Says the Rambam [Keilim 9-1]:
כָּל כְּלֵי מַתָּכוֹת שֶׁיֵּשׁ לָהֶן שֵׁם בִּפְנֵי עַצְמָן מְקַבְּלִין טֻמְאָה חוּץ מִן הַדֶּלֶת וְהַנֶּגֶר וְהַמַּנְעוּל וְהַפּוֹתָת שֶׁתַּחַת הַצִּיר וְהַצִּיר וְהַקּוֹרָה וְהַצִּנּוֹר מִפְּנֵי שֶׁאֵלּוּ עֲשׂוּיִין לְקַרְקַע אוֹ לְשַׁמֵּשׁ אֶת הָעֵץ אֵינָן מְקַבְּלִין טֻמְאָה וַאֲפִלּוּ קֹדֶם שֶׁיִּקָּבְעוּ. וְכָל כְּלִי מַתָּכוֹת שֶׁיֵּשׁ לוֹ שֵׁם לְוַוי אֵינוֹ מִתְטַמֵּא בִּפְנֵי עַצְמוֹ מִפְּנֵי שֶׁהוּא כְּמִקְצָת כְּלִי. כֵּיצַד. עַקְרָב שֶׁל פְּרוּמְבְּיָא טָמֵא וּלְחָיַיִם שֶׁלָּהּ שֶׁעַל לְחָיֵי הַבְּהֵמָה מִכָּאן וּמִכָּאן טְהוֹרִין.
All metal keilim that have independent names are susceptible to impurity except a door, a bolt, a lock, a holder for a door hinge a hinge, a beam, and a drainpipe. The latter are not susceptible to impurity, because they serve the earth or wood. This applies even before they are affixed to a building or to wood.
Any metal k'li that has an auxiliary name does not contract impurity independently because it is only part of a k'li. What is implied? The "scorpion" of a muzzle is impure and the iron plates placed on the cheeks of the animal on both sides are pure, because they do not have an independent name. When all these articles are connected to the reins, everything is susceptible to impurity.
Not clear: Why is the עקרב של פרומביא Tamei when it seems to have a שם לווי?
