I was taught, 3/4 pipe is 3/4 I.D. IF YOU SAY 5/8 PIPE THEN DON'T CALL IT 3/4! I'm not trying to argue with you, but theirs a reason for this terminology! Now, I have no Idea how the metric world describes the mess that they created!
The pipe OD is based on the original OD of the British cast iron pipe of that nominal ID. As they transitioned to steel, they didn’t need as much material to hold the pressure, but they did need to maintain the OD so the pipes could thread together. Tubing is called out by the OD, and wall thickness. That is why, you can’t use a pipe bender to bend tubing, or a tubing bender to bend pipe. The ODs are too different for the sides of the shoe to keep them from kinking.
Listen I'm definitely not trying to argue with anything both of you say, I'm stoked your both helping me learn and I'm very grateful for both your knowledge, thanks both of you. This is all very much new to me, before my carry, other than the little I did to me 82 sambar, I've never stepped into auto mechanic world lol but I refuse to pay someone big $ if I believe I can do it myself! And so far, with the help of you two and a few others here on the mini truck talk forum, I've been able, it's pretty sweet! This site is amazing and believe me when I say this, I'm very grateful for your input and help. I know I was never taught proper lingo for 5/8 pipe or 1/2 pipe. This is just something I assumed, when someone says they put a 2 inch exhaust, I thought it was measured from one outside edge to the other. Now that your explaining it, I understand and I've learned something new and very valued, you guys rock!
With exhaust tubing your looking at a very thin walled tubing. So, it is very close either way. And Exhaust “pipe” is tubing so it is measured by OD. If your welding up your own exhaust manifold, or header then the runners are sized by an equation that uses the valve diameter, lift, and rpm at which you want peak torque. I did some looking and found a calculator: http://www.wallaceracing.com/header_length.php
