{"product_id":"royal-enfield-521237-air-filter-assembly-thunderbird-350","title":"Royal Enfield 521237 – Air Filter Assembly for Thunderbird 350 (UCE – TBTS BS3 \u0026 BS4\/ABS “NTB 350”) – Genuine OEM","description":"\u003cp data-start=\"246\" data-end=\"814\"\u003eRestore factory breathing and protect your engine with the genuine \u003cstrong data-start=\"313\" data-end=\"359\"\u003eRoyal Enfield 521237 – Air Filter Assembly\u003c\/strong\u003e. This is the \u003cstrong data-start=\"373\" data-end=\"405\"\u003ecomplete OE air cleaner unit\u003c\/strong\u003e for the \u003cstrong data-start=\"414\" data-end=\"437\"\u003eThunderbird 350 UCE\u003c\/strong\u003e platform—listed by dealers specifically for \u003cstrong data-start=\"482\" data-end=\"496\"\u003eTBTS (BS3)\u003c\/strong\u003e and \u003cstrong data-start=\"501\" data-end=\"528\"\u003eBS4\/ABS Thunderbird 350\u003c\/strong\u003e (often shown as \u003cstrong data-start=\"545\" data-end=\"558\"\u003e“NTB 350”\u003c\/strong\u003e in parts listings). Because it’s OEM, the housing, sealing lip, mounting points and filter media match the original, ensuring \u003cstrong data-start=\"685\" data-end=\"707\"\u003edust-tight sealing\u003c\/strong\u003e, correct intake noise, and \u003cstrong data-start=\"735\" data-end=\"752\"\u003eplug-and-play\u003c\/strong\u003e fit on the stock airbox bracket—no drilling or adjustments.\u003c\/p\u003e\n\u003cp data-start=\"816\" data-end=\"1368\"\u003eIf your bike shows \u003cstrong data-start=\"835\" data-end=\"894\"\u003ereduced mileage, dull throttle response, or intake howl\u003c\/strong\u003e after a fall or deep clean, a deformed\/dirty filter housing can be the culprit. Swapping to the \u003cstrong data-start=\"991\" data-end=\"1017\"\u003e521237 Air Filter Assy\u003c\/strong\u003e restores the designed flow and filtration level. We’ve verified the part name and application across multiple sources that list \u003cstrong data-start=\"1146\" data-end=\"1176\"\u003e“AIR FILTER ASSY – 521237”\u003c\/strong\u003e for \u003cstrong data-start=\"1181\" data-end=\"1217\"\u003eThunderbird 350 (TBTS \/ NTB 350)\u003c\/strong\u003e and as a Royal Enfield genuine part. Only the \u003cstrong data-start=\"1264\" data-end=\"1284\"\u003econfirmed models\u003c\/strong\u003e below are included to keep returns near zero.\u003cspan class=\"\" data-state=\"closed\"\u003e\u003cspan class=\"ms-1 inline-flex max-w-full items-center relative top-[-0.094rem] animate-[show_150ms_ease-in]\" data-testid=\"webpage-citation-pill\"\u003e\u003c\/span\u003e\u003c\/span\u003e\u003c\/p\u003e\n\u003ch1 data-start=\"1370\" data-end=\"1384\"\u003eKey Features\u003c\/h1\u003e\n\u003cul data-start=\"1385\" data-end=\"1864\"\u003e\n\u003cli data-start=\"1385\" data-end=\"1490\"\u003e\n\u003cp data-start=\"1387\" data-end=\"1490\"\u003e\u003cstrong data-start=\"1387\" data-end=\"1402\"\u003eGenuine OEM\u003c\/strong\u003e Royal Enfield \u003cstrong data-start=\"1417\" data-end=\"1449\"\u003eAir Filter Assembly – 521237\u003c\/strong\u003e. \u003cspan class=\"\" data-state=\"closed\"\u003e\u003cspan class=\"ms-1 inline-flex max-w-full items-center relative top-[-0.094rem] animate-[show_150ms_ease-in]\" data-testid=\"webpage-citation-pill\"\u003e\u003c\/span\u003e\u003c\/span\u003e\u003c\/p\u003e\n\u003c\/li\u003e\n\u003cli data-start=\"1491\" data-end=\"1608\"\u003e\n\u003cp data-start=\"1493\" data-end=\"1608\"\u003eDirect replacement for \u003cstrong data-start=\"1516\" data-end=\"1539\"\u003eThunderbird 350 UCE\u003c\/strong\u003e air cleaner unit (complete). \u003cspan class=\"\" data-state=\"closed\"\u003e\u003cspan class=\"ms-1 inline-flex max-w-full items-center relative top-[-0.094rem] animate-[show_150ms_ease-in]\" data-testid=\"webpage-citation-pill\"\u003e\u003c\/span\u003e\u003c\/span\u003e\u003c\/p\u003e\n\u003c\/li\u003e\n\u003cli data-start=\"1609\" data-end=\"1675\"\u003e\n\u003cp data-start=\"1611\" data-end=\"1675\"\u003eCorrect OE \u003cstrong data-start=\"1622\" data-end=\"1653\"\u003esealing \u0026amp; mounting geometry\u003c\/strong\u003e for dust-tight fit.