Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.
Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.
Summary
Four things have to match before a replacement bolt is the right bolt: diameter, thread pitch, length, and grade. The first three are measurements anyone can take with a caliper, a pitch gauge and five minutes. The fourth is the one that hurts, because a lower-grade bolt looks identical in the hand, threads in exactly as sweetly, holds fine on the bench, and then lets go under load without any warning at all. The only evidence of grade you have in front of you is the marking stamped on the head — and the FAA's blunt version of this, written about aircraft hardware, is worth carrying into a garage: bolts with no markings are low strength.
This page will teach you the method and will not print you a chart. That is deliberate and it is worth explaining. The systems that assign meaning to those head markings live inside published standards — SAE J429 for inch-series steel fasteners, SAE J1199 and ISO 898-1 for metric ones — and those standards are copyrighted documents you buy, not facts in the public domain. Procedures and facts are not copyrightable and a marking table is neither. So what follows is how to read a fastener, how to record what you find, and exactly who to ask for the number the marking stands for: the equipment manufacturer first, the standard second. If you came here for a grade chart, the honest answer is that the people who wrote it should be the ones who sell it to you.
Necessary equipment
| Item | Note |
|---|---|
| Caliper | A caliper gives the diameter and the length of the bolt. |
| Thread pitch gauge | A pitch gauge has one set of blades for inch threads and one set for metric threads. |
| Wire brush | Clean the threads and the head before you measure them. |
| Eye protection | Wear eye protection when you use a wire brush. |
| Camera | A photograph of the head keeps the marking after you lose the bolt. |
| Documentation for the equipment | The manufacturer of the equipment gives the correct fastener. |
| Bright light optional | A marking on a used head is often full of paint or corrosion. |
| Labelled bags or a parts tray material | Keep the bolts of one joint together. |
Record the joint before you remove a bolt
Estimated 4 min to 8 min
PHOTOGRAPH THE JOINT BEFORE YOU REMOVE A BOLT.
One joint can hold bolts of more than one length.
- Photograph the joint before you remove a bolt from it.1mReport a problem with step 1, Record the joint before you remove a bolt
- Show the position of each bolt in the photograph.
- One joint can hold bolts of more than one length.
- Put the bolts of one joint in one labelled bag.2mReport a problem with step 2, Record the joint before you remove a bolt
- Write the name of the joint on the bag.
- Write the position of each bolt on the bag.
KEEP THE OLD BOLT UNTIL THE NEW BOLT IS IN POSITION.
The old bolt is the only reference that you have.
- Keep the old bolt.<1mReport a problem with step 3, Record the joint before you remove a bolt
- The old bolt is your reference for each of the four properties.
- Discard the old bolt only after the new bolt is in position.
Clean the bolt
Estimated 5 min to 11 min
WEAR EYE PROTECTION BEFORE YOU USE A WIRE BRUSH.
A wire from the brush can go into your eye and injure it.
- Clean the threads of the bolt with the wire brush.2mReport a problem with step 1, Clean the bolt
- Move the brush away from your body.
- Dirt in a thread gives a wrong pitch measurement.
- Clean the top of the head of the bolt.3mReport a problem with step 2, Clean the bolt
- Remove the paint and the corrosion from the top face.
- A marking on a used head is often difficult to see.
- Use a bright light at a low angle to the face.
Measure the three dimensions
Estimated 14 min to 25 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
- Measure the diameter across the threads with the caliper.2mReport a problem with step 1, Measure the three dimensions
- Measure at a part of the thread with no damage.
- The measurement is a little below the nominal diameter.
- Write the measurement down.
- Compare the measurement with a nut of a known size.3mReport a problem with step 2, Measure the three dimensions
- An inch bolt and a metric bolt can measure almost the same.
- A nut that turns on freely for its full depth is the correct size.
- A nut that stops after two threads is the wrong size.
DO NOT PUT AN INCH BOLT IN A METRIC HOLE.
The bolt can turn for two threads and feel correct. Then the threads fail under load and the parts can fall and injure you.
- Measure the pitch of the thread with the pitch gauge.3mReport a problem with step 3, Measure the three dimensions
- Try each blade until one blade sits fully in the threads.
