I got an "end-plug" brake lever with that will fit bars with internal diameter 18.5–21mm. Does this mean I need a road bar (ie 26mm/23.8mm clamp/grip diameter) or can I use a MTB bar (ie 25.4mm/22.2mm clamp/grip diameter)?

It won't fit inside the MTB bar I have now (internal diameter 18mm), but I don't know if this is typical for MTB bars or if mine just has a thick wall. I don't want to sand it down due to the integrity of the carbon and I'd prefer to stick with the 25.4mm clamp diameter for greater flexibility in stem choice.

Internal diameter of MTB style bars varies considerably. In aluminum generally speaking the the more expensive the bar the thinner walled it is due to use of stronger grade aluminum and heat treatment given to the more expensive bars.

Not sure about CF bars. Unclear to me from your post as to whether the bar or brake levers are CF or both.

BTW seeing as how the external diameter of all MTB bars in the lever/shifter clamping area is the same, the OD at the stem has minimal influence on the ID at the end of the bar.

i got my diacompe lever to fit into my origin-8 mustache bars by replacing the original alloy sleeve with something thinner (i used cardboard, though a soda can is probably better).

The levers are CF, but I'd like to upgrade to a CF bar. The problem is that I don't want to spend a lot of money on a bar only to find out it won't work.

Does anyone know what a typical wall thickness is for a CF bar (if there is a typical thickness at all)?

Well, typically bars with a 25.4mm stem clamp are 22.2mm at the lever/shifter.

At a guess most bars that fit MTB levers will not fit end plugs. There are exceptions like the Soma Sparrow bar, where the inside of the bar is machined at the ends, but I wouldn't want to do that to carbon.

You want to stick end plug brake levers on a MTB bar? Why? That doesn't seem very practical to me. Why not just use MTB brake levers? I would think that you have a higher potential for damaging the levers because the plug ends are sticking out the sides of the bar more than the handlebars. It is not like they are on a TT bar facing forward or on a mustache like bar facing back.

So are 31.8mm stem clamp area MTB or similar flat bars. I have those on my Civia Hyland. All take the same outside diameter brake levers and shifters. 26.0mm diameter stem clamp area bars are typically drop bars and are used with 23.8mm clamp diameter brake levers, typically so called road levers. I do not recall seeing 26.0mm MTB bars though they may exist.

Nitto does offer 25.4mm stem clamp area drop bars with 23.8mm main diameter for use with road levers.

Willing to bet that there is little or no standardization of bar internal diameters regardless of material. As I mentioned both my Civia Hyland and Swobo Dixon aluminum bars are 22.2mm outside diameter in the controls clamping area but are very different internal diameters at the end of the bars. The Civia bars use much thinner wall aluminum.

Well my current bar is 22.2mm OD and a 18MM ID at the end, which is only 0.5mm too small for my end plug lever. Since this is a pretty cheap bar, I'm inclined to think that there are many MTB bars in which the plug should fit because I'm only short half a mm and, as noted by tatfiend, lighter bars should have a bigger ID.

So I think it's a reasonable guess to assume that my end plug levers would fit most quality 22.2mm OD aluminum bars. Therefore, my current question is if I can expect a carbon bar of the same OD to have a thinner wall than aluminum or not. I'm inclined to think not because my impression is that carbon walls are usually thicker, but if anyone has reason to think otherwise then please advise.

Hmmm, my current bar also has 0 degrees bend (it's just a straight tapered tube). I'd also like to keep that shape, but I'm having difficulty locating carbon bars without bend (only aluminum or steel).

So perhaps I'd be better off finding a wide drop bar and cutting off the drops. I don't need a bar more than 450mm wide or so. Do you forsee any problems with this plan?

As I mentioned earlier most drop bars are larger outside diameter than MTB bars so will not accept MTB bar shifters and other clamp on the outside of the bar type MTB accessories.

Cutting off drops to give a straight bar will not give even 450mm width as road bars are typically listed as measured width center to center on the drops and the bar top forward curve starts well inside that point. I do not know of a drop bar wide enough to do what you want.

The CF bars I have looked at are typically relatively thick wall construction. The light weight is due largely to the low density of the carbon fiber material.

Thanks. I think this answers my question.

Though to be clear the OD doesn't matter in my case because the only controls I have are end plug brake levers.