I'm replacing an ancient Moen shower cartridge at my girlfriend's house and I'm stuck. I got the old one out surprisingly easily and the new one (#1225) slid right in. However, I can't manage to get the flat brass locking pin (kind with two legs) to slide in to lock the cartridge in place. It feels like I'm getting solid resistance on both sides despite having lined the notches up as carefully as possible and making sure the new cartridge is seated all the way in. It almost feels as if the new cartridge is a mm or so longer than the old one.

I'm considering using a file or hacksaw blade to widen the slot for the locking pin a little. Any other ideas?

Hello… the issue is with my first floor Moen tub/shower single H/C pull handle that was leaking profusely down into the tub as well as behind the wall into the basement. So, I purchased a new "1225" cartridge. I thought it was going to be fairly easy, but it turned out that the cartridge wasn't budging. So, I went back to the store and purchased a "puller" to remove the stubborn cartridge, which surprisingly worked. I then installed the new one and the water stopped pouring out through the single handle. But, after turning the main house water valve back on, a steady water stream started from the faucet below, that I'm now unable to stop. The one thing that I wasn't able to reinstall, was the "retainer clip", because it broke when I forgot about it when pulling out the cartridge. Is that what's causing the "new" leak from the faucet? If not, then I’m at a loss for getting the water to stop. I hope the details helped and didn't confuse anyone. I just didn't want to overlook anything. Thanks in advance for any assistance.