Exhaust flange fabrication
Last Updated on July 22, 2024 by Mutiara
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To fabricate an exhaust flange one first needs to acquire good metal material. This can be done by finding a local metal supplier that has a good selection or can order various types of plates. A general thickness of 3/8ths should be used, any thinner and the exhaust flange may warp from the heat, while going any thicker would be overkill and unnecessary weight. The plate can be any size as long as all of the ports and bolt holes can fit on it, with a little bit of room to spare around them. Cold rolled is preferred for it’s lower carbon content and straighter edges, versus hot rolled steel. In the following examples the cold rolled plate was acquired from Alro Metals for around $ 10.
The following assumes that the cylinder head is removed from the vehicle, as this allows for the best possible port matching to the flange and also a good chance to port the rest of the head at the same time.
Tracing the pattern
Work area demonstrating tools used (punch in background), template created, and holes drilled
The first and most important step is to trace the exhaust ports and bolt pattern. This can be done with paper and dirty hands, pencil, or dirty gloves by simply rubbing the paper and paying special attention to the ports to give a nice sharp line int he paper. Having paper long enough to trace the entire pattern can be nice, however it’s possible to do a section at a time. In this case two sets were made from standard legal paper, one of each edge, then the overlapping parts carefully aligned to ensure accurate matching.
Cut out the ports carefully and accurately with a razor blade over a cardboard surface. Then place the paper template over the metal plate and carefully go over with a permanent marker, drawing the shape of each port. A metal marking tool can also be used but will be more difficult to create smooth results with. Try to place one edge of the template by the edge of the plate to avoid extra cutting (assuming the edges are perfectly straight and untouched).
For each bolt hole, accurately place a punch tool over the center, and inspect it from all angles to ensure it’s not off. Then proceed to either click it in place or hit it with a hammer (depending on type of punch tool) to make a good indentation in the metal, where the drill press will be used to make a hole. Repetitive action of the punch tool is recommended to ensure a good deep mark.
Drilling port outlines
To remove a general outline of metal in each port, and prepare the flange for the next stages of fabrication, using a drill press is a good idea and not as time consume as one would generally think. While some such as Kevin have obtained good results using an ordinary jigsaw blades for metal, this technique is not as consistent because often times it’s hard to find good blades that will not become dull too frequently to complete the project. For the following examples Bosch metal cutting blades were used and dulled almost instantaneously. The decision was made to switch to using a drill press.
Any drill bit can be used for this task, however it should be at least the same thickness as the die grinder burr, or thicker. The thicker drill bits will leave more metal in between all the holes, but will be able to overlap easier if done at a slow pace.
When making the outlines, leave 1 mm or so between each one if using a smaller drill bit. However this does not have to be perfect as the die grinder will take care of it all afterwords. Larger drill bits can overlap the holes, but smaller drill bits will flex and slide off into the neighboring hole if they do not have a wall of metal all around them. This can be quite annoying and can even result in broken drill bits. Whatever the case may be, always try to ensure that you do not overlap the traced exhaust ports, it’s safer to leave a bit of excess metal to later remove by the die grinder than to drill slightly outside of the port and ruin the exhaust flange. Keep in mind the holes will always be a little larger than the drill bit seems, so it’s easy to underestimate the clearance that is needed around the potential hole.
Removing the port centers
This can be performed quite simply by inserting a die grinder with a carbide bur into each port and patiently moving it to the next hole, it will not take too long to remove most of the ports. The hardest part of this task is keeping the bur from creating teeth in the metal and traveling around the hole. Sometimes it will do this and rather than staying and grinding away at one area, travel around the hole and cause a lot of vibration. Over time it’s easy to adjust to this and feel the proper stance of the die grinder. If one has created teeth in the hole, start the die grinder slowly in the direction you were going, with only half of the bit inserted, and slide it in further as you speed up, this will allow you to smooth out the metal and eliminate the teeth for the burr to grab on to so you can get back on track.
Drilling exhaust manifold mount holes
This is quite easy and one of the quickest parts of making the flange, however proper attention should be paid to make sure the markings in the previous template step were done with the best accuracy possible. Using the drill press carefully align the tip of the drill bit with the marks, press in a little bit, make sure they are drilling in the right area, and continue to make the entire hole. If done properly, all of them will be perfectly on target when mating the unfinished flange to the head. If some are slightly off it is possible to correct them with the die grinder burr, or to drill the holes slightly larger than the bolts so that the flange can be aligned. However this is not best for port matching, having perfect accuracy will allow a the flange to be in perfect alignment each time it is installed.
