can a mod 25 45 acp cylinder be used in a 625 45 colt frame wihtout modifications. i'd like to be able to shoot both calibers in my Mountain Gun. thanx

Yes, no, or maybe. It depends on the dash number. You'll have to measure the two cylinders.
Even if it fits, the frame lug will allow lots of slop when the cylinder is open.
I have a 1917 that I fitted a 45 Colt cylinder to. It took a lot of work to make it fit, and of course it has the slop I mentioned.

Longarm, have the colt cut for moon clips then you can shoot 45 acp in it alone with 45 colt and 45 schofield. Makes a real good gun to have.

I agree with John above.
But I went the opposite way with a 1917 in 45 acp. I had the chambers reemed deeper for 45 colt. Yes, there is more headspace but the Long colts fired reliably. I shoot acp with clips (you have to anyway unless you like to pick out acp cases with your finger nails) and I can shoot long colt, 45 schofield and 45 auto rim that don't need clips. The first 10,000 1917s didn't even have shoulders in the chambers for acp. They are called charge holes and bored straight thru like the old heeled bullet blackpowder guns. I could not even tell the difference in 45 acp or auto rim accuracy with the longer 'bullet jump in the longer shouldered chambers.

The advantage of starting with a 45 acp chamberd gun is that the cylinder stop in the right hand lower corner of the cylinder window in the frame (when looking at the left side of the gun) is longer (towards the front) for the shorter acp cylinder. But when you face off your long colt cylinder, you will create the slop that someone else talked about. There's two ways to fix that:

1. Whoever faces off your cylinder can leave just the slightest rim on the outer edge to contact the cylinder stop. It will be a very then lip and you'll have to make sure your 1/2 moon clips fit down in it. or...
2. Have the gunsmith install a new longer cylinder stop from an ACP gun. Since you have a 625 you won't even have to worry about bluing it.

I like the way you think!
JIM

P.S. Better yet, use full moon clips and you can leave a wider rim around the cylinder edge when faced off. With full moon clips the rounds slip in from the outside edge.
jim