A Hi-Lok is a two-piece aerospace fastener made of a threaded pin and a self-locking collar. Howmet, which makes the system, describes it as a two-part fastener whose collar has a regulated break-off groove: the pin is held still by the installation tool while the collar is turned, and at the designed torque the hex portion of the collar shears off [2]. That built-in break-off gives every fastener a consistent preload.
Because the collar shears at a set torque, Howmet notes the fastener can't be over-torqued, which eliminates the need for torque inspection [2]. LISI Aerospace, another Hi-Lok licensee, adds that the pins install from one side, by one person, even in restricted areas [3].
WHAT MONROE STOCKS
Monroe stocks alloy steel HL18PB through HL21PB pins, plus aluminum, stainless, A-286, and titanium collars. Browse Hi-Lok head pins, or find matching collars in Hi-Lok and collars.
For a printable pin and collar cross-reference, download Monroe's Hi-Lok & Collar Quick Reference Guide (PDF).
HI-LOK PIN TYPES
Pin part numbers tell you the material and head style. Based on LISI's published pin table, the alloy steel pins Monroe stocks break down like this [3]:
HL18 - Alloy steel, protruding head, shear.
HL19 - Alloy steel, 100° flush head, shear.
HL20 - Alloy steel, protruding head, tension.
HL21 - Alloy steel, 100° flush head (MS24694 style), tension.
LISI lists all four with 160 ksi tensile and 95 ksi shear strength [3]. The HL18 standard sheet specifies alloy steel and 95,000 psi shear [5]. The same table lists HL10 as a 6Al-4V titanium protruding shear pin [3].
Oversize pins exist for repairs. For example, LISI lists HL62 as the first-oversize version of HL18 and HL218 as the second oversize [3].
HOW TO READ A HI-LOK PART NUMBER
The HL18 standard sheet uses HL18PB8-8 as its example [5]:
HL18 - Basic pin part number.
PB - Finish code. On HL18, PB means Type II cadmium plate with Hi-Kote 2 solid film lubricant [5].
First dash (8) - Nominal diameter in 1/32nds of an inch. 8/32 = 1/4 in. [5]
Second dash (8) - Maximum grip length in 1/16ths of an inch. 8/16 = 1/2 in. [5]
Finish codes vary by pin standard, so check the sheet for the exact pin you're buying.
MATCHING COLLARS TO PINS
A collar must match the pin's diameter, shank size (nominal or oversize), and load type. LISI's collar table groups them like this [4]:
HL70 - 2024-T6 aluminum, shear, nominal shank. HL79 and HL84 are the first and second oversize versions.
HL86 - 300 series stainless steel, tension, nominal shank. HL87 and HL93 are the first and second oversize versions.
HL1087 - 300 series stainless steel, tension.
HL94 - 300 series stainless steel, shear.
HL97 - A-286, shear.
HST71 - 6Al-4V titanium, shear.
The HL18 standard sheet lists HL70, HL79, HL82, HL94, HL97, and HL175 as typical collars for HL18 pins [5]. It also notes two rules that matter when you order [5]:
- The lower-strength part (pin or collar) sets the strength of the system.
- Non-lubed pins must be used with lubed collars.
NASA's fastener installation spec says the same about lubricant: buy either the pin or the collar with lubricant, but not both [1].
INSTALLATION RULES
These requirements come from NASA's process specification PRC-9007 for collared (Hi-Lok) fasteners [1]. Follow your drawing or structural repair manual first.
- Hole angle: Specify hole perpendicularity of 90° ±1/2°. Self-aligning Hi-Loks can be used on sloped surfaces [1]. LISI lists self-aligning collars for slopes up to 7° [4].
- Washers: Hi-Loks are generally used without washers because the collar has a built-in washer [1].
- Grip: Measure the joint and pin with a Hi-Lok grip gage before installation. Changing the pin grip length or washer count during installation isn't allowed [1]. Choosing the right grip pin keeps threads out of bearing [1].
- Prep: Deburr holes before installing fasteners [1].
