or surface damage — the instructions also require that tubing “shall be free of surface imperfections.” Fix the tube end instead.
Related guides
- The best copper press fittings
- ProPress vs soldering
- NIBCO press vs Viega ProPress
- Type L vs Type M copper pipe
- Can you press a wet line?
- Why a tubing cutter cuts crooked
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