How to Cut Mandrel Bends
It's easy to build a system from mandrel bends but it's not idiot proof. It requires dogmatic adherence to one technique. Luckily it's a simple one.
Unless you're intentionally breaking the rules (which is rare) then always cut tubes perpendicular (90 degrees) to their centerline. Then the cut end will emerge perfectly round and will match the wend of any other tube cut perpendicular to its centerline whether straight or bent.
It's easy enough to plot a perpendicular line on a straight tube: just lay a combo square head on the tube and draw along the ruler. But that's not possible in a bend. In these cases we have to reference the radius centerline. The radius centerline represents the line from the radius' center point as plotted across the tube.
There are several ways to do that but a template makes life so much easier. It takes a board, compass, square, protractor, pencil, nail, and a length of string. A template can be used for multiple pipe diameters and radii. Just draw the appropriate reference points for each pipe's dimension.
22. Finally he clamped the system and hung it. Many enthusiasts dismiss simple exhaust-shop hangers but they isolate the system from the rest of the car real well, let the hot pipes expand with heat, and last for years without any maintenance.
23. This is the way the system looks without the header flanges. Woolery sent it to Don Meth at Show Quality Metal Finishing for the flash plating as seen in the first shot. Though not quite as durable as ceramic it costs far less and looks like it was ripped out of an old magazine.
24. Strike X and Y axes on a board and add marks at 15, 30, 45, and 60 (shown) or better yet print out a compass and glue it to the board. Draw a circle the size of the bend's inside radius. Pop a short nail in the center point and tie a string around it. The parallel lines make alignment simpler.
25. Align the tube over the parallel lines and circle, pull the string over the tube to the desired angle, and draw the line. Remember to cut on the side of the line closest to the string.
26. Mark the tube properly and its cut end will emerge just like this, perfectly round and ready to meet another tube at any angle. It can't get much easier than that!