I have a 2006 trek 4900 with rockshox j1(80mm). if i were to uprade the fork how much travel could i get.
also how do i tell if is a threadless or not

if you have 80mm i wouldn't go over 100mm

I would...........

you could put a very long travel fork on it but the geometry is gonna get screwy, even a 100mm will change it but it won't be drastically diff, but w/e throw somthin long on there if u want

I haven't looked up the specs but I am sure it's threadless. Does your stem have 3 bolts, 2 side and 1 on top? That would be threadless.

yay my stem has 2 bolts on the side and 1 on the top

Yay indeed.
But make sure that new fork improves over the old in a meaningful way.
More travel does not mean better fork.
I'd rather have a predictable 10mm fork than a 100mm leaky piece of twisty junk.
Check the reviews.

Dont get a dart. One of my spare rotors has been ruined by leaking oil from my old dart

Oily rotor + alcohol = good rotor.

Hmm, well then, I guess you should get a dart. I never read the end of that thread.

how about the rockshox recon 351 Coil

Recon = good

will the recon fit on my bike

Yes

Are you going to DIY? I recommend getting a shop to do it - it has a few steps.

what are the steps on replacing it

I would throw on a tora solo air or a reba. Both are good forks for the price and tunable. You can run 100mm travel without too much issue but there isn't a point to put more unless you're doing riding other than xc, in which case I would consider getting a different bike.

As far as installation, check out park tool's mechanics corner.
https://www.parktool.com/repair/readhowto.asp?id=111

The steps are as follows. Remove bolts from stem. Remove topcap from headset. Pull out fork. Remove brake attatchment. Install headset race on new fork. Cut fork to proper length. Install new starnut into fork. Slide fork into headtube after installing bearings from headset. Tighten topcap and stem bolts. Attatch brake.
These steps are not exactly in order. Installing the race on the fork requires a specialty tool but I've done it twice using various methods. Same with the star nut. You risk damaging the race and setting the star nut wrong with the shadetree method. If your not confident, take it to a shop. They can also recommend a suitable fork.

Well, I followed your instructions...but after you cut the fork to proper length, how do you get the front wheel to stay on?

Silly person . . . you're supposed to cut the stanchions, not the sliders .

Really? You're going to pick that? As if anyone would actually cut both legs?
Very weak. Not your best effort I'm afraid, but I'll concede stranger things may have happened. You'd have to be all sorts of dumb though. Just thick headed as heck. The mind boggles imagining such an individual.
For the record, cut the steer tube.

You can always cut the bottom of the drop outs to be more shallow to shed some weight...

Really what?

I'm gonna pick what?

Hence the spirit of the neener in the air.

At least I got a nibble.

I always give 110% to my wonderful BF.net compadre's.

Didn't you know?

You obviously haven't spoken with my beloved wife.

Just give me a minute...I will astound you.

Went w/o saying...so why waste all this good sarcasm?

^ Wow.

Jameson-

Hey, follow Ed's lead! Yer not havin' nearly as much fun as he is!