class class listen you take a piece of string wrap it tightly around the hose wherever is best for you make a mark on the string where two come together--measure length..then divide by pi 3.14 will give you ID of hose
That would be the O.D. measurement, wouldn't it?
Also, the hose on my truck has a different size on each end, and is hard to tell where in the hose it changes size. Some are easy to see the change, like a flare right at one end or the other, mine had more of a broad taper.
So when I cut my hose section out, one end was bigger than the other and I almost got f'd because it didn't match both ends of the heater tube.
So anybody who does this mod has to pay special attention to where your hose size changes along the tube if you happen to have different sized ends.
And it is important to mount it on the rising size of the hose and not in a hump of the hose.