I am looking to replace the fork on my old mtb commuter. The current fork has a 140mm steerer and I was wondering if a fork with a 150mm tube would work. Could I take up the 10mm difference with spacers or is that a no-no. Decent forks with 1 inch threaded steerers for cantis and 26 inch tires are not easy to find anymore. Thanks.

As long as the threaded section extends far enough for you to properly adjust the threaded upper cup, the rest of the space can be occupied by spacers. If the threaded section falls short of what you need, it can be extended with the proper die. This is usually a bike shop job, as the die and die stock tend to be expensive for a one-off job.

Great, thanks. I think I will be ok, most stems seem to have some adjustability as far as the amount of threads available. I was just worried about using too many spacers. Thanks again.

The extra 10 mm should cause no problem. You want to be sure the stem's expander is below the threaded length but spacers above the headset's top race and below the locknut are fine. I have one bike with a "too long" threaded steerer and have 20 mm of spacers between the top race and locknut and it works just fine.

Why not just cut the steerer?

I like a bit of extra Steerer tube , because Im not 23.

they said 10mm not 10cm

Nitto Technomic stems are just the thing in these situation so the quill wedge is under the threads cut.

I will be running a Nitto 100mm Dirt Drop stem that has plenty of length to get below the threads. I agree about the Technomic stem, I have one on my 85 Cannondale tourer and it really helps keep the weight off your hands.