Question 1

What size socket do I need for a bolt?

Accepted Answer

Socket size is the across-flats measurement of the hex head, not the bolt diameter. Common metric heads: M6 = 10 mm, M8 = 13 mm, M10 = 16 or 17 mm, M12 = 18 or 19 mm, M14 = 22 mm. For SAE, a 3/8 inch bolt usually takes a 9/16 inch wrench. Measure flat-to-flat with a caliper, or use the wrench and socket finder.

Question 2

How do I read bolt grade markings?

Accepted Answer

SAE bolts show raised radial lines on the head: no lines is Grade 2, three lines is Grade 5, and six lines is Grade 8 — more lines means stronger steel. Metric bolts are stamped with a property class number such as 8.8, 10.9 or 12.9, where higher numbers are stronger. Class 8.8 is roughly SAE Grade 5 and 10.9 is roughly Grade 8.

Question 3

What is the difference between metric and SAE bolts?

Accepted Answer

Metric bolts are sized by shank diameter in millimetres (M6, M8, M10) with pitch in millimetres; SAE bolts are sized in fractional inches with pitch in threads per inch (TPI). They are not interchangeable — threading SAE into metric cross-threads and strips. Nearly every passenger vehicle built after 1992 uses metric, including domestic makes.

Question 4

What torque should I use for a bolt?

Accepted Answer

Use the manufacturer spec when you have one. Without it, torque depends on grade and size: a clean, dry Grade 5 / class 8.8 bolt is a safe general baseline, and Grade 8 / class 10.9 takes more. Lubricated, anti-seized or plated threads need roughly 15 to 25 percent less torque than the dry value because lower friction produces more clamp load at the same reading. Always use a calibrated torque wrench.

Question 5

In what order do I tighten lug nuts and wheel bolts?

Accepted Answer

Always tighten in a star or criss-cross pattern, never in a circle, so the wheel is pulled down square against the hub. Use an X on a 4-lug wheel, a star on a 5-lug, and opposite pairs on a 6-lug. Snug by hand, run a half-torque pass, then a final pass to the full spec, and re-torque after 50 to 100 miles.

Question 6

How do I measure a bolt?

Accepted Answer

Four numbers identify any bolt: length (measured under the head, except for flush countersunk heads which include the head), major diameter (across the thread crests), thread pitch (mm for metric, TPI for SAE), and head or wrench size (across the flats). Written out it looks like M8 x 1.25 x 40 mm or 1/2 inch-13 x 2 inch.

Question 7

What is thread pitch and how do I measure it?

Accepted Answer

Thread pitch is the spacing of the threads — millimetres between crests for metric (M10 coarse = 1.5 mm, fine = 1.25 mm) or threads per inch for SAE (3/8 inch UNC = 16 TPI, UNF = 24 TPI). Measure it with a thread pitch gauge by finding the leaf that seats with no rocking, or count the crests across a known length.

Question 8

Are these charts and the widget free to use on my own site?

Accepted Answer

Yes. The reference charts and the bolt-finder widget are free to embed and the PDFs are free to download and print, for personal or commercial use. The only ask is that you keep the visible link back to WhatSizeBolt that comes with each embed — that attribution is how the data stays free and maintained.

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