2005 Avalanche. 5 GT
 

What kind of front Derailleur do I need, it was missing. I was considering a SHIMANO DEORE XT FD-M771 or a M771-d,it has a Deore XT on the rear.

Some specs call for a Deore 9 Spd Triple Front Derailleur 34.9mm, my tube is 32 mm

it's actually a 31.8mm clamp size that you need... the der. you mentioned usually comes with a spacer when new.

here's a 751 in the proper size...

Shimano Deore XT FD-M751 MTB Front Derailleur 31.8mm Triple 9s Silver | eBay

find a 771 with the spacer still with it, and you should have no problem.... do not get a "direct mount" type, and make sure it's a bottom pull or dual pull type, since i'm fairly certain your bike has the cable running under the bottom bracket, right?

a lot of bike shops will have the spacers hanging around since not all bikes require them.

The bike in question have the cables running across the toptube , therefore he need a top pull derailleur .

ahhh... just looked up that year Avalanche... seems the cable location has changed, then changed back thru the years... the three avalanches i've sold this season were all bottom pull... the dual pull der. will be fine.

I agree the dual pull will be fine , I just service one today and it cables were on the top tube .

Yes, the cable comes off the top tube and is hanging down.

so a top pull, or dual pull der. will work.

make sure the der. you get will match up to your chain ring sizes/range... we've already addressed clamp sizing.

if you get a new der., the red tape is for adjusting height of the der. over the large chain ring.... make sure the der. is perfectly aligned to the ring... then, set the cable up as taught as possible... then use the adjuster on the shifter for final adjustment.