They make that aero bar to fit both 31.8 and 26.0 handle bars. Is it possible you've got the wrong one? I'd check any markings on the aero bar, info on packaging and receipt details if you got it online. But if you have the 26.0mm version that would explain it.

Edit - in fact in the last photo you can see the diameter of the top clamp is too small for that handlebar. It would just sit on the 31.8 part of the handle bar and not fit it at all so it must be the 26mm version of the aero bar.

My guess is you asked for the 31.8mm Airstrykes but were given the 26mm version. They make them for both sizes - I presume the clamp fittings are the only difference.

It's obvious from the photos that the clamps don't wrap very far around the bar and that the bolts are either too short or just barely long enough. I can't tell from the photos how the curve of the clamp and the bars match up. It should be fairly easy to tell if the clamp is made for the diameter of the bar. If it's not sitting snuggly against the bars it's the wrong size. If it's the 26mm size I would expect it to only touch the bars at either end of curve near the bolts. It would have a gap in the middle because the radius of the curve is too small to sit properly onto the bar. That won't grip the bar safely and will produce a point load damaging the bar. Don't try and use them.

Thankyou very much to you both. I bought the aerobars second hand off ebay and the size wasn't mentioned and I didn't know to ask. No great loss but a lesson learnt. Appreciate the help.

You could always check if Profile sell the clamps as a spare part. If they do I'd imagine you can use everything else and just swap out the wrongly sized clamps.

Per Ai_1's suggestion, Profile does sell the bracket. They used to sell it as just the flip-up bracket without the pads, so you might be able to find a set floating around in someone's stock. If you are not tied to the flip-ups, the L1 and L2 brackets should work for you as well (to position extensions below the bars).

Thanks again. I emailed them and they responded promptly. I'm certainly not set on this particular bar and only paid $20 for it. But given the cost of replacement brackets, I may just keep an eye out for another 2nd hand one on ebay, even if only for the 31.8 brackets.

We do sell the brackets that you are looking for and the pricing is below.

Forged Flipup Bracket Kit 31.8 - $39.99
Bracket-Extension Threaded 31.8 - $12.00 (each)