How to Notch (Fishmouth) Tube Without a Notcher

A hole-saw notcher is the quick way to fishmouth tube, but plenty of good exhaust and chassis work is done without one. What matters is marking the right profile in the right place, then cutting to it accurately. Here are three ways to do it, how angled notches differ from a 90° T, and how to get a fit that's ready to TIG.

What a fishmouth, saddle or cope is

A fishmouth is the curved profile cut on the end of one tube (the branch) so it sits flush against the side of another (the run). Pipe fitters call it a saddle cut; chassis and roll cage builders call it a cope. Same cut.

The profile has two points that wrap down the sides of the run and two saddles that sit over the top. Its shape depends on both ODs and the angle between the tubes. Change any one and the profile changes.

You need one wherever tube meets the side of tube:

• T-joints and angled branches, like a wastegate tube rejoining the downpipe
• Balance pipes (H-pipes) between twin exhaust runs
• Crossover pipes between the banks of a V engine
• Runners joining a tubular intake plenum
• Brackets, frames and roll cages

Three ways to notch tube without a notcher

Method	Good at	Weak at
Paper wrap template	Any size combination, any angle, offset joints	Print scaling, paper creep, speed
Freehand with a grinder	No extra kit	Repeatability, over-cutting
Slip-on notch sleeve	Repeat joints, speed, no printing	One size and one angle per sleeve

Paper wrap templates

An online tube notch template generator draws the flat pattern for almost any combination. Enter both ODs and the angle, print it, wrap it round the branch, tape it and trace.

Pros: free, and it handles unequal diameters, odd angles and offset joints.

Cons: it's only as accurate as the print. Print at 100%, not "fit to page", and check the wrap length equals π × OD (119.7 mm on 1.5" tube). If the ends overlap or leave a gap, the scale or the OD you entered is wrong. Paper also creeps while you trace, and a slight spiral puts the notch out of square.

Freehand with a grinder

Mark the saddles and points at quarters round the tube, cut a V into each saddle with a cut-off disc, then shape the curve with a flap disc until it fits the run.

Pros: no extra kit.

Cons: it's done by eye, so it's hard to repeat and easy to over-cut. The gap usually ends up wandering.

The two-cut shortcut. When both tubes are the same OD and meet at 90°, the outside edge of the fishmouth lies exactly on two flat planes at 45°. Two 45° cuts meeting in a ridge across the diameter give you the profile, with the ends of the ridge as the points. The cuts must mirror each other exactly, but on a bandsaw it's a fast way to rough out a T. Equal-diameter angled joints are two flat cuts too: 30° and 60° off square for a 60° joint.

Slip-on notch jig sleeves

A notch sleeve fits over the end of the branch with the notch profile built in, so you scribe round it instead of drawing the profile yourself.

Pros: the profile is right for the size and angle, and identical every time, which matters on both ends of a balance pipe or a set of matching branches. Ours are a loose slip fit with no stop, so take a moment to hold the sleeve square while you scribe.

Cons: one size and one angle per sleeve. Bendlabs sleeves are made for a branch and run of the same diameter. Unequal combinations can be made to order; for a one-off offset joint, a paper template is quicker.

Step by step: notching with a guide sleeve

1. Cut the branch to length, plus a little. On equal-diameter tube, at any angle, the tips of the points land level with the run's centreline, beside where the two centrelines cross. Measure along the branch centreline to that crossing point and add a couple of millimetres.
2. Draw an orientation line. Lay a length of angle iron on the tube, V-side down, and draw along its edge for the full length of the tube. Clock every notch and bend on that tube from this one line.
3. Fit the sleeve. Slide it onto the end of the branch and hold it square. There's no reference mark, so line up the middle of a saddle with your orientation line by eye. For an angled notch, decide which way the branch must lean first.
4. Scribe the profile. Mark round the shaped end of the sleeve. Fine permanent marker shows up well on stainless; marking blue and a scribe give the sharpest line on mild steel. Hatch the waste side.
5. Cut outside the line. Take the sleeve off (it's plastic and for marking only) and rough out the waste, leaving about a millimetre. On a bandsaw or with a thin cut-off disc, take a V out of each saddle. Wear eye protection, ideally a face shield.
6. Finish to the line. Take the saddles down with a flap disc, then finish with a half-round file. Stroke the file in the direction the run tube will lie, so its curve mimics the run. Then relieve the inside of the points (see below).
7. Test fit. Offer it up to the run, or an offcut of it, and look for daylight. It should sit without rocking or leaning sideways. To find high spots, colour the cut edge with marker, rub it on the run and file the shiny spots.
8. Deburr, and open up the run if it carries gas. For exhaust and plenum work, scribe round the inside of the fitted branch onto the run, cut the hole no bigger than that line, and deburr it. For frames and brackets, leave the run whole.

