I have and old "road" size bar which measures exactly 26mm, and its mating stem (both halfs (handlebar end)) fit perectly to a pair of handlebars which are 25.4mm, are these compatiable? As different companies make different size handlebars 25.4, 25.8, 26. Do different stem manufactuers make different size bar clamping diameters? what is compatiable. Thanks

You cannot run a 26.0 bar in a 25.4 stem.

Well, it sounds like the OP has a stem with removable face plate (as he mentions two "halves") so he probably could put a 26.0 bar in his 25.4 stem. Don't do it, though, OP. Get yourself a 26.0 stem if you like your bar and your general well being.

Yes, almost all manufacturers make stems with various clamp diameters.

The most common are 25.4 (MTB, old cheap road bars and a few drop bars made for MTBs with drops), 26 old road standard, 31.8 newer MTB and road standard.

26.4 is ye olde Cinelli standard and there is at least one 35mm bar/stem out there.

Even if the faceplate has enough room to tighten on a 25.2 bar, it isn't compatible. The curvature is different so the clamping force won't be evenly distributed, and will distort the clamp plate or handlebar causing stress above what either is designed for.

An analogy can be drawn to an egg shell. If you put the egg in your hand and squeeze evenly it'll be hard to crack, but if you squeeze just on two sides it'll be fragile as, well, an egg.

If you can't bring yourself to replace the bar or stem, try to locate some material exactly 0.4mm thick to use as shim stock to make up the difference and match the bar to the stem.