- Look at the gauge against a bright background for a gap.
- Do not calculate the pitch from the diameter.
- Record the system of the pitch with the number.2mReport a problem with step 4, Measure the three dimensions
- The blade of an inch gauge gives the number of threads for each inch.
- The blade of a metric gauge gives the millimetres between two threads.
- The same bolt has one number in one system and a different number in the other.
- Measure the length of the bolt with the caliper.2mReport a problem with step 5, Measure the three dimensions
- Measure a hexagon head bolt from below the head to the end.
- Measure a countersunk head bolt from the top face to the end.
- The convention is not the same for each type of head.
- Record the type of head and the type of drive.2mReport a problem with step 6, Measure the three dimensions
- Examples of a head are hexagon, socket, flange and countersunk.
- Examples of a drive are hexagon, hexalobular, cross and slot.
Read the marking on the head
Estimated 7 min to 13 min
Source: AC 43.13-1B, Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair, Chapter 7 (Aircraft Hardware, Control Cables, and Turnbuckles), Section 3 Bolts · tier 1 · accessed 2026-07-29
DO NOT USE A BOLT WITH NO MARKING TO REPLACE A BOLT WITH A MARKING.
FAA states that bolts with no markings are low strength.
- Look at the top face of the head for a marking.2mReport a problem with step 1, Read the marking on the head
- FAA states that code markings on the bolt heads give the identity of a bolt.
- FAA states that these markings generally denote the material of the bolt.
- FAA states that bolts with no markings are low strength.
- Record each element of the marking separately.3mReport a problem with step 2, Read the marking on the head
- Count the lines and record their positions.
- Record any number, letter or symbol.
- One element is usually the mark of the manufacturer.
- Photograph the head at a low light angle.2mReport a problem with step 3, Read the marking on the head
- The photograph keeps the marking after you lose the bolt.
- Put a rule in the photograph for the scale.
Find the meaning of the marking
Estimated 26 min to 1 hr 16 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
Source: SAE J429_201405 — Mechanical and Material Requirements for Externally Threaded Fasteners (catalogue entry) · tier 1 · accessed 2026-07-29
Source: SAE J1199_201302 — Mechanical and Material Requirements for Metric Externally Threaded Steel Fasteners (catalogue entry) · tier 1 · accessed 2026-07-29
DO NOT INSTALL A BOLT OF AN UNKNOWN GRADE IN A JOINT THAT CARRIES A LOAD.
A bolt of a lower grade fails with no warning. The parts can fall and injure you.
- Do not read a grade from the marking without a reference.1mReport a problem with step 1, Find the meaning of the marking
- The inch system and the metric system do not use the same marks.
- This page does not publish a table of marks and grades.
- The tables are inside standards that you must buy.
- Look for the fastener in the parts list for your equipment.5mReport a problem with step 2, Find the meaning of the marking
- A parts list gives a part number for each fastener.
- A part number from the manufacturer removes all four questions at one time.
- If the documentation gives no fastener, speak to the manufacturer.10mReport a problem with step 3, Find the meaning of the marking
- Send the photograph of the head and your three measurements.
- Ask for the grade and for the torque together.
DO NOT SELECT A HIGHER GRADE THAN THE MANUFACTURER GIVES.
A different grade changes the way that the joint fails. FAA tells you to use like bolts in replacement.
- If the manufacturer gives no answer, buy the standard for your system.10mReport a problem with step 4, Find the meaning of the marking
- SAE J429 covers the steel fasteners of the inch series.
- It covers a bolt, a screw, a stud and a U-bolt.
- It covers sizes to 1-1/2 in.
- SAE J1199 covers eight property classes of metric fasteners.
- SAE J1199 covers the sizes M1.6 to M36.
- You must buy each of these documents from the publisher.
Buy and install the replacement
Estimated 15 min to 35 min
Source: AC 43.13-1B, Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair, Chapter 7 (Aircraft Hardware, Control Cables, and Turnbuckles), Section 3 Bolts · tier 1 · accessed 2026-07-29
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
- Buy a fastener with the same four properties as the old one.5mReport a problem with step 1, Buy and install the replacement
- The four properties are diameter, pitch, length and grade.
