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

Grip is the thickness of material the fastener is designed to clamp, and the makers tie a wrong grip to a bad installation: Monogram says stem break-off outside its limits "may indicate the use of the wrong grip fastener" (MBF2003 §10.1), and Cherry lists wrong grip among the causes of high or low stem breaks (CA-1013, CA-1015). Where the stack varies, the grip is 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, and each maker tabulates that band. CherryMAX: "the last dash number indicating the maximum grip length in 16ths", so a -04 covers .188-.250" (CA-1011) [1]. Composi-Lok II MBF2112: -300 covers .250-.300" (drawing grip table), and MBF2003 says to favor the max-grip part at a borderline reading ("a 0.250 reading uses a -250 grip part") [2]. Other families tabulate differently: Visu-Lok II dash numbers are the nominalgrip, e.g. -5 = .282-.344" (brochure Table 3) [3]; Cherry Maxibolt lists grip limits per dash, e.g. -04 = .219-.282" (CA-1009) [6]; Radial-Lok MRL3210 -500 covers .425-.525" while MRL3212 -500 covers .450-.500" (drawing grip tables) [7]. Read the table for the exact base part number.

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 dash in thousandths of an inch. MBF2110-6-300 is 3/16" (-6) with a .250-.300" grip range; Composi-Lok II comes in "incremental 0.050" grip lengths" (MBF2003 §2.1).
• 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

Monogram's rule is to gauge before installing: "Prior to installation, the grip length should be checked with a grip gauge" (MBF2003 §6.1) [2], and the Composi-Lok 3 spec adds "All sheet gap should be removed prior to gauging for proper grip" (MBF2301 §6.1) [8]. Where the blind side is tapered, "the grip length must be determined by the depth at the centerline of the hole", and the taper must not exceed 7° (MBF2003 §6.2) [2]. For flush heads, "the minimum grip for a flush-type shall in no case ever be less than the head height" (§6.3) [2]. Where the grip "varies due to tolerance build-up", favor the maximum grip (§6.3) [2]: across a bonded or shimmed hole pattern that means a thickness range, 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. Buying margin on the wrong dash numbers leaves unused hardware on the shelf while the line waits on the 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 package.

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 (AC 43.13-1B Fig. 4-6(a)) [5].

Always verify against the controlling drawing and your stress group.