I'm looking to replace my handlebar/headset bolts as they are rusted. It's a Trek Madone and the handlebars are bontrager. Is there a universal size of the bolts to replace them? And any recommendations to buy online? Don't have a LBS near me so not so straight forward to pop into a shop.
Thanks in advance.

I don't know about stem bole sizes... You should be able to pop one out and bring it to your local hardware store to see what size they are. Sizes are typically metric, not SAE.

Most of these bolts are either M5x0.8 mm or M6x1.0 mm and any decent hardware, auto parts or home improvement store should have them in a variety of lengths and head types. As recommended, take one of the old bolts with you to get a match.

Look for stainless steel or you will be doing it again sometime soon.

BTW, if your local hardware store doesn't stock metric SS (mine doesn't) you can find them in small lots on ebay...

Auto parts stores carry metric bolts ..

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Bring the headset cap screw and the stem with you to a hardware store and check things out. Cap bolt will be M6; the rest of the stem will be M5. I say to bring the stem because if I'm remembering correct, all the cap heads are inset and standard bolt head diameters might not fit...

I have run into stems using 7mm diameter bolts... a real oddball. Calipers and a pitch gage are your friends.

Also many stems seem to use bolts with a reduced diameter head; standard hardware will not fit these stems. I haven't seen any sources for these bolts, unless you want to buy them in OEM quantities.

I don't know why manufacturers go to the trouble of using non-standard hardware.

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+1

I'm in the process of replacing my bolts on the stem holding the handlebars. Right now it takes just two bolts to take off (easy) and cut 4 cables (even easier) and someone would have a nice handlebar, bar warp and a nice set of Ultegra 10 sp shifters for bout 3-4 minutes of work. It's way too easy for this to happen. Even though I don't park overnight anywhere, I have already gotten nicked parked at stores I frequent and once even at 9am in the morning! I am replacing some of my bolts with a kit from Specialized, anti-theft bolt kit , granted this kit could be pooh, but hoping not. Specialized is repping this, so it gives me some hope.

Or a shop that sells marine hardware.