Merge collector template generator: a printable cutting template for 2, 3, 4, 6 or 8 into 1

A merge collector joins the primaries of a header into one pipe: the tubes come together at a shallow angle round the collector's centre line, and each one is cut so that it fills its own slice of the circle and its cut faces sit flat against its neighbours. The cut is a long curve on every tube, the spike, and the usual ways to get it are a fixed-size paper template or a cutting jig for one tube size. This page draws the template for any tube, any number of primaries and any merge angle, and prints it at full size.

Everything runs in your browser: nothing to install, no account, and the template prints on ordinary A4 or Letter paper, tiled when it is longer than one sheet. The collector cone itself is a cone in sheet, with its own pattern.

Merge collector template generator

Type the outside diameter of the primaries, pick how many go into one, and type the merge angle: the angle between each primary and the collector's centre line. The collector inlet is optional: with it, the generator says how far from the tip of the spike the collector sits, and draws that line on the template. Print gives the template at 1:1.

Embed this calculator on your site

Free, and it works on any site: paste the code where you want the calculator. It links back to this guide.

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How to use the template

1. Print at actual size, 100 %, with no fit-to-page, and measure the check bar on the sheet: it must read 100 mm or 4 in. Glue the sheets on the repeated strip when the template took more than one.
2. Cut out the template along the curve, wrap it round the end of the tube with the straight edge square to the tube and tape it. The wrap is the outside circumference of the tube, so the two ends meet.
3. Put the 0° station on the line of the tube that will face away from the collector's centre line: it is the long point of the spike. 180° faces the centre line. Every primary gets the same template.
4. Mark the curve, take the template off and cut: band saw, cut-off wheel or plasma. Leave the line, then dress each face flat with a disc.
5. Tack the tubes together: each cut face meets the matching face of the next tube, and the long points meet on the centre line at the tip of the spike. Check the merge angle of each tube before welding the seams.
6. Slide the collector over the spike to the dashed line. A round inlet touches the outer line of each tube there and leaves a gap between the tubes: close it with weld, with small filler pieces or with a formed inlet that follows the tubes.

The formula

Take R as the outside radius of the primaries, n as how many there are and α as the merge angle. The axes of all the primaries cross the collector's centre line at one point, and each tube is cut by the two planes through the centre line half way to its neighbours, at ±180°/n from its own. The line of the tube at angle φ round it, counted from the outer line, reaches those planes at

Cut line

s(φ) = R × [ |sin φ| × cot(180°/n) − cos φ × cos α ] / sin α s runs along the tube's axis from the point where the axes cross, and the tube keeps the side where s is larger. The template height at each station is the largest s minus s(φ), plus the margin to the straight edge.

Each plane cuts two neighbouring tubes, and the two cuts are mirror images in that plane, so the faces mate exactly. The outer lines of all the primaries meet on the centre line at R / sin α beyond the crossing point: that is the tip of the spike. From there the outer lines open as a cone of half angle α, so a collector of inside diameter Dc reaches them at Dc / (2 tan α) from the tip, as long as the cut has not ended by then.

The template is drawn on the outside of the tube, which is where the faces meet on thin-wall exhaust tube. On thick wall, dress the inside edge of each face after cutting.

Worked example

Four 1-3/4 in primaries (44.45 mm) into one at 10°, into a 3-1/2 in collector (88.9 mm inside):

1. The wrap is π × 44.45 = 139.6 mm.
2. At the outer line, s(0°) = 22.225 × (0 − cos 10°) / sin 10° = −126.0 mm; the highest point of the cut is 179.6 mm, near 135°. The cut runs 305.7 mm along the tube.
3. The tip of the spike is 22.225 / sin 10° = 128.0 mm beyond the point where the axes cross.
4. The collector sits 88.9 / (2 × tan 10°) = 252.1 mm from the tip.

Merge angle and cut length

A shallower merge makes a longer spike and puts the collector further from the tip. For four primaries into one:

Frequently asked questions

How do you make a merge collector template?

Type the primary tube OD, the number of primaries and the merge angle in the generator above and print at 100 %. Wrap the template round each tube with 0° on the outer line, mark and cut: every primary gets the same template.

What is a merge collector spike?

The point the primaries make where they meet: each tube is cut on two planes through the collector's centre line, and the long points of all of them meet on that line. For four 1-3/4 in tubes at 10°, the cut runs 305.7 mm along each tube.

What merge angle should I use?

The template works for any angle up to 45°. Shallow angles make long spikes: four 1-3/4 in tubes cut 382.9 mm at 8°, 305.7 mm at 10° and 254.1 mm at 12°. Pick the angle the header layout allows and check the length against the space you have.

Does it work for 2 into 1 or 3 into 1?

Yes, from 2 to 8 primaries. In a 2 into 1 the two tubes are cut on one plane between them; with three or more each tube is cut on two planes.

Where does the collector go?

The outer lines of the primaries open from the tip of the spike as a cone of half angle α, so a collector of inside diameter Dc meets them at Dc / (2 tan α) from the tip: 252.1 mm for a 3-1/2 in collector at 10°. Type the collector inlet and the generator draws that line on the template.