Help on front door mortise lock

I lost my last key for the entrance door. I have another cylinder that i would like to replace it with. I took off the 4 screws and then inserted a screw driver into the key slot and tried to turn the cylinder into the opposite direction, but it would not budge. On the inside there is the latch that locks the door from the inside with a bolt...looks like its attached to that cylinder. If the last resort is to get a locksmith to make keys from this lock, is it worth it or should I just get it repaired? The cost, if known? Thanks
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Locksmith. Have him (her) come out to the house. You sound like this is outside your experience. If you were close by me, it would be $80. If you havn't totally damaged the lock by now. Yes, I'm a locksmith with 20 plus years experience.
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Christopher A. Young
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Stormin Mormon posted for all of us... I don't top post - see either inline or at bottom.

And still breaking jambs and taking advantage of drunks and old women.
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Tekkie

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There should be scalp plate on the strike side of the door. Remove the scalp plate, usually held with 2 very short screws- be careful they are hard to come by and they are very short.
you should now see a set screw that lines up with the cylinder. Back this screw out a few turns and screw your cylinder out. Notice on your new cylinder that there is a groove cut in the side through the threads. This groove is what the screw grabs.
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