Should these be interchangeable between the two models? I sent three reels, 2 C3s and a C4 to Dads Ole Tackle for some parts and they came bach with different spool caps. They look like they are for the brake side of a 7000, with a flat checkered top surface. The guy at Dads told me he changed them out because he wasn't able to get the reels spooling as free as he thought they should be. Anyway, I was unable to get any drag at all on the spool so I put the original caps(they were returned) back on the C3s, but the cap seems to not reach the threads on the C4 and I'm unable to install it. Both model reels have a rubber o-ring below the threads, so that can't be what's keeping it from reaching the threads and screwing on. What about it, Abu guys?

The c3 is domed and the c4 is flat. You can use either with either reel. You just have to add shims and the clip or remove the clip. If your using the flat on the c3 you might have to remove the clip and or shims.

Looking at the schematics, they would both have to be the same because the part numbers are the same. I just don't know why I can't put one back on a C4.

Take the clip out and put the clip on the axle then screw it down or u can leave the clip off. The axle is not going in the clip allowing u to screw it down.

Here a pic with the clip off and attached to axle then screw the cap on