Fireproof Clamp Manufacturing for Aerospace: MS21919 Guide

Fireproof clamps are essential hardware in aircraft fuel, hydraulic, pneumatic, and electrical systems. These clamps secure fluid lines cables, and wiring bundles in engine compartments, APU bays, and wheel wells where exposure to fire is a credible risk. Manufactured primarily from corrosion-resistant stainless steel (AISI 301, 304, or 321) with fire-resistant rubber or silicone cushion inserts, fireproof clamps must survive 1100°C flame exposure for 15 minutes without losing their mechanical retaining function. This guide covers the full manufacturing process — from material selection and stamping through fire testing and passivation — for aerospace fireproof clamps meeting MS21919, MS27291, and similar specification standards.

Material Selection and Specifications

Fireproof clamps for aerospace applications are governed by military and industrial standards that define material grades, dimensions, and fire resistance requirements:

Specification	Material	Typical Gauge (mm)	Fire Rating	Cushion Material	Temperature Range
MS21919 (standard)	AISI 301 / 304 SS	0.5 – 1.0	1100°C / 15 min	Silicone rubber	−55°C to +200°C
MS27291 (heavy duty)	AISI 321 SS (stabilized)	0.8 – 1.5	1100°C / 15 min	Fire-retardant silicone	−55°C to +250°C
NAS1747	17-7PH CRES	0.4 – 0.6	1090°C / 15 min	PTFE / Viton	−65°C to +260°C
SAE AS2981	CRES 301 (1/2 hard)	0.6 – 1.2	1100°C / 15 min	Silicone (fireproof)	−55°C to +200°C

AISI 301 stainless steel in the 1/2-hard condition (HRC 25 – 35) is the most common material for MS21919 fireproof clamps. Its combination of spring-like elasticity (for clamp retention force) and formability (for stamping complex clamp geometries) makes it ideal. The 0.5 – 1.0 mm gauge provides sufficient fire resistance while allowing the clamp body to be formed with standard tooling.

Stamping and Forming Processes

Fireproof clamps are produced in a multi-stage stamping and forming sequence. Depending on volume, manufacturers choose between progressive die stamping (high volume, >50,000 parts/year) or CNC wire bending and forming (low to medium volume):

Progressive Die Stamping. For high volumes, a 6 – 12 station progressive die produces the complete clamp body in a single press stroke per cycle. Stations sequentially: blank, pierce mounting holes, form cushion-retention tabs, bend the clamp loop to its final radius, and cut off from the carrier strip. Press capacity of 30 – 80 tons is adequate for gauges up to 1.2 mm. Production rates of 30 – 60 parts per minute are typical. CNC Wire and Strip Forming. For prototype runs, low-volume production, or large clamp sizes beyond die capability, CNC benders form the clamp profile from pre-cut strip stock. A 5 – 7 axis CNC wire former bends the strip progressively, achieving ±0.1 mm dimensional accuracy on the clamp radius. Cycle time per part is 20 – 60 seconds, significantly slower than die stamping but with zero tooling investment. Cushion Insert Molding. After the metal clamp body is formed, the fire-resistant cushion is injection-molded or compression-molded directly onto the inner diameter. Silicone rubber (with 30 – 50% ceramic fiber filler for fire resistance) is injected at 150 – 200°C into a mold cavity that surrounds the formed clamp body. The rubber bonds mechanically to the clamp's tab-perforated inner surface, creating a permanent assembly. Cycle time per shot is 60 – 120 seconds in a multi-cavity mold producing 4 – 16 parts per cycle.

Fire Testing and Certification

Fire testing is the definitive qualification for fireproof clamps. The standard test protocol follows FAR 25.853, AC 20-135, or the applicable customer specification:

Test Parameter	Requirement	Typical Result (CRES 301 / 0.6 mm)	Test Method
Flame temperature	1100°C ± 50°C	1100°C measured at burner face	2-probe type R thermocouple
Exposure duration	15 minutes	Pass — clamp intact	Actual flame exposure
Post-test function	Clamp retains conduit	No release under 10 N axial load	Load test after cooling
Cushion burn-off	Max 30% weight loss	18 – 22% weight loss (silicone)	Weight before / after
Smoke generation	Dₛ < 200 (4 min)	Dₛ = 85 – 120 (4 min)	NBS smoke chamber

During the fire test, the silicone cushion burns and chars, but the stainless steel clamp body must remain structurally sound. Acceptable criteria: the clamp may distort but must not release the contained tube or cable bundle. After cooling, a 10 N axial load is applied — the clamp must continue to retain the line. Type certification of a new clamp design requires testing three samples; if any fails, the design is rejected and must be redesigned.