\u003c\/p\u003e\n\u003c\/li\u003e\n\u003cli data-start=\"1676\" data-end=\"1746\"\u003e\n\u003cp data-start=\"1678\" data-end=\"1746\"\u003eFactory-spec filtration media for \u003cstrong data-start=\"1712\" data-end=\"1743\"\u003eengine protection \u0026amp; mileage\u003c\/strong\u003e.\u003c\/p\u003e\n\u003c\/li\u003e\n\u003cli data-start=\"1747\" data-end=\"1814\"\u003e\n\u003cp data-start=\"1749\" data-end=\"1814\"\u003eRestores stock \u003cstrong data-start=\"1764\" data-end=\"1780\"\u003eintake sound\u003c\/strong\u003e and airflow—no tuning required.\u003c\/p\u003e\n\u003c\/li\u003e\n\u003cli data-start=\"1815\" data-end=\"1864\"\u003e\n\u003cp data-start=\"1817\" data-end=\"1864\"\u003eShips in OE packaging; authenticity guaranteed.\u003c\/p\u003e\n\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003ch1 data-start=\"1866\" data-end=\"1904\"\u003eCompatibility Table \u003c\/h1\u003e\n\u003cdiv class=\"_tableContainer_1rjym_1\"\u003e\n\u003cdiv class=\"group _tableWrapper_1rjym_13 flex w-fit flex-col-reverse\" tabindex=\"-1\"\u003e\n\u003ctable data-start=\"1906\" data-end=\"2293\" class=\"w-fit min-w-(--thread-content-width)\"\u003e\n\u003cthead data-start=\"1906\" data-end=\"1962\"\u003e\n\u003ctr data-start=\"1906\" data-end=\"1962\"\u003e\n\u003cth data-start=\"1906\" data-end=\"1914\" data-col-size=\"md\"\u003eModel\u003c\/th\u003e\n\u003cth data-start=\"1914\" data-end=\"1934\" data-col-size=\"sm\"\u003eEngine \/ Platform\u003c\/th\u003e\n\u003cth data-start=\"1934\" data-end=\"1953\" data-col-size=\"sm\"\u003eBS Stage \/ Years\u003c\/th\u003e\n\u003cth data-start=\"1953\" data-end=\"1962\" data-col-size=\"md\"\u003eNotes\u003c\/th\u003e\n\u003c\/tr\u003e\n\u003c\/thead\u003e\n\u003ctbody data-start=\"1981\" data-end=\"2293\"\u003e\n\u003ctr data-start=\"1981\" data-end=\"2114\"\u003e\n\u003ctd data-start=\"1981\" data-end=\"2010\" data-col-size=\"md\"\u003e\u003cstrong data-start=\"1983\" data-end=\"2009\"\u003eThunderbird 350 (TBTS)\u003c\/strong\u003e\u003c\/td\u003e\n\u003ctd data-start=\"2010\" data-end=\"2022\" data-col-size=\"sm\"\u003e346cc UCE\u003c\/td\u003e\n\u003ctd data-start=\"2022\" data-end=\"2045\" data-col-size=\"sm\"\u003e\n\u003cstrong data-start=\"2024\" data-end=\"2031\"\u003eBS3\u003c\/strong\u003e (≈2009–2012)\u003c\/td\u003e\n\u003ctd data-start=\"2045\" data-end=\"2114\" data-col-size=\"md\"\u003eListed explicitly for TBTS. \u003cspan class=\"\" data-state=\"closed\"\u003e\u003cspan class=\"ms-1 inline-flex max-w-full items-center relative top-[-0.094rem] animate-[show_150ms_ease-in]\" data-testid=\"webpage-citation-pill\"\u003e\u003c\/span\u003e\u003c\/span\u003e\n\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr data-start=\"2115\" data-end=\"2293\"\u003e\n\u003ctd data-start=\"2115\" data-end=\"2173\" data-col-size=\"md\"\u003e\n\u003cstrong data-start=\"2117\" data-end=\"2146\"\u003eThunderbird 350 (BS4\/ABS)\u003c\/strong\u003e \u003cem data-start=\"2147\" data-end=\"2172\"\u003e(“NTB 350” in listings)\u003c\/em\u003e\n\u003c\/td\u003e\n\u003ctd data-start=\"2173\" data-end=\"2185\" data-col-size=\"sm\"\u003e346cc UCE\u003c\/td\u003e\n\u003ctd data-start=\"2185\" data-end=\"2208\" data-col-size=\"sm\"\u003e\n\u003cstrong data-start=\"2187\" data-end=\"2194\"\u003eBS4\u003c\/strong\u003e (≈2017–2020)\u003c\/td\u003e\n\u003ctd