Unable to open mouth more than 1/2 an inch after root canal?

Why? Why couldn't you open your mouth more than that with the first dentist? Did you open your mouth further for junior? If you did, you had something against the first one, whether you were conscious of it or not. Fear or anger has many faces. With regard to 'after root canal work' there is something, definitely wrong if can't open mouth more than half inch. Go back.
Can happen. Sometimes when your mouth has been opened for a long period of time, your jaw muscles can spasm leading to limited opening. I assume you have no TMJ issues. With some nsaids, possibly a muscle relaxant, most patients do fine.
May be trismus. Not uncommon...Probably trismus, which can unfortunately last days/weeks. Nsaid medications, warm compresses to the side of your face receiving the root canal, chewing sugar-free gum can all help "loosen" the muscle that's tight. You should see your dentist to diagnose whether this is in fact the cause of your symptoms.

Related Questions

Unable to open mouth more than two fingerbreaths for root canal so endodontist quit!?! had to see his jr partner.

Why? Why couldn't you open your mouth more than that with the first dentist? Did you open your mouth further for junior? If you did, you had something against the first one, whether you were conscious of it or not. Fear or anger has many faces.
Patience is key. Acute dental infection often causes "trismus"-- limitation in the jaw's range of motion, by inducing spasm in the jaw muscles. It is often helpful if your endodontist administers antibiotic and waits for the more acute manifestations of your infection to resolve, at which time you will be able to more easily open your mouth. This will allow endodontic treatment to proceed more quickly and easily.