Hi,
I want to mod a Girvin Flexstem and I'm wondering whether most M6 bolts will use the same sort of thread specifications. Is the type used on bikes a specific specification. . . I guess it must be? I need a T bolt which obviously isn't a typical bike part and need to make sure the threads match those of the stem.

Had a 60's SAAB, built with a variety of hardware sources, various european companys
there are a variety of thread pitches , fine to coarse, though they are to be perhaps both 6, as you say, MM

5 is commonly 0.8 thread per mm, 6 , 1 per mm
though theres a number of 5mm holding on the faceplate of my Azonic stem ,
its more a coarse thread than standard .8, perhaps to hold better in aluminum, than a finer thread would.

Broad generalizations fail, measure the pitch.

Specific specification?

All threaded interfaces belong to some specification, there's no getting away from that. This means that although one thread can be most usually found in a certain application, the thread is rarely proprietary and can be found elsewhere too. I think there are a couple of screws/bolts on a bike that despite being a regular M-standard have a different pitch, but the only thing special I've ever noticed about stem screws was on one occasion where thread size didn't correspond to the most common Allen head size fofr that thread.

I thought, for instance, that all brake boss bolts were standardized? I'll have to read up on thread pitch but I know that whatever my v-brake bolts have the correct thread for the stem. But it seems finding the right sort of T bolt might not be easy.

Threads are highly standardized. They have to be because so that folks who make fasteners and those who need them can be on the same page. Here's a link to more technical data than you need.

M6 is easy because it only comes one way 6x1. (larger screws come with fine or coarse threads) The only concerns are length, style of head, material, and whether it's threaded to the head (most shorter screws) or has an unthreaded length. For your purposes, if it looks right it is.

A good hardware store or any machine shop will have a thread pitch gauge that you can use to confirm the thread pitch.

The T you want to match, take it with you to the hardware store
and try screwing on nuts from labeled drawers.

Hate to pop your bubble but 6mm comes in .5 and .75 pitch also.Wiki isn't as smart as everyone thinks.

If it's threaded into aluminum,it's 6 x 1.0(coarse thread) coarse is almost always used on aluminum,unless it has an insert of some type.

Well for most things it's good enough. It's the Cliff Notes or Idiot's guide to a lot of topics and summarizes most things well. But definitely not comprehensive enough on technical items. Engineers aren't the best at writing and they spend enough time doing it for their projects that they certainly aren't going to spend extra time doing it for free.

One of the issues with that Wiki article on threads is it's theoretical. Real-world threads are cut and rolled with a more rounded profile and have a pre-determined gap between the male & female threads to allow for production variations. This gap varies based upon the size of the fasteners and their material.

I had one heck of a time finding M7 12-point bolts at a hardware store out in the middle of nowhere.

Booger1 is not quite correct: the M6 to which the OP refers is one specific thread standard, the M in front of the 6 means that it uses ISO metric standard coarse pitch (1mm).

Booger's general thrust is correct in that there are other metric standards and some of them occasionally pop up on bikes: an example is the thread on the dust caps of your cranks if you still use square taper. These are metric standard fine, usually 22mm x 1mm or 24mm x 1mm.

BTW there are two "i"s in aluminium; although the second one is often not pronounced by Septics it's still there.

Not in the US. You must be in the UK.

Not according to IUPAC. Or at least at first, they caved to American pressure after a few years and recognized our right to mispronounce elements.

I expect Alcoa, Kaiser and Reynolds had more influence than the academics at IUPAC.