Skip to main content
Machinist Free Pro Features Available

Thread Repair Insert Calculator: HeliCoil Drill & Tap Sizing Guide

Size Thread Repair Inserts for UNC, UNF, and Metric with Drill and Tap Lookup

Free thread repair insert calculator for machinists and maintenance mechanics. Select the original thread size in UNC, UNF, or Metric, choose insert length (1D, 1.5D, 2D, 2.5D, or 3D), and get the required drill size, STI tap size, insert part number, and installation depth. Covers all common sizes from #4-40 through 1"-8 UNC and M3 through M24 Metric.

\n

Thread repair inserts (HeliCoil, Recoil, E-Z Lok, Key Insert) restore damaged or stripped threads to original or better strength. This calculator eliminates the guesswork of finding the correct oversize drill and special tap for each insert size. Based on manufacturer tabulated data per industry standards.

Pro Tip: Use 1.5D insert length as your default for most repairs. It provides the same thread engagement as the original and is standard for through-holes. Go to 2D or 2.5D in blind holes, soft materials like aluminum and magnesium, and any application where the original thread stripped under normal torque. In spark plug ports and other critical locations, always use 2D minimum.

Calculate tap drill sizes and engagement percentages

Tap Drill Calculator →

Find any drill size by number, letter, or metric

Drill Size Finder →

Look up drill and tap combinations

Drill & Tap Calculator →

Reference bolt torque values for repaired threads

Bolt Torque Calculator →

PREVIEW All Pro features are currently free for a limited time. No license key required.

Thread Repair Insert Sizing Guide
Pop Out

How It Works

  1. Identify the Damaged Thread

    Determine the original thread size: UNC, UNF, or Metric, with diameter and pitch. Use a thread pitch gauge or the original bolt specifications.

  2. Select Insert Length

    Choose 1D (thin materials), 1.5D (standard), 2D (high strength), 2.5D, or 3D (maximum retention). Length is expressed as a multiple of the nominal thread diameter.

  3. Review Drill Size

    The calculator shows the oversize drill required to bore out the damaged thread. This drill creates the correct minor diameter for the STI (Screw Thread Insert) tap.

  4. Review STI Tap Size

    Get the correct STI tap specification. STI taps cut the oversize thread that accepts the wire insert. These are NOT the same as standard taps.

  5. Install and Verify

    Drill, tap, install insert, break tang (if tanged type), and verify with a standard thread gauge. The repaired thread accepts the original bolt size.

Built For

  • Maintenance mechanics repairing stripped bolt holes in aluminum castings (engine blocks, gear cases, pump housings)
  • Aviation mechanics installing HeliCoil inserts per manufacturer specifications
  • Machine shop operators repairing threaded holes in fixtures and workholding
  • Automotive technicians fixing stripped spark plug holes, head bolt holes, and oil pan threads
  • Fabricators adding thread inserts to soft materials for improved thread life
  • Field service technicians carrying a thread repair kit with pre-identified drill and tap sizes
  • Manufacturing engineers specifying thread inserts for production assemblies in aluminum and plastics

Features & Capabilities

UNC, UNF, and Metric Coverage

All common thread sizes: UNC #4-40 through 1"-8, UNF #4-48 through 1"-12, Metric M3 through M24. Correct drill and tap for each combination.

Five Insert Lengths

1D, 1.5D, 2D, 2.5D, and 3D multiples of nominal diameter. Longer inserts for higher pullout strength in soft materials and blind holes.

Drill Size Lookup

Shows the exact oversize drill diameter with nearest standard drill in fractional, number, letter, and metric systems.

STI Tap Identification

Identifies the correct Screw Thread Insert tap for each thread size. STI taps are thread-specific and must match the insert.

Depth Calculations

Calculates minimum drill depth, tap depth, and insert installation depth for blind hole applications.

PDF Export

Export insert sizing data as a branded PDF for repair work orders or kit inventory reference.

Frequently Asked Questions

In most materials, thread inserts provide equal or greater strength than the original thread. In aluminum and magnesium castings, inserts typically exceed original thread strength because the wire coil distributes load over a larger area of the parent material. Inserts also resist vibration loosening better than threads cut directly in soft metals.
No. STI taps cut a thread that is larger than the nominal size by the wire diameter of the insert. A standard tap will cut the wrong thread and the insert will not install. The STI tap thread class is specific to each insert size.
Tanged inserts have a notched driving tang at the bottom that is broken off after installation. Tangless inserts use a notch drive that eliminates the tang. No tang to break off and no FOD risk. Tangless costs more but is required in many aerospace and defense applications.
Solid inserts (E-Z Lok, Key-Sert, Keensert) are better for applications requiring high torque resistance and locking capability. They use a press-fit or threaded external body with an internal thread. Wire coil inserts are lighter, cheaper, and sufficient for most maintenance repairs.
Yes, but you must calculate drill depth (insert length + 2 pitches minimum), tap depth (insert length + 1 pitch), and ensure the insert seats fully without bottoming out. Use the 2D or 2.5D length for blind holes to maximize thread engagement.
Disclaimer: Insert sizing data is based on industry-standard tabulated values. Always verify drill and tap specifications against the specific insert manufacturer's installation instructions. For aerospace, military, or safety-critical applications, follow the applicable specification (NAS1130, MA1562, etc.) and use certified inserts.

Learn More

Machinist

Thread Repair Inserts: Helicoil, E-Z Lok, and Keensert Compared

Types of thread repair inserts, when to use wire vs solid inserts, minimum engagement rules, and installation tips to avoid galling.

Read Guide →

Related Tools

Machinist Live

Chip Load Calculator

Calculate chip load per tooth for milling, drilling, and turning. Forward and reverse modes with material-specific recommendations, chip thinning factor, and MRR. Metal and wood modes.

Launch Tool →
Machinist Live

Thread Pitch Identifier

Identify unknown threads from caliper measurements. Cross-references UNC, UNF, Metric ISO, NPT, BSP, and ACME thread standards with tolerance matching and confidence scoring.

Launch Tool →
Machinist Live

Tolerance & Fit Calculator

ISO 286 tolerance zone calculator for shaft/hole fits. Calculates clearance, transition, and interference fits with visual tolerance zone diagram and common fit presets (H7/g6, H7/h6, H7/p6, etc.).

Launch Tool →