Gemara

(Mishnah): There are stringencies of Kodshim above Terumah. One may be Tovel (in a Mikveh) Kelim inside Kelim for Terumah, but not for Kodshim.

21a (Gemara) Question: Why is this forbidden for Kodesh?

Answer #1 (R. Ila): The inner Keli weighs down on the outer Keli; this is a Chatzitzah.

Question: The Seifa discusses a Chatzitzah. This implies that the Reisha does not!

(Seifa): Terumah is unlike Kodesh. For Kodesh we untie (a garment, to avoid Chatzitzos), dry and immerse, and afterwards tie. For Terumah we (may) tie and immerse.

Answer: The Reisha and Seifa both discuss Chatzitzos. Both are needed: Had the Mishnah taught only the Reisha, one might have thought that we are concerned only for a Chatzitzah that weighs down on a Keli, but one need not untie even for Kodesh;

Had it taught only the Seifa, one might have thought that we are concerned for Chatzitzah only when something is tied, but we are not concerned for what weighs down even for Kodesh, for water lifts Kelim.

Answer #2 (to Question (2) - Rava): Since the Seifa discusses a Chatzitzah, the Reisha does not! Rather, the Reisha is a decree lest one immerse needles or small forks in a Keli in which the opening is narrower than ki'Shfoferes ha'Nod (the circumference of a hollow reed used to draw out liquid from a pouch).

(Mishnah): Two Mikva'os join only if the opening between them is at least ki'Shfoferes ha'Nod, (a circle) in which two fingers can rotate around each other.

Question: Practically, what do Rava and Rav Ila argue about?

Answer: They argue about Kelim immersed in a basket or Gargusni (a large basket used to strain juice that flows from the winepress). R. Ila's concern lest the inner Keli weigh down and be a Chatzitzah applies. Rava's decree lest one immerse in a Keli with a narrow opening does not apply.

Rava's teaching is consistent with what he taught elsewhere.

(Rava): If one filled a basket or Gargusni with Kelim and immersed them, they are Tehorim.

(Rava taught that immersion in a Keli whose opening is narrower than ki'Shfoferes ha'Nod is invalid.) This is only if the outer Keli was Tahor, but if it was Tamei, Migo (since) Tevilah is Metaher the outer Keli, it is also Metaher the Kelim inside;

(Mishnah): If one filled a Keli with Kelim and immersed them, they are Tehorim;

If it was not immersed, the water (inside) is considered connected (to the Mikveh) only if the opening is ki'Shfoferes ha'Nod.

Question: What does it mean 'if it was not immersed'?

Answer: If (the outer Keli) did not need to be immersed, the water is considered connected only if the opening is ki'Shfoferes ha'Nod.

Tana'im argue as Rava and Rav Ila do:

(Beraisa): If one filled a basket or Gargusni with Kelim and immersed them, they are Tehorim for Kodesh or Terumah;

Aba Sha'ul says, they are Tehorim for Terumah, but not for Kodesh.

Rishonim

Rambam (Hilchos Avos ha'Tum'ah 12:1): One may be Tovel Kelim inside Kelim for Terumah, but not for Kodshim. This is a decree lest the opening of the (outer) Keli be narrower than the circumference of a standard pouch (for liquids), in which case the inner Kelim are immersed in a Keli, not in a Mikveh.

Rambam (Hilchos Mikva'os 3:26): If one filled a big Tamei Keli with other Kelim and immersed them all, they are Tehorim, even if the big Keli has a very narrow opening. Since water enters, and the Tevilah helps for the big Keli, it helps for those inside. If one tilted the big Keli on its side and immersed it, or if it was Tahor, Tevilah does not help for the inner Kelim unless the opening is at least ki'Shfoferes ha'Nod. This is for Terumah, but for Kodshim we do not immerse Kelim inside Kelim, not even in a basket or box.

Rambam (6:8): One may not immerse (Kelim) in a chest or box in the sea, unless it has a hole the size of ki'Shfoferes ha'Nod. One may immerse inside a bag or (wicker) basket.

Source (Kesef Mishneh): We learn from Mikva'os 6:5. R. Shimshon explains that since the bag or basket is full of holes and does not hold water, the water inside is Batel to the Mikveh.

Rosh (Bava Kama 7:3): One can be Tovel in a Keli if it is connected to the Mikveh, like the Gemara in Chagigah (21b) says, if the opening is ki'Shfoferes ha'Nod.

Poskim

Shulchan Aruch (201:9): If one filled a big Tamei Keli with other Kelim and immersed them all, they are Tehorim, even if the big Keli has a very narrow opening. Since water enters, and the Tevilah helps for the big Keli, it helps for those inside.

Taz (17): Just like Tum'ah entered through the opening, Taharah enters through it.

Shach (28): When the big Keli is in the Mikveh, all the water inside is like the Mikveh. If it merely touches from the side, only what is even with the Mikveh is considered like the Mikveh.

Shulchan Aruch (ibid.): If one tilted the big Keli on its side and immersed it, or if it was Tahor, Tevilah does not help for the inner Kelim unless the opening is at least ki'Shfoferes ha'Nod.

Source (Beis Yosef (DH u'Mah she'Chosav Rabeinu): The Tosefta (Mikva'os 5:1) says that if he tilted the big Keli on its side, the Kelim inside remain Temei'im. R. Shimshon (Mikva'os 6:5) says that we are not concerned for the size of the opening only if the big Keli is upright. We must say that the big Keli was Tamei.

Question (Chazon Ish Mikva'os Tinyana 3:3): When the outer Keli is Tamei, why is there any need for ki'Shfoferes ha'Nod? Perhaps the Rambam explains that when the outer Keli is on its side it is not entirely submerged, but he agrees that if it was immersed any opening suffices. One may be lenient.

Beis Yosef (DH v'Da): The above Tosefta says that for a Mayan (spring), any amount suffices. R. Shimshon (ibid.) explains that if one did Hashakah to a Mayan, we are never concerned for the size of the opening. The Rambam and Rosh do not distinguish between a Mikveh and Mayan. It seems that they reject the Tosefta because it argues with the Mishnah. The Mishnah requires ki'Shfoferes ha'Nod regarding a sea, and R. Yosi considers a sea to be like a Mayan. If he did not require a Shi'ur regarding a sea, the Mishnah would have taught this.

Rema: One may immerse Kelim in a basket or bag. Since it does not hold water, it is better than a hole the size of ki'Shfoferes ha'Nod.