I just built up a Trek6500 (2008?)with isis Middleburn RS7 cranks and a short bottom bracket, but the XT front mech (FD-M700A) doesn’t reach the granny ring

Is there anything I can do about this?
I tried adjusting it with the limit screw and cable.
I am still 8speed but going to 10 speed soon
Do I need a special front mech?
I’d like to keep using this BB

Road mech.

are all road mechs suitable?

You’d have a far better set up if you switched to the correct BB.
I’m assuming you’re running a 108mm BB and it needs a 113mm?

Alternatively, do you have scope to fit a 2.5mm spacer to push the BB over to the drive side and still allow the NDS cup to support the BB unit? If so, and you have NDS crank arm/chainstay ckearance, this should correct your chainline enough for your ft mech to work.

No idea. I’ve used a d-a braze on mech mated to a carbon band to get a square taper xt double to work. Chain line is the killer. IIRC road uses about 5mm narrower than mtb.
Any road mech that you can fit to the frame (hence braze on and band clamp) should give you another few mm of clearance. You can also modify the inner stop of some mtb mechs. Detailed dremell work!

that mech doesnt look like I can modify it.
The chain line looks pretty good though
May be able to fit a spacer in if I had one
Meanwhile I have another mech I can try

Might need an older mech too. New ones use am odd cable pull. Older ones can be used with minor fettling

just fitted an old xt mech, it seems to be fine now.
Itching to get on it now, its not raining…

it was fine in the back garden, but a different story riding it, it wouldn’t drop down

I have the mech touching the frame now, but there is still 9mm between the downtube and mech cage, which is too much

Do road mechs have the same cable pull ratio as mtbs? (8speed and 10speed)

No. Road mechs now have a different/odd pull. You’ll have to work out what shifter your going to use and go from there.

I have a Zee shifter.
What about SRAM?

Why not just fit the right BB length to save it all being a bodge?

i dont want to buy another Ti ISIS BB

Did you try re-spacing it as above?

not yet. I dont have a spacer yet

It does look like there is just enough room for a 2.5mm spacer, but that would mean screwing the BB cup inside the BB shell 2.5mm

Can you angle the front mech so that the back of the cage is pointing inwards to try and knock the chain off onto the inner ring?
As above though, new BB would be better.

I tried that and it makes a difference, not sure 100% yet, but it may well be enough

If its zee (9 or 10 iirc) you’ll just need a road mech. 8/9/10 will all work. Pretty sure you can get some really cheap braze on ones. Think I’ve got a Tiagra somewhere, probably cost about 9 quid new.

Oh yeah, top pull or bottom pull frame. Road mechs are all bottom pull. Unless you can source one of the shimano cx range. Or dick around with pulleys.

Last frame I did it with was a mid 90s steel frame. Gears routed on the down tube……

https://www.rosebikes.com/article/shimano-cyclocross-fd-cx70-f-front-derailleur/aid:509812

Far preferable to riding with one leg sticking out.