How to determine blind fastener grip length — and order the right mix

Grip is the thickness of material the fastener is designed to clamp. Get it wrong in one direction and the blind head forms inside the hole; wrong in the other and the stem breaks proud or the collar bottoms out. On bonded structure the stack isn't even one number — it's a range, and that range is what you order against.

Grip is a range, not a point

Every dash number covers a band of stack thickness. For most blind fastener families the usable band runs from the catalogued maximum grip down to the previous increment: a fastener with a 0.300" max grip in an 0.050" series covers roughly 0.250–0.300" of stack. Cherry publishes this per part in the CA-1011 catalog legend; Monogram's catalogue does the same for Composi-Lok. The design rule is simple: measured stack thickness must fall inside the grip band, ideally not at its very edge.

How the dash numbers encode grip

The three big numbering schemes disagree on units, which is a routine source of ordering mistakes:

• Composi-Lok (MBF) and Radial-Lok (MRL): grip in thousandths of an inch. MBF2110-6-300 is a 3/16" fastener with 0.300" max grip; the series steps in 0.050" (MRL in 0.100").
• Cherry (CR) and Hi-Lok (HL): grip in sixteenths of an inch. CR3242-6-04 covers up to 0.250" (4/16); HL10-6-8 up to 0.500" (8/16).
• Solid rivets (MS20470): the trailing dash is length, not grip — a different quantity entirely, sized from stack plus the material needed to form the shop head.

Paste any part number into the decoder to read its diameter and grip band, or browse the part-number index for the full grip chart of each series.

Measure, don't assume

On sheet-metal assemblies the stack is the drawing dimension and a grip gauge through the hole confirms it. On bonded or liquid-shimmed structure the effective stack includes the adhesive: a bondline that is nominally 0.010" can vary several times that across a part, and co-cured or infused skins add their own thickness tolerance. The installed reality is a thickness distribution across the hole pattern, not one value.

The straddle problem

When a zone's thickness range crosses a grip boundary, no single dash number covers every hole. A zone at 0.280 ± 0.040" in an 0.050"-step series needs three grips on the shelf, and grip must be confirmed per hole at install. This is where fastener budgets quietly die: per-grip pricing varies enormously, and buying worst-case margin on the wrong dash numbers means shelves of unused hardware while the line waits on the one grip that ran out.

The grip length planner does this arithmetic: enter each zone's design grip and tolerance (or measured min/max), and it returns the covering dash numbers, flags boundary-straddling zones, and produces a buy list with your chosen margin — which you can send straight to the RFQ builder.

Ordering rules of thumb

• Order to the measured range, not the nominal, and carry margin on every grip the range touches — not just the middle one.
• Weight margin toward boundary grips: straddle zones consume the adjacent dash numbers unpredictably.
• Confirm the unit convention (sixteenths vs thousandths) before the PO goes out; a "-4" and a "-400" are not neighbors.
• For solid rivets, remember the dash is length: use stack plus roughly 1.5× diameter of protrusion for the shop head, per AC 43.13 practice.

Sources: Cherry CA-1011 catalog legend; Monogram/PennAero Composi-Lok catalogue; Hi-Lok Spec 342 grip tables; FAA AC 43.13-1B for solid-rivet length practice. Always verify against the controlling drawing and your stress group.