- Tools: Use proper hand or pneumatic Hi-Lok equipment [1]. The FAA handbook notes that related collared fasteners can be installed with an Allen wrench and a box-end wrench "just like the Hi-Loks" [7].
- Sequence: Unless the drawing says otherwise, tighten in a star pattern for circular bolt patterns and a spiral pattern for rectangular ones [1].
- No reuse: Collars must not be removed and reinstalled [1].
INSPECTION
After installation, NASA PRC-9007 requires checking that [1]:
- The gap under collars or pin heads is no more than 0.002 in.
- Collars aren't loose, cracked, split, or showing abnormal shear fracture.
- Pin heads don't have cracks or dimples.
- Pin protrusion falls within the limits of the table below.
Hi-Lok pin protrusion limits, NASA PRC-9007 Table 1 (inches) [1]:
-05 (0.164) min 0.302 max 0.384
-06 (0.190) min 0.315 max 0.397
-08 (0.250) min 0.385 max 0.467
-10 (0.312) min 0.490 max 0.572
-12 (0.375) min 0.535 max 0.617
-14 (0.437) min 0.625 max 0.707
-16 (0.500) min 0.675 max 0.757
HI-LOKS VS. RIVETS AND BOLTS
Howmet notes the Hi-Lok can match the best qualities of a bolt and nut or a pin and swaged collar [2]. The FAA handbook covers it alongside other special fasteners such as Hi-Shear pin rivets and Hi-Tigue fasteners [7]. To compare, see how Hi-Shear pin rivets are installed, or compare Hi-Loks with Jo-bolts.
FAQ:
Q: Can a Hi-Lok be over-torqued?
A: No. Howmet states the collar's hex shears off at the designed torque, so the fastener can't be over-torqued and doesn't need torque inspection [2].
Q: Can I reuse a Hi-Lok collar?
A: No. NASA PRC-9007 says collars must not be removed and reinstalled [1].
Q: What do the numbers in HL18PB8-8 mean?
A: The first 8 is the diameter in 1/32nds (1/4 in.), and the second 8 is the maximum grip in 1/16ths (1/2 in.). PB is the finish code [5].
Q: Which collar goes with an HL18 pin?
A: The HL18 standard lists HL70, HL79, HL82, HL94, HL97, and HL175 as typical collars [5]. Match the collar to your pin's shank size and your drawing.
Q: Do Hi-Loks need washers?
A: Generally not. The collar has a built-in washer [1].
Shop: Hi-Lok | Hi-Lok Head Pins | Hi-Lok & Collars
Related guides: Aircraft Rivets | Special Purpose Bolts | Threaded Fasteners
This guide is part of the Monroe Aerospace Guides library. Browse the full library, or contact us for help pairing pins and collars.
References
[1] NASA JSC, PRC-9007C, Process Specification for Installation of Threaded and Collared Fasteners: https://www.nasa.gov/wp-content/uploads/2023/03/prc-9007-current.pdf
[2] Howmet Aerospace, Hi-Lok Fastening System: https://www.howmet.com/products/aerospace/pin-systems/hi-lok-fastening-system/
[3] LISI Aerospace, Hi-Lok and Hi-Lite Fasteners: https://www.lisi-aerospace.com/en/product/hi-lok-and-hi-lite-fasteners/
[4] LISI Aerospace, Hi-Lok and Hi-Lite Nuts and Collars: https://www.lisi-aerospace.com/en/product/hi-lok-and-hi-lite-nuts-and-collars/
[5] HL18 Hi-Lok Pin Standard Sheet: https://www.aaestore.com.au/media/specs/HL18.pdf
[6] Monroe Aerospace, Hi-Lok & Collar Quick Reference Guide: https://catalog.monroeaerospace.com/Asset/HI-LOK---Collar-Quick-Reference-Guide.pdf
[7] FAA, Aviation Maintenance Technician Handbook - General, Chapter 5 (Special Fasteners), via AvStop: http://avstop.com/ac/Aviation_Maintenance_Technician_Handbook_General/5-66.html