Mountain Biking - How much steering tube do I need on my fork?

I'm thinking of buying a fork off of eBay and there's one in particular I'm looking at that has 195mm of steering tube left. How do I know if this is enough for my setup. From what two points on my bike should I measure to determine how much I need without removing my fork?

How much steerer tube is available with a brand new fork and how much gets typically cut?

Excellent question, I've been wondering about that too.

well the headtube length on a hardrock 19" is 120mm. i'm planning on getting a 180mm steer tube fork, so that leaves about 6cm for headset, spacers, and stem.

my fork had about 9 inches of steerer, i cut off about and inch and 3/4"... just measure the steerer on your fork now, and go by that.

Measure from the bottom of your bottom headset cup to the top of your stem clamp. Anything within a 1/4" from the stem cap is good, any shorter and it will be too short and not enough of your stem will clamp the steerer.

Measure from the bottom of your bottom headset cup to the top of your stem clamp. Anything within a 1/4" from the stem cap is good, any shorter and it will be too short and not enough of your stem will clamp the steerer.

Thanks - so looking at the two pictures below, I really need only between 180 and 185mm?

Yes thats all you need. You could also take out your spacers if need to gain even more room and just get a very thin one.