2000 Super Air with Apex 5.7... knock sensor threaded hole problem/question

[RandyBrokeSomething]

I'm de-winterizing my 2000 Super Air... it has the fairly uncommon 5.7 Apex motor, but this may be a universal question. The hole that the knock sensor threads into... how "deep" should the threads be in that hole? Is there only a few threads there to catch, and then a sort of flange up in there that the knock sensor bumps into when you tighten it?

I pull the sensor out when I winterize obviously, but this year when I went to put it back in, I couldn't get the threads to catch. Looking at it, there was a solid wall up in there... not good. I poked at it with a dental pick, and it turns out it was a rust/crud blockage that didn't let all the water drain out. I keep the boat in a garage that doesn't get too super cold, so I'm hoping I didn't have any major damage due to the water (it wasn't a ton of water)... but now I'm trying to clean those threads and get the knock sensor back in, and I can't get it. At the very least I have thread problems and need to tap that hole. Is that supposed to be a clean hole of the same diameter all the way in? Mine looks like there's only a small section of threads, and then a smaller diameter flange up in there that the sensor bumps into, but this could also just be rusty crud that just looks like a flange. I don't want to do any damage trying to tap that hole so the sensor will thread back in. Thanks for any advice...

[RandyBrokeSomething]

Following up to clarify... I'm not talking about the wall of the water jacket, I can feel that up there. I think I had a weird rusty wall forming in there that just corked that hole, and it looked like a flange of some sort after I punched a hole in it with a screwdriver.
I did some homework and made progress on this, so following up here: I found out that the knock sensor uses 1/4" -18 NPT threads like a standard pipe thread. I got the appropriate tap, and was able to get it threaded in there and cleaned the threads up enough to re-install the knock sensor. Shockingly everything went pretty smoothly I assume I was just seeing rust up there, and all that crud broke away when I re-tapped the hole. The boat started, and the water that was stuck in there all winter doesn't seem to have done any damage.
I guess from now on when I pull that knock sensor out, I'll make sure to poke something up into that hole and make sure I've cleared any blockage. Scary afternoon discovering that...

[jpwhit]

Yes, you always have to stick something in those holes and clean stuff out when you remove the knock sensor for winterization. Otherwise, all the water won't drain out. I typically use a L shaped end of a small allen wrench.