Hi all,I have a Orange evo 2 , the bars need changing so got the calliper out to measure and it’s 24.3 mm or 0.96 inches , this seems small to me so I will have to change the headset as well so my question is can I just fit a new headset that will take a fatter diameter handlebar, hope that makes sense, thanks for reading.

You will need to change the stem, not the headset.

But yes, you can get a different stem which will fit the new handlebar diameter. Swapping the stem is reasonably easy.

brilliant , thanks for your quick answer.

24.3mm is not at standard size, you have 3 options for an MTB, 25.4mm (older, now uncommon on anything but low end) 31.8mm pretty much current industry-standard & 35mm (new-ish standard, struggling to gain ground) for almost everything on a modern bike metric is used for measurement

If your changing stem, you will need to match the handlebars, as TimothyH notes; given the age of the bike (well over 10 years as it has a 25.4mm stem) you may find getting matching length stem to your current one hard to find (or very easy if on clearance)

If you take the bike off-road, you may want to look at going shorter than the original stem, as generally, bikes will ride better off-road with short stems than were used in the past.

Have a look here at a post with shows this Accidental build: Orange Evo2 | Retrobike

That is kind of an odd size, did you measure the end of the bar or the area where the stem clamps the bar?

Sounds like he's measuring the steerer tube.
Olden days style 1" which you can get a shim for so you can use nowadays style stem.
Shim the fork steerer tube so you can use contemporary size (1 1/8") stem and you can use modern type (31.8mm) handlebars.

There seems to be a lot of confusion about what exactly the OP was measuring. Everyone here wants to be helpful, so if the OP could explain exactly what they were measuring all the rest of us could offer better advice

No, the OP measured 24.3mm the only place close to this on a modern bike (the OP's model was introduced in 1998ish) is the stem/handlebar diameter interface.

It can't be the steerer, as by 1998 everything (MTB) was 1 1/8" (with the exception of a very few DH bikes), for the stem/handlebar, it's too early for 31.8mm or 35mm.

It can't be the brake lever/shifter mounting point, as these are 22.2mm

The OP does need to re-measure as they appear to have under/miss read the caliper (hope this was what they were using) or just take the handlebar / stem apart, (as the exact size should be stamped/laser etched/printed on both these parts), although, given we know what bike the OP has there are no real alternatives for the correct reading to be 25.4mm, also, given that the OP used a mix of metric and SAE sizes, that further indicates a measuring and possibly conversion issue.

hi, thanks for all your posts, the 24.3 was measured at the widest point on the handle bars
( middle) when I started to look for a wider bar to replace it and could not find any thing
that was the same diameter . looks like a bigger job than I first thought.

It shouldn't be that much bigger of a job. All you need is to get a new stem to match your new handlebars. If you're already replacing the handlebars, the incremental amount of work required to also replace the stem is close to zero (it's basically two extra bolts to deal with). Most of the work will be involved in moving all the control levers and grips over to the new bars. You'll probably spend more time researching which stem to get than actually installing it. Unless of course the bike has an oddball steerer diameter to match the oddball handlebar diameter.

The only thing that might cause headaches is if you're trying to change the fit of the bike by raising or lowering the bars, in which case you may need to mess around with stem spacers (not a big deal) or even cut down the steerer (here's where it starts getting more involved).

I recently changed out my stock handlebars for some others. I found that I had to go back to the bike shop & get some half-cylindrical shims to make the dia. of the new HBs match the stock ones. It was an unexpected inconvenience.

thanks for all your feedback its been very helpful , I,ve ordered new stem and bars ,
so fingers crossed they will go straight on .