Hayward C1750 Cartidge Filter.. problem removing top cover

[shorembo]

I seem to be having a problem with unscrewing and screwing down the top of an old Hayward C1750 Cartridge Filter.

I took a look at the bolt sticking up and it had hard build up on the threads. I removed the build up and it seemed to work slightly better.

I am thinking that the same buildup might be on the inside of the locking knob. Is there anyway I can clean the inside threads? (I know how to remove it but I can't figure out a way to clean it well).

And... would this really make a difference? Or is it most likely the threads are stripped and I just have to get a new locking knob?

Thanks for any feedback.

[PoolDoc]

Is there anyway I can clean the inside threads?

Those are probably standard threads. If you've got a machinist or job friend, you might be able to borrow the correct tap to 'chase' the female thread in the nut. If not, you could probably ID the threads by taking the connecting rod out, and comparing to bolts at Lowes, etc.

I'd guess it's one of these two:
Century Drill and Tool 95107 High Carbon Steel Fractional Plug Tap, 3/8-16 NC (http://www.amazon.com/exec/obidos/ASIN/B004UUAM3G/scouscho-20/)
Century Drill and Tool 95109 High Carbon Steel Fractional Plug Tap, 7/16-14 NC (http://www.amazon.com/exec/obidos/ASIN/B004UUAMA4/scouscho-20/)
http://ecx.images-amazon.com/images/I/519GBkhEsiL._SX522_.jpg

Remember, you'll want a "plug tap", like those, and not a threading tap, which would bottom out before you'd cleaned the threads.

Or, you can simply buy a new nut:
Hayward Star-Clear Plus Filter Locking Knob CX1750G (http://www.amazon.com/exec/obidos/ASIN/B003ZTUIL4/scouscho-20/)
http://ecx.images-amazon.com/images/I/31HvkZauxkL.jpg

[shorembo]

Thanks for the response. I ended up getting a 1/2 inch pipe cleaner along with some pool o-ring lube. Between cleaning out the threads and lubing the o-ring well, the top comes off easily now.

[PoolDoc]

Didn't think of that, but a 1/2" wire reamer would have fit. Be careful, though. That wire is MUCH harder than the brass threads.

Glad it worked for you, though.