Building up a frame right now with 5.3-6.3" of travel and it's currently set at the 5.3" setting. Would it be ok to put on a fork with 80-100-120mm travel if the fork was set at the 120mm (4.7") mark? I've been around the block long enough to know that as a general rule, the fork should have as much if not more travel than the frame for balancing purposes. I'm just wondering if a fork with this minimal of a difference in travel would be a huge issue for actual riding.

Which frame, and what dot he manufactures spec?

Would also stick to using mm as suspension travel (front and rear) is now almost universally quoted in mm

Don't do it. It will not only steepen your head angle too much, it will also screw with BB height and small-bump and square-edge-hit angle of the rear suspension.

Check what axle-to-crown fork the frame was designed around, what the a to c is of the proposed fork at 120mm and what that will do to actual geometry with the fork sagged into its normal riding height; I think you'll see what I mean.

According to BikePedia, the originally spec'd fork was 127mm (5") and the fork that I'm looking at now is 120mm (4.7") at it's max travel setting.

I knew about the bb height thing and such, I was just hoping that someone would tell me that the 7mm difference wasn't really much of a difference after all... I'm just trying to finish building up my frame without having to spend money on the bigger parts (fork, wheels) when I already have some laying around. I will if needed of course, but spending money on something that isn't necessary seems somewhat pointless.

Compare actual a-to-cs; if you're still under 10mm difference, ya, you're within fudging room.

Now if you wanted a 7" dual crown for it, I have a couple of those laying around .

Haha. My only bike worthy of a dual crown fork (Giant DH Team) has one on it already. The frame in question that I'm trying to build up is a 2002 Giant AC1 frame. I'm debating putting a Rock Shox Psylo SL fork on it (hence this thread).

I found HERE (a link worth salting away) that the Marz Shiver SC that came on that frame was 510mm. Hope that helps.

The same chart shows a 2007 Psylo SL uturn at exactly 510 as well. What do you know?