- FAA states that it is of extreme importance to use like bolts in replacement.
- Compare the new bolt with the old bolt before you install it.3mReport a problem with step 2, Buy and install the replacement
- Put the two bolts head to head.
- Compare the two lengths.
- Put a nut on the new bolt.
- Turn the nut for the full length of the thread.
- Compare the marking on the two heads.
USE THE TORQUE THAT THE MANUFACTURER GIVES.
FAA states that an overtorque can cause failure of a bolt or a nut from overstress of the threaded areas.
- Install the new bolt to the torque that the manufacturer gives.4mReport a problem with step 3, Buy and install the replacement
- This page publishes no torque figure.
- FAA states that the torque is for the size of the bolt shank and not the wrench size.
- FAA states that the threads must be clean and dry, unless the manufacturer gives a different instruction.
- Write the four properties on a label in the equipment record.3mReport a problem with step 4, Buy and install the replacement
- Keep the photograph of the head with the record.
- The next replacement then takes two minutes.
Terminology
This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.
- bolt
- Also called: screw, machine screw, cap screw, fastener, set screw, stud, that bolt thing
- thread pitch
- Also called: pitch, threads per inch, TPI, thread count, coarse or fine, thread size
- fastener grade
- Also called: bolt grade, grade 5, grade 8, property class, class 8.8, class 10.9, bolt strength, the lines on the head
- head marking
- Also called: bolt head markings, lines on the bolt head, radial lines, the numbers on a bolt, grade marking
- nominal diameter
- Also called: bolt size, diameter, gauge, what size is this bolt, M8, quarter inch
- thread pitch gauge
- Also called: pitch gauge, thread gauge, screw pitch gauge, thread checker, the comb thing
When to use a professional
Stop if the fastener is in a brake system, a steering system, a suspension system or a restraint system. Speak to a qualified technician. A wrong fastener in one of these systems is a risk to life, and this page does not cover them.
Stop if the fastener holds a part of a building or a part of a structure. Speak to a qualified engineer. The grade and the size of such a fastener come from a design and not from a measurement.
Stop if the old bolt has a stretched shank, a damaged thread or a crack. Do not put it back. Some fasteners are single-use parts, and the manufacturer of the equipment is the only source of that information.
Stop if you cannot find the grade from the manufacturer or from the standard. Do not estimate the grade from the marking. Do not use a fastener from a mixed container of used bolts.
Do not cut a bolt to length. Do not make a hole larger to accept a bolt of a different diameter. Speak to the manufacturer about the correct part.
Common questions
What do the lines on a bolt head mean?
They are a grade marking, and this page deliberately does not print the table that turns them into a number. The systems that assign meaning to head markings are defined inside published standards — SAE J429 for inch-series steel fasteners, SAE J1199 and ISO 898-1 for metric ones — and those are copyrighted documents sold by their publishers, not public-domain facts. What we can tell you, and what actually matters, is the method: photograph the head, count and place every element you can see, then get the meaning from the equipment manufacturer first and from the standard second. A marking chart copied off a third-party site is a number that travelled, and rule one of this site is that a safety-critical value that cannot be traced does not get printed.
Why won't you just show me a grade chart like everyone else does?
Because the chart is not ours to give you. Procedures, methods and facts are not copyrightable, which is why the whole of the method above is here in full. The specific expression of a standards body's table is a different thing, and reformatting somebody else's table does not change what it is. There is a second reason that matters more: a chart on a repair site is only as current as the day somebody typed it, and fastener standards get revised. SAE J429 has been revised repeatedly since it was first issued in 1949. Sending you to the source protects you from a stale copy as well as protecting us.
Is a grade 8 bolt always better than a grade 5?
No, and this is the single most expensive misconception about fasteners. A higher grade is harder and stronger in tension, and it is also less ductile — it bends less before it breaks and it is less forgiving of shock loading and of stress concentrations. Some joints are designed so the fastener is the part that gives way, and substituting something stronger moves the failure somewhere much worse. The FAA's rule for aircraft hardware is that it is of extreme importance to use like bolts in replacement, and that principle carries across. Fit the grade the manufacturer specifies, not the biggest number on the shelf.
