The NGP 320S is the silicone seal version of the NGP 320 Series automatic door bottom, specifically designed for hollow metal doors that have the standard 1-5/8 inch by 3/4 inch mortise channel cut into the bottom of the door from the factory.

The 320S drops into that channel. Four #4 by 5/8 inch sheet metal screws secure it. Maximum drop when the seal engages is 1/2 inch. When the door opens and the plunger releases from the door frame, the seal lifts and clears the floor. For maximum sealing performance, the 320S is used together with a threshold on the floor below. If the installation has no threshold and the floor surface is carpet, NGP recommends a nylon brush seal instead of the automatic door bottom in that configuration. For all NGP gasketing and builders hardware, browse our builders hardware catalog.

Why Silicone Over Neoprene on Fire-Rated Hollow Metal Doors

The NGP 320 Series comes in three seal materials: 320N (NGP-TPV thermoplastic vulcanizate), 320S (silicone, this product), and 320V (vinyl). Silicone rubber outperforms neoprene and vinyl at high temperatures, which matters on fire-rated door assemblies where the seal is expected to maintain integrity under positive pressure fire conditions. The silicone seal (black) stays flexible across a wider temperature range and resists the compression set that causes neoprene seals to lose their sealing profile over time under sustained load.

The 320S is specifically rated for hollow metal fire doors. Fire ratings on the 320 Series apply to metal doors only. If the application involves a wood fire door, confirm the correct model from NGP's wood door automatic door bottom lineup.

Ordering Length and Field Sizing

The NGP 320S is furnished 1/2 inch shorter than the ordered length to fit within the net door width. A 36-inch order ships as 35-1/2 inches. This is standard across the NGP automatic door bottom line.

For 36-inch and longer units, field cutting up to 4 inches shorter is allowed. Units under 36 inches should not be field cut; order the exact net length required. If the installed channel in the door is deeper than the standard 3/4 inch, a furring strip can be used to lower the automatic door bottom to the correct position within the deeper channel.

NGP 320S Fire and Smoke Certifications

The 320S carries the full certification stack required for fire-rated hollow metal door assemblies:

UL to ANSI/UL10B and CAN/ULC-S104: Hollow metal fire doors up to 3 hours, wood fire doors up to 90 minutes. Positive Pressure to UL 10C: Hollow metal fire doors up to 3 hours, wood fire doors up to 90 minutes, compliant with IBC, NFPA 80, and NFPA 252. Smoke and Draft Control Category H to UL 10C and UL 1784: For use on "S" label positive pressure hollow metal fire doors up to 3 hours, IBC and NFPA 105 compliant. Air Infiltration: Tested to ASTM E283. ANSI/BHMA A156.22 Grade 1: Certified to 1,000,000 cycle performance test.

NGP 320S Specifications

• SKU: 320S

• Also known as: NGP 320S, 320S-36, ngp 320 silicone, ngp auto door bottom silicone, ngp door bottom hollow metal 320s

• Series: NGP 320 Series

• Type: Mortise automatic door bottom for hollow metal doors

• Seal: Silicone (black), Aluminum alloy 6063 T5 temper carrier

• Channel: Fits 1-5/8" x 3/4" standard hollow metal door channel

• Length ordered: 36" (furnished 35-1/2")

• Available lengths: 24" to 48"

• Max drop: 1/2"

• Field cut: Up to 4" on 36" and longer units; order net length on units under 36"

• Furring strip: Available to lower unit in deeper channels

• Fasteners: 4 x #4 x 5/8" SMS furnished; end caps furnished standard

• Fire ratings: Metal doors only

• UL/Fire: UL 10B (HM to 3 hrs; wood to 90 min); UL 10C positive pressure (HM to 3 hrs)

• Smoke: Category H, UL 1784, NFPA 105, "S" label HM doors up to 3 hrs

• BHMA: A156.22 Grade 1, 1,000,000 cycle

• ANSI/IBC: NFPA 80, NFPA 252, IBC compliant

• Resources: NGP Gasketing Catalog PDF in Resources section

320S vs 320N vs 320NA: Series Comparison

320N uses NGP-TPV thermoplastic vulcanizate seal in mill aluminum finish. 320S (this product) uses silicone seal in mill aluminum finish. 320NA is the 320N in clear anodized aluminum finish. All three fit the same 1-5/8" x 3/4" channel and share the same installation specs. Specify 320S when the fire door application requires silicone seal performance; specify 320NA when the door finish matches clear anodize and neoprene is acceptable for the temperature rating.