Measuring your header and flex for 2 cycle racing

by Jeff Bosset
of TS Racing, Inc.

Reprint from Inside Karting Magazine, June 92

We need to take a look at how to measure the ever important flex length. Flex length is the measurement in inches from the back side of the piston to the end of the flex pipe.
First we will look at the exhaust port length. This measurement is taken from the back side of the piston straight back to the header mounting surface. (Example 1) The standard measurement in the industry for a Yamaha is 2".

Example 1

Example 2

Next, we will look at the header itself. The measurement used for the header is a chord dimension, which is the average of the shortest measurement and the longest measurement if an angled header is being used (Example 2) If a straight header is used, one measurement is enough. But for accuracy the center line measurement would be required. These two measurements should be taken from the inside of the header. Once the measurements of the exhaust port length and the header length have been made, add the two together and scribe or engrave the dimension on the header so it can be easily seen by you. Now place your tape measure at end of the header flex cup at the dimension you marked, and measure to the end of the flex. By adding or reducing the length of your flex pipe, you can achieve your desired flex length (Example 3).
I hope this helps you in making your flex measurements easier and faster. And be careful, your pipe may be HOT!