How do I tell a metric bolt from an imperial one?
Not by eye, and not reliably by diameter — a 1/4 inch bolt and an M6 are close enough to fool anyone, and so are 5/16 and M8. Measure the pitch with a thread pitch gauge, which has separate sets of blades for inch and metric threads: the blade that seats fully with no light under it tells you both the system and the number. The second check is a nut of known size, run down the full length of the thread by hand. A nut that goes on for a turn or two and then binds is telling you the systems do not match, and forcing it destroys both parts.
Where exactly do I measure the length of a bolt from?
It depends on the head, which is why people order the wrong thing. For a hexagon or other protruding head, length is measured from under the head to the end of the bolt. For a countersunk head that sits flush, it is measured from the top face — because the head is inside the material, not on top of it. Get this wrong and the bolt is a head-thickness too short or too long, which either bottoms out in a blind hole or leaves you short of thread engagement. If the head is an unusual shape, take the convention from the manufacturer's own parts drawing.
The bolt head has no markings at all. What does that mean?
Treat it as low strength until something authoritative tells you otherwise. The FAA states this plainly for aircraft hardware — bolts with no markings are low strength — and the same reasoning applies anywhere: a manufacturer who has met a grade requirement generally marks it, because the marking is how anyone downstream can tell. The related warning from the same document is about traceability: hardware that is not traceable, or has been improperly altered, may be substandard or counterfeit, because its physical properties cannot be substantiated. An unmarked bolt out of a coffee tin of old fasteners is not a known quantity.
Can I just take the old bolt to the shop and match it?
It is a good start and it is not enough on its own. Matching by eye and by feel gets you diameter, pitch and length, which are three of the four things — and it gets you them reliably, because the counter will have a thread checker. What it does not get you is grade, unless somebody reads the head marking and finds out what it means. So take the old bolt, and take a photograph of its head, and take the model number of the equipment. The last of those three is usually the one that actually answers the question.
How do I get the marking to show up on a corroded head?
Clean the top face first — a wire brush and eye protection, then a wipe. After that the trick is lighting rather than cleaning: put a bright light almost flat across the face rather than shining straight down, so the raised or stamped elements throw shadows. Photograph it at that angle too, because a photograph will often show a faint mark your eye skated over. If paint has filled a stamped mark, a light rub with fine abrasive across the face can bring the contrast back, though on a plated fastener be aware that you are removing the plating.
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- Tell a metric hex key or socket from an imperial one before it rounds the fastener
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Sources
Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.
- Tier 1AC 43.13-1B, Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair, Chapter 7 (Aircraft Hardware, Control Cables, and Turnbuckles), Section 3 Bolts
- Tier 1SAE J429_201405 — Mechanical and Material Requirements for Externally Threaded Fasteners (catalogue entry)
- Tier 1SAE J1199_201302 — Mechanical and Material Requirements for Metric Externally Threaded Steel Fasteners (catalogue entry)
Revision history
Current version 1 · updated 2026-07-29 · next review due 2027-07-29
- v1 · 2026-07-29First draft. This record deliberately publishes NO grade marking table and NO grade value, and says so on the page. Fastener grade is a safety-critical value under rule 1, and the marking systems that assign meaning to head markings are defined inside SAE J429 and SAE J1199 — copyrighted standards that must be purchased — so reproducing their tables would be a rule 10 breach. The catalogue entries for both standards were read on 2026-07-29 at saemobilus.sae.org and are cited by standard number, title, scope and purchase status only. The method of reading a fastener is published in full, because a method is not copyrightable. FAA AC 43.13-1B Chapter 7, a United States federal work in the public domain, supports the marking principle, the unmarked-bolt statement, the like-for-like replacement rule and the torque warnings. The record explicitly refuses the common 'higher grade is safer' advice. ISO 898-1 and ASTM F568M could not be read (iso.org and astm.org both returned HTTP 403) and are therefore not cited as sources; ISO 898-1 is named in the FAQ only as the standard a metric reader is directed to buy.
Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.
Comments and corrections
Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.
Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.