Opening width of Sanke Kettle

Question..... I'm planning to build my immersion chiller this weekend with 50' of 1/2 OD copper and I currently have a 13 inch wide brew pot. I would like to use this IC for a sanke kettle that I plan on building in the near future. My question is... what is the typical width of the opening made when a sanke top is cut off? I realize the keg itself is roughly 15 inches wide but I was unsure what the cut diameter is.

Also, I'm currently brewing my second Brewer's Best extract kit and they seem to have trouble getting down to the suggested FG. I had plenty of fermentation the first 24hours but then after that it craps out. The wort had plenty of oxygenation and I used two dry yeast packets this time to try to help the situation with no luck. The specific gravity seems to be stuck at 1.018 for the past few days. Any suggestions? Thanks

I cut the top off mine and it was about 12" so you'll want to keep that in mind when coiling your chiller.

Which yeast does your brewer's best kit come with? Also what temp are you at?

For your current brew you may want to put it someplace warmer now that it is almost finished fermenting and try to rouse the yeast by swirling it.

For the top of the keggle, 12 inches is a good size. The copper coils can always be coiled just a bit tighter by hand to make it fit. I must say I really like the counter flow chiller. It would only require 50 ft of hose, and Phils Fittings. 212 degrees going in, 70 degrees going out. And finally, the last few gravity points take the longest. Keep it warm, and if monitor the gravity. If it does not change in a few days, gently swirl the bucket or carboy to redistribute the yeast, while trying your absolute best not to incorporate air into the beer.

12" is about as tight as you can get on the cutout if you're using an angle grinder (larger hole is possible with a plasma cutter). In any case, 12" is reasonable and you can use a corny keg as your template for coiling the chiller.

Thanks everyone for the tips! The extract kit I used came with Munton's yeast. I think their general run of the mill dry packaged yeast. It seems to be hardy but also floculates very easily..... maybe the gentle stirring idea is the best to combate this to help finish the fermentation. Would it be necessary to use yeast nutrient? I have some but I used no adjuncts that I feel would warrant using it. Anyways back to the bucket to take another sample and gravity! Thanks again.