What does AF mean on spanners?

AF stands for "Across Flats" — it is the measurement across the two parallel flat sides of a bolt head or nut. This is the measurement that determines what spanner you need. A 1/2" AF spanner has jaws that open to exactly 1/2 inch (12.70mm) across. AF is the standard used for imperial bolt heads on British and American machinery from the 1950s to 1980s. It is distinct from Whitworth, which sizes spanners by the bolt shank diameter rather than the head.

What is the difference between AF and Whitworth spanners?

AF (Across Flats) sizes the spanner by the width of the bolt head. Whitworth sizes the spanner by the diameter of the bolt shank. This means a "1/2" Whitworth" spanner and a "1/2" AF" spanner are completely different sizes — the Whitworth is significantly larger. A 1/2" Whitworth spanner has jaws of 21.0mm, compared to 12.70mm for 1/2" AF. Never interchange Whitworth and AF spanners — the sizes are completely different and using the wrong one will round the bolt head.

Can I use a metric spanner on an imperial bolt?

Sometimes, with caution. When the metric size is close to the imperial AF size — within about 0.3mm — a metric spanner can work in an emergency. For example, 19mm works well on 3/4" AF (0.05mm difference), and 13mm works on 1/2" AF (0.30mm difference). However, when the difference is larger — 14mm on 9/16" AF (0.29mm — borderline), or 17mm on 11/16" AF (0.46mm — risky) — using the wrong size risks rounding the head, especially on corroded bolts. Always use the correct size where possible.

What spanner size fits an M12 bolt?

A standard M12 metric bolt has a 19mm hexagon head and takes a 19mm spanner or socket. This is also very close to 3/4" AF (19.05mm). On older machines where imperial bolts have been replaced with metric, or vice versa, the 19mm/3/4" AF pairing means both sizes will fit the same head — though for precision work always confirm which thread standard you are dealing with.

What spanner do I need for a 1/2" BSP fitting?

A 1/2" BSP fitting takes a 27mm spanner across the hex flats — equivalent to 1-1/16" AF. This is one of the most common hydraulic fitting sizes on UK farm machinery. For 3/4" BSP fittings, you need a 32mm spanner (1-1/4" AF). The BSP spanner sizes are covered in detail in the AgriOps BSP Spanner Size Chart.

What is the correct spanner for a Ferguson TE20?

The Ferguson TE20 uses BSW (Whitworth) and BSF (British Standard Fine) threaded fasteners throughout. You need a proper Whitworth spanner set to work on a TE20 — imperial AF spanners will not fit correctly. The most common sizes on a TE20 are 3/8" Whitworth (16.5mm head, takes 11/16" AF), 1/2" Whitworth (21.0mm head, takes 13/16" AF) and 5/8" Whitworth (26.8mm head, takes 1-1/16" AF). A dedicated BSW/BSF spanner set is an essential purchase for anyone working regularly on pre-1960 British machinery.

What are the most common spanner sizes on a modern tractor?

Modern tractors (1985 onwards) are fully metric. The most common spanner sizes are 13mm (M8 bolts), 17mm (M10 bolts) and 19mm (M12 bolts) — these three sizes cover the majority of fasteners on a modern tractor cab, engine and transmission. A 24mm spanner covers M16 bolts used for heavy structural applications. For hydraulic fittings, 27mm (1/2" BSP) and 32mm (3/4" BSP) are the most frequently encountered sizes.

Why does my metric spanner not fit my imperial bolt properly?

Because metric and imperial bolt head sizes do not align exactly. When the difference between a metric spanner and the imperial size it is being used on is more than about 0.3mm, the fit becomes sloppy and the spanner can round off the bolt head corners under load. This is especially dangerous on corroded or tight fastenings. The solution is to use the correct imperial AF spanner — or in an emergency, to use a metric size that is slightly smaller rather than slightly larger, so it grips rather than slips.

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Workshop Reference

Metric to Imperial Spanner Conversion — AF, Whitworth and Bolt Head Sizes

What spanner fits which bolt. Metric bolt head sizes, nearest AF equivalents, Whitworth sizes for old machinery — and direct answers to the most common workshop questions.

8 min read·Updated May 2026·Workshop Reference

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If you work on farm machinery, you need to know three things: what spanner your metric bolts take, whether your old tractor uses AF or Whitworth, and which metric spanner to grab when the right imperial one is missing. This guide answers all three.

For the full metric-to-fractional conversion table and the era guide showing which thread standard applies to which tractor decade, use the Metric Imperial Conversion Chart tool. This post focuses on the specific practical questions that come up most often in a farm workshop.

What Does AF Mean?

Across Flats

The measurement across the two parallel flat sides of a bolt head or nut. A 1/2" AF spanner opens to exactly 12.70mm. This is the standard for imperial bolts on British and American machinery from the 1950s onwards.

Whitworth

BSW / BSF

Sizes the spanner by the bolt shank diameter — NOT the head. A 1/2" Whitworth spanner is 21.0mm across — much larger than 1/2" AF (12.70mm). Used on pre-1960 British machinery. Never interchange with AF.

Metric

ISO Standard

Sizes the spanner by the actual head dimension in millimetres. A 19mm spanner fits a 19mm bolt head. Standard on all machinery manufactured from 1985 onwards and most European machinery from the mid-1970s.

The key rule

Pre-1960 British machinery

Whitworth set required

1960s–1980s UK tractor

Imperial AF

1985 onwards

Metric throughout

What Spanner Fits Each Metric Bolt?

Standard ISO metric bolt head sizes — the spanner size is determined by the bolt's hex head, not its thread diameter. A "M12 bolt" has a 19mm head and takes a 19mm spanner.

Bolt Size	Head (mm)	Nearest AF	Common Use
M4	7 mm	—	Small fastenings, instrument panels
M5	8 mm	—	Small fastenings, guards
M6	10 mm	—	Very common — sensors, brackets, small assemblies
M8	13 mm	1/2"	Extremely common on modern machinery
M10	17 mm	11/16"	Very common — mid-sized fixings throughout
M12	19 mm	3/4"	Common structural bolt — loader arms, linkage
M14	22 mm	7/8"	Less common — some heavy structural applications
M16	24 mm	15/16"	Heavy structural fastenings
M18	27 mm	1-1/16"	Less common — some hydraulic fittings
M20	30 mm	1-3/16"	Heavy structural — drawbars, chassis
M22	32 mm	1-1/4"	Heavy fastenings
M24	36 mm	1-7/16"	Very heavy structural applications
M27	