I have some questions about stems.
So if the P3C has a 1 1/8" steer tube, what combination of stem/aerobars are you going to use?
What is the difference between 26.0mm and 1" ? How about 31.8mm and 1 1/8" ?
Does the Visiontech sizemore stem solve the problem with having a 1 1/8" steertube and a 26.0mm bar? (ie HED bar) Are there any other stems that solve this problem?
You’re looking at two different ends ofthe stem here. The 1" vs. 1 1/8" is for the steerer end, the 26.0mm vs 31.8mm is for the bar clamp. Most stems, even those with 26.0mm bar clamps, will fit on a 1 1/8" steerer nowadays.
We can’t even come to an agreement to use the same system of measurement on one part? 
So my one piece Profile aerobar with a 1" steerer might fit onto the 1 1/8" steer tube?
well you could put the clamp measure in inches but millimeters are just more fun.
To clarify Gerard the 1 1/8 and 1 inch measurements are the diameters of the forks steer tube, the rod that attaches the fork lowers goes through the headset bearings and you clamp the stem onto. Most stems now are 1 1/8 inch stems and if you want to use them with a smaller 1 inch steer tube you use a shim to size them down.
The 26.0 mm refers to the clamp on the other end of the stem, the end that clamps to your handlebars. 26.0 is standard fitting most bars while there is a relatively new oversize bar around as well. For triathletes most aerobars are meant to fit on a standard 26.0 bar.
Your profile aerobar with a 1 inch stem (assuming you are using a 1-piece integrated bar like the Carbon-X) will NOT fit on a 1 1/8" steer tube. Make sure that your bars are 1" though as I didn’t think they made a 1" version.
We can’t even come to an agreement to use the same system of measurement on one part?
So my one piece Profile aerobar with a 1" steerer might fit onto the 1 1/8" steer tube?
I think what Gerard is saying is that most stems made these days are made to fit 1 1/8" steerer tubes, and come with a shim to allow the use of 1" steerer tubes. You are stuck with 1" steerer tubes with your one piece bar.
I believe that Profile only makes their Carbon X bars in 1-1/8" but they come with a shim for 1" steerer tubes. So, you should be okay… as long as the length is right to fit you on the P3C.
My Profile carbon X’s are a little old, cerca 2001. I really think they are 1". I will go measure.
Yep, definitely 1". It looks like they will be going up for sale soon if anybody is interested.