Angled notches: what the angle means

The notch angle is the angle between the two tube centrelines: 90° is a straight T and 85° is nearly square. Bendlabs angled sleeves run from 45° to 85° in 5° steps.

Don't measure the lean from square: a branch leaning 30° off vertical on a horizontal run meets it at 60°. Measure between the centrelines on the side where they close together (the acute side).

A 90° fishmouth is symmetrical: two identical points and two identical saddles. On equal-diameter tube each saddle is half the OD deep, measured along the branch from the tips of the points. An angled notch is lopsided. The acute-side saddle gets deeper and the obtuse-side saddle gets shallower. At 60° on equal-diameter tube they're about 0.87 × OD and 0.29 × OD. On 1.5" (38.1 mm) tube that's roughly 33 mm and 11 mm, against 19 mm for both saddles on a 90° T.

That's why clocking matters more on an angled notch. Turn the mark 180° and the branch leans the other way.

Getting the fit right for TIG

TIG rewards a good fit. On thin-wall exhaust tube you want the notch closed up, with no daylight and an even fit all the way round. A gap that opens at the saddles or points means more filler, more heat and more distortion.

Wall thickness matters most at the points. A template or sleeve marks the outside of the tube, but at each point the inside edge of the wall touches the run first. That holds the branch up and opens a gap at the saddles. File a back-bevel on the inside of each point until the outside edge sits down. The thicker the wall, the bigger that bevel, so expect more work on SCH10S or thick chassis tube than on thin exhaust tube. Our sleeves mark the outside profile, so the same applies. On equal-diameter joints the points finish as fine tips; blunt them slightly rather than chase a knife edge.

Clean off the marker and degrease before tacking. Tack one point, then the opposite point, then the saddles, so the branch doesn't pull over. Back purge stainless for a clean root, and use fume extraction when welding it.

Common mistakes

Notch clocked wrong. The profile is right but rotated, so the branch points the wrong way or a bend at the far end sits in the wrong plane. Both ends of a balance pipe must be clocked to each other. Mark everything from one orientation line.

Notch not square to the tube axis. A sleeve that isn't fully seated, or a paper template that spirals, leaves the points out of line and the branch leaning. To check, wrap a straight-edged strip of paper round the tube, line the edge up with itself, draw a square line and measure both points back to it.

Cutting inside the line. You can't put material back. Over-cut a saddle and the branch drops, the gap moves and the length changes.

Wrong size. 1.5" OD tube is 38.1 mm; 1.5" NPS pipe is 48.3 mm. Check which you have with our SCH10S pipe vs OD tube sizes guide.

Tools that help

Bendlabs makes two sets of slip-on notch sleeves for OD tube, made to order in our UK workshop:

• Angled Notch Pipe Cutting Jig Sleeves: 45° to 85° in 5° steps, in sizes from 1.5" to 4.5" OD.
• Equal Angle Notch 90° Pipe Cutting Jig Sleeves: for square T-joints, in sizes from 1.5" to 4" OD. "Equal angle" means a square 90° T between tubes of the same size.

Both are in the angled notch guides collection, and other sizes, angles and unequal combinations (such as a 2" branch onto a 3" run) can be made to order. Working in schedule pipe? 3" NPS pipe and 3.5" OD tube share the same 88.9 mm OD, and the sleeves fit by OD. Ask us if you're unsure which to order. For tight bends in the branch, see how to pie cut exhaust tube.

FAQ

Can you fishmouth tube with just an angle grinder?
Yes. Mark the profile first, rough it out with a cut-off disc and finish with a flap disc and file.

How deep is a fishmouth on a 90° T?
On equal-diameter tube, half the OD, measured along the branch from the tips of the points to the bottom of the saddles. A smaller branch onto a bigger run needs a shallower notch.

Can I use a same-size template or sleeve for a smaller branch?
No. An equal-tube profile is too deep for a bigger run, so the points land first and leave gaps at the saddles. Print a template for the actual combination, or ask us for a sleeve made to suit.