How to
Measure
Bolt Size
When you're ordering individual titanium hardware by size, getting the measurement right the first time matters. This guide covers the three measurements every metric bolt requires and shows you how to size nuts and washers from those same numbers.
Reading a Metric Bolt Size
Every metric bolt is described by three numbers in a standard format. Understanding what each number means before you pick up a gauge makes the whole process faster.
Thread Diameter
Thread diameter is the width of the bolt's threaded shaft measured across the peaks of the threads. For metric hardware it's expressed as an "M" size — M5, M6, M8, M10, M12 — where the number is the diameter in millimeters.
To measure it, look straight at the threaded section and measure across the widest point of the shaft — not the head. Most engine bay hardware on Japanese and European vehicles falls in the M6 to M8 range.
Using a gauge removes any guesswork. Slide the bolt into each labeled slot until you find one where it seats snugly with no side play. That slot's size label is your diameter.
Slide the bolt into each labeled slot from the top of the gauge. Find the slot where the bolt seats cleanly with no wobble. The size printed next to that slot is your thread diameter.
Thread Pitch
Thread pitch is the distance between each individual thread on the bolt, measured in millimeters. Two bolts can share the same diameter but have different pitches — and they will not thread into each other or into the same hole.
The most common pitch for M6 bolts found in engine bays is 1.0mm. For M8, the standard pitch is 1.25mm. Fine-thread versions exist but are less common in automotive applications.
Pitch is the hardest measurement to take accurately by eye — even with a ruler it's easy to misread. A thread pitch gauge takes the guesswork out entirely and is the main reason the combo tool is worth having.
Flip the bolt gauge to the pitch side. Press the bolt threads into each set of grooves until you find the one where the threads lock in cleanly — no wobble, no gaps. The pitch value stamped underneath those grooves is your thread pitch.
Thread Length
Thread length — also called bolt length — is measured from the underside of the bolt head to the tip of the threaded shaft. Do not include the head height in your measurement. This is the most common measuring mistake.
The one exception is countersunk (flat head) bolts, where the head sits flush with the surface — for those, measure total length from the top of the head to the tip.
Length is always expressed in millimeters. A bolt that measures 20mm from under its head to the tip is a 20mm bolt, regardless of diameter or pitch.
Place the bolt along the ruler side of the gauge with the bottom of the head at the zero mark. Read where the tip of the bolt reaches. That number is your thread length in millimeters.
Sizing Nuts & Washers
Once you have your bolt measurements, sizing nuts and washers is straightforward — you're working from numbers you already have.
A nut must match both the thread diameter and thread pitch of its bolt. If you have an M6 × 1.0 bolt, you need an M6 × 1.0 nut. Length doesn't apply to nuts — only diameter and pitch matter.
Washers only need to match the thread diameter — pitch and length don't factor in. An M6 bolt takes an M6 washer regardless of its pitch. The washer just needs to fit over the bolt shaft without binding.
Bolt Head Types
Fast Turn Fittings carries four head styles. Head type doesn't change your diameter, pitch, or length measurements — but it determines the tool you'll use and how the bolt sits in the application.
Common Metric Bolt Sizes
Most automotive engine bay hardware falls within this range. Use this to cross-check your measurements or identify a size when the markings have worn off.
| Size | Diameter | Thread Pitch | Common Automotive Use |
|---|---|---|---|
| M5 | 5mm | 0.80mm | Small trim brackets, interior panels, widebody/overfenders |
| M6 Most Common | 6mm | 1.00mm | Engine bay covers, fenders, strut towers, headlights |
| M8 Most Common | 8mm | 1.25mm | Intercooler, intake manifold, suspension brackets |
| M10 | 10mm | 1.25mm · Some European vehicles use 1.50mm | Larger engine brackets, subframe components |
| M12 | 12mm | 1.50mm | Suspension, structural components |
Frequently Asked Questions
Common questions about measuring metric hardware. Don't see your question answered? Reach out at sales@fastturnfittings.com and we'll help.
What's the difference between coarse and fine thread? +
Coarse thread is the standard pitch for a given bolt diameter — for example, an M6 bolt with 1.0mm pitch is coarse. Fine thread bolts have a smaller distance between threads (an M6 fine thread would be 0.75mm pitch). Fine thread offers slightly higher clamping force and is less likely to vibrate loose, but coarse thread is far more common in automotive applications. Most engine bay hardware on Japanese and domestic vehicles uses coarse thread.
Do I need to know the bolt grade or material? +
For sizing purposes, no — diameter, pitch, and length are all you need. Bolt grade refers to the material strength rating (such as 8.8, 10.9, or 12.9 for steel), and Fast Turn's titanium hardware uses 6AL-4V Grade 5 titanium across all kits and individual sizes. Grade 5 titanium is stronger than most steel grades commonly used for OEM dress up hardware locations.
Can I use a longer bolt than the original? +
Sometimes — but only if there's clearance behind the mounting hole for the extra length, and only if the longer bolt doesn't bottom out before the head clamps down on the surface. A bolt that bottoms out won't actually clamp the part it's holding. When in doubt, match the original length. Going slightly shorter is generally safer than going longer.
Why does my European car use different thread pitches than my Japanese car? +
European manufacturers often use different pitch standards for certain bolt diameters — most notably M10 × 1.50mm where Japanese and domestic vehicles typically use M10 × 1.25mm. Always verify pitch before ordering for European applications.
What if I don't have a bolt gauge? +
A bolt gauge gives the most accurate measurements and is worth the small investment if you're ordering hardware by individual size. In a pinch, you can measure diameter with calipers (measure across the threads at the widest point), thread pitch by counting threads over a known distance, and length with a ruler from under the head. The Fast Turn bolt gauge handles all three measurements in one tool for $9.99.