data-start=\"2208\" data-end=\"2293\" data-col-size=\"md\"\u003eDealers show 521237 for BS4 ABS T’bird 350. \u003cspan class=\"\" data-state=\"closed\"\u003e\u003cspan class=\"ms-1 inline-flex max-w-full items-center relative top-[-0.094rem] animate-[show_150ms_ease-in]\" data-testid=\"webpage-citation-pill\"\u003e\u003c\/span\u003e\u003c\/span\u003e\n\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003c\/tbody\u003e\n\u003c\/table\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003cblockquote data-start=\"2295\" data-end=\"2510\"\u003e\n\u003cp data-start=\"2297\" data-end=\"2510\"\u003e\u003cstrong data-start=\"2297\" data-end=\"2330\"\u003eCHECK PART NUMBER BEFORE PURCHASE\u003c\/strong\u003e\u003c\/p\u003e\n\u003c\/blockquote\u003e\n\u003ch1 data-start=\"2512\" data-end=\"2522\"\u003eVariants\u003c\/h1\u003e\n\u003cul data-start=\"2523\" data-end=\"2661\"\u003e\n\u003cli data-start=\"2523\" data-end=\"2661\"\u003e\n\u003cp data-start=\"2525\" data-end=\"2661\"\u003eN\/A for LH\/RH. If you need only the \u003cstrong data-start=\"2561\" data-end=\"2579\"\u003efilter element\u003c\/strong\u003e, order the element PN specific to your variant; this listing is the \u003cstrong data-start=\"2648\" data-end=\"2660\"\u003eassembly\u003c\/strong\u003e.\u003c\/p\u003e\n\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003ch1 data-start=\"2663\" data-end=\"2669\"\u003eFAQs\u003c\/h1\u003e\n\u003cp data-start=\"2670\" data-end=\"2888\"\u003e\u003cstrong data-start=\"2670\" data-end=\"2736\"\u003eQ1. Is this the full air filter box or just the paper element?\u003c\/strong\u003e\u003cbr data-start=\"2736\" data-end=\"2739\"\u003eThis listing is the \u003cstrong data-start=\"2759\" data-end=\"2782\"\u003eAir Filter Assembly\u003c\/strong\u003e (OE housing with filter as supplied by RE for the Thunderbird 350).\u003cspan class=\"\" data-state=\"closed\"\u003e\u003cspan class=\"ms-1 inline-flex max-w-full items-center relative top-[-0.094rem] animate-[show_150ms_ease-in]\" data-testid=\"webpage-citation-pill\"\u003e\u003c\/span\u003e\u003c\/span\u003e\u003c\/p\u003e\n\u003cp data-start=\"2890\" data-end=\"3073\"\u003e\u003cstrong data-start=\"2890\" data-end=\"2943\"\u003eQ2. Will 521237 fit my Thunderbird 350 ABS (BS4)?\u003c\/strong\u003e\u003cbr data-start=\"2943\" data-end=\"2946\"\u003eYes—dealers list \u003cstrong data-start=\"2963\" data-end=\"2973\"\u003e521237\u003c\/strong\u003e for \u003cstrong data-start=\"2978\" data-end=\"3005\"\u003eThunderbird 350 ABS\/BS4\u003c\/strong\u003e (often labelled \u003cstrong data-start=\"3022\" data-end=\"3033\"\u003eNTB 350\u003c\/strong\u003e). =\u003c\/p\u003e\n\u003cp data-start=\"3075\" data-end=\"3230\"\u003e\u003cstrong data-start=\"3075\" data-end=\"3121\"\u003eQ3. Does it need tuning after replacement?\u003c\/strong\u003e\u003cbr data-start=\"3121\" data-end=\"3124\"\u003eNo. It’s OEM; install and ride. Check idle after replacement and ensure the snorkel and clamps are seated.\u003c\/p\u003e\n\u003cp data-start=\"3232\" data-end=\"3355\"\u003e\u003cstrong data-start=\"3232\" data-end=\"3270\"\u003eQ4. How often should I replace it?\u003c\/strong\u003e\u003cbr data-start=\"3270\" data-end=\"3273\"\u003eInspect every service; replace per your RE schedule or sooner in dusty conditions.\u003c\/p\u003e","brand":"UNDER THE HOOD","offers":[{"title":"Default Title","offer_id":50624337772839,"sku":null,"price":598.0,"currency_code":"INR","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0878\/7447\/0183\/files\/OE_air_cleaner_housing_521237_for_TBTS_BS4_ABS.png?v=1762283457","url":"https:\/\/underthehood.co.in\/products\/royal-enfield-521237-air-filter-assembly-thunderbird-350","provider":"UNDER THE HOOD","version":"1.0","type":"link"}