Dimensional Range and Standard Sizes

Fireproof clamps are produced in a wide range of diameters to accommodate every tube and cable size in an aircraft:

MS21919 Dash Number	Nominal Diameter (mm)	Tube OD Range (mm)	Band Width (mm)	Bolt Hole Spacing (mm)	Max Load Rating (N)
−2	6.4 (1/4")	4.0 – 7.0	9.5	20.6	45
−4	12.7 (1/2")	9.5 – 13.5	12.7	25.4	90
−6	19.1 (3/4")	15.0 – 20.0	14.3	28.6	135
−8	25.4 (1")	21.0 – 26.5	15.9	31.8	180
−12	38.1 (1.5")	34.0 – 39.5	19.1	38.1	225
−20	63.5 (2.5")	58.0 – 65.0	25.4	50.8	300

The MS21919 standard encompasses over 1,000 distinct dash numbers covering tube diameters from 3.2 mm (1/8 inch) to 127 mm (5 inches). Each dash number specifies the clamp inner diameter, band width, mounting hole pattern, and cushion material. Manufacturers maintain tooling libraries of 200 – 500 active die sets to cover the most common sizes.

Passivation and Surface Treatment

Stainless steel fireproof clamps require passivation per ASTM A967 or AMS-QQ-P-35 to restore the chromium oxide protective layer after forming. The passivation process is critical because stamping and forming operations can embed iron particles or damage the natural oxide layer:

Passivation Procedure. Parts are first cleaned in an alkaline detergent (60 – 70°C for 10 – 15 minutes) to remove forming lubricants. The passivation bath is nitric acid (20 – 50% by volume at 20 – 50°C for 20 – 40 minutes or 65 – 85°C for 10 – 20 minutes). A copper sulfate test (ASTM A967 Test Method C) verifies that all free iron has been removed — no copper precipitate should appear on the clamp surface. Alternative Treatments. For clamps used in extreme corrosion environments (engine nacelles exposed to de-icing fluids), electropolishing is sometimes specified. This removes 0.005 – 0.015 mm of surface material, producing an ultraclean surface with Ra 0.2 – 0.4 µm finish. Electropolishing adds 15 – 25% to the unit cost but provides superior corrosion resistance.

Quality Control and Inspection

Quality assurance for fireproof clamps follows AS9100, with specific inspection criteria per the applicable procurement specification (MS21919, SAE AS2981, etc.):

Dimensional Inspection. Key dimensions verified per the specification: inner diameter after cushion molding (±0.25 mm), band width (±0.20 mm), mounting hole diameter (±0.05 mm), hole center distance (±0.15 mm), and clamp gap (the opening between clamp ends, ±0.25 mm). An optical comparator or vision measurement system is used for 100% inspection of critical safety-of-flight clamps. Mechanical Testing. Clamp retention force is verified by installing the clamp on a mandrel of specified diameter and applying a calibrated axial pull force. The clamp must not release or permanently deform beyond 5% of its original diameter at the specified test load. A spring-back test ensures the clamp returns to within 95% of its original inner diameter after opening to twice its nominal diameter — essential for reusability during maintenance. Cushion Bond Integrity. The bond between the silicone cushion and the stainless steel body is verified by a 90° peel test per ASTM D429. Minimum peel strength is 5 N per 10 mm of bond width. A 100% visual inspection checks for gaps, voids, or delamination between cushion and metal.

Conclusion

Fireproof clamp manufacturing for aerospace applications demands precise stamping and forming of thin-gauge stainless steel, reliable injection molding of fire-resistant silicone cushions, and rigorous fire testing to FAR 25.853 standards. The MS21919 specification library with over 1,000 dash numbers requires manufacturers to maintain extensive tooling inventories and flexible production capability. Passivation per ASTM A967 ensures long-term corrosion resistance, while dimensional and mechanical testing per AS9100 guarantees clamp performance in the demanding aircraft environment. Engineers selecting fireproof clamps should verify that the manufacturer holds current fire test certification, maintains full MS21919 tooling coverage for the required dash numbers, and operates under Nadcap-accredited passivation and testing processes.