I have a single handle Delta bath faucet valve whose cartridge needs to be replaced because it is dripping. It is about 4 years old or so. I can find no part number anywhere on it.

I got all the parts removed, down to removing the brass collar that holds the cartridge in place. NO GO! I put a big monkey wrench on it - nothing. I soaked it with silicon lube - nope. I then tried white vinegar, in case water deposits were the culprit -- NOPE! I thought I was going to rip the plumbing out of the wall.

I'm stuck. No more ideas. I hate to call an expensive plumber because I can't get this thing off. What can I do?

You made me laugh . Sucks to be a plummer sometimes. After the brass collar was of did you see a U clip that actually holds the cartridge in. some designs have set screws This clip would slip straight up or down which holds the cartiage in if not it would blow out under pressure and hit you in the belly cartiage and handle The clip is perpendicular to the cartiage.
Handle off
Collar off.
Clip off. Even then its wow at times That's why the make pullers . I try pump pliers and ten shock the pump pliers to get a start.
When you put the next one in get plumber's grease and that help stops these problems and down the road if it get a bit stiff re grease. Many of broken single tub and shower faucets break because thecartiage needs re greased and all other cartridge faucets
Signed 21 boat

The delta cap is thin brass, a good grip causes it to be out of round and not turn. A strap wrench remedies this problem and you will find the cap nut is actually not very tight.

the delta cap is thin brass, a good grip causes it to be out of round and not turn. a strap wrench remedies this problem and you will find the cap nut is actually not very tight.

Thanks, doug238. I had tried twisting it by hand and when it didn't budge, I brought in the big guns!

TOO big, apparently, for when I tried to use the shower this morning, when I hit the spout valve to send water through the shower head, instead, the water went into the wall and down into my basement. Looks like I'll be calling that plumber after all.

Appreciate the help.