Cutting off all excess metal
Prior to mounting the flange on the cylinder head, all of the excess metal should be removed for proper fitment. While the bottom of the flange did not have to be cut, due to the perfect edge of the cold rolled steel plate, the top in the following example was protruding by about 1 cm. Making a straight cut can be achieved with nothing more than an angle grinder, a vise, and some C-clamps.
Marking proper amount of metal is easy by placing a second piece of the same cold rolled steel (it was the half cut off) vertically, just barely touching the bolt holes, and drawing a line on the other side. This provides 3/8ths of clearance around the bolts, and is identical to the clearance at the bottom of the flange, creating a good appearance and structure.
There is a good trick to making the straight cut with an angle grinder. Place the other identical piece of cold rolled steel on top of the flange, with the edge just slightly below the marked line where the metal will be cut. Clamp the two together, and put them in a vise. Then apply the angle grinder to the area being cut, but do not stay in one place during the whole process of cutting. Rather glide the cutting wheel back and forth along the whole flange as you cut it, using the other piece of cold rolled steel as a guide. This will allow the blade to always stay in a straight line, and due to the constant movement over the whole area, prevent uneven sections from forming.
In the picture to the right, the cut was made from the back side. The piece of metal clamped behind the flange was used as the guide for the cutting wheel to travel by, while making a straight cut. The cut off piece of metal can be seen resting on the clamps. The results achieved could never be done by simply cutting the flange alone with an angle grinder by hand, and rival that of even a bandsaw! A sanding wheel was used (installed in the photo) to clean up and polish the final area after the cut. With the guide plate removed, quickly slide the grinder with the sanding wheel at a 45 degree angle around the edges to create a clean finish.
The whole setup was done outside to avoid getting metal all over everything, especially as the flying sparks will damage windshields and paint jobs on cars, among other things. A 7 in cutoff disc was used, however smaller 4-5 in discs can also be fine, but will be consumed too quickly and take a long time to cut. Most angle grinders (except cheap Ryobi models) can handle cutting with a 7 in disc, but will require the removal of the safety shield. Protective eye wear must be worn at all times during cutting, small particles can make their way in the eye and become lodged. If this occurs they should be removed by an optometrist (or self if possible) immediately else rust circles will begin to form an hour or two later, which will create serious vision problems later on.
The results are extremely straight and appear close to the quality of getting it machined with more expensive tools.
Finishing the port shapes
Flange mounted on a Ford 2.0 CVH SPI SOHC cylinder head.
Mount the flange on to the cylinder head. If any of the bolt holes don’t line up now would be a good time to resolve that with methods previously mentioned. Align the angle grinder perpendicularly to the flange with the best of your ability, and begin to remove the excess metal. This task is actually not as difficult as it would seem, and only took about 1-2 hours to complete. Oddly enough it’s the same as would take to do aluminum because aluminum being soft clogs up the bur and prevents it’s full use.
While cutting off straight metal with a guide is good by moving the angle grinder back and forth, moving the die grinder in a similar fashion should be avoided. This will remove too much metal at the ends, and prevent you from making straight even lines. Although in some areas it can be beneficial in small amounts. When cutting out the ports the die grinder with the bur should instead be progressed slowly and evently through the highs and lows. You can try to practice on some pieces of metal first to develop a feel for it, but eventually it will be easy to carve out a straight line even through all the inconsistencies of the cuts made by drilling. Make sure to keep the angle grinder as perpendicular to the flange at all times as possible, else this will sometimes create the appearance of an even port although in reality it’s tilting towards one side. Some aluminum will be touched during the process, and this is ok for proper port matching. Just be sure to stop right away when you do this to prevent taking off any more than is necessary.
In the end you will be left with a flange the likes of those found on professional header systems, and matched better to your cylinder head than ever could be. All that is left to do is to get properly bent tubes, or cut off the flange from another header, then weld the two together with a MIG or TIG. If the welding equipment and skills are not sufficient to create a clean assembly, then taking the parts to a professional welder is recommended, and should not be too expensive since the hardest part which is prep work has already been accomplished.
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Prior to finishing everything off, it’s a good idea to port and polish the head while the flange is still separate from the headers. The photo above is taken prior to doing that, revealing the type of burr and die grinder used.
