I’m working on these 2GRFE headers for a friend. Some long equal length 700mm stepped primaries have been spec’ed, and given limited space there are a number of bends.
When it came to checking my work, I was having a hard time measuring lengths. Using a tape measure gave a different result every time as I looked for a happy medium between measuring inside radius and outside radius. I then raided the kids toy box for some gigantic 1″ diameter foam pipe cleaners, cutting them to the right length and then threading them (with some difficulty) through the tubes. That method as well proved inconsistent, as the pipe cleaners found different routes through the much larger 2.125″ primaries.
In the end, I picked up a $6 bag of play sand from Home Depot, filled an appropriate length of straight tubing with the sand, taped one end of my primaries up, and dumped the sand out of the straight tubing into my primary. Then I marked the level of the sand in that primary and cut accordingly. Of course, it’s not that simple as the primaries still all need to meet in the same place to be collected into the secondary… but for better or worse, at least I actually have a good understanding of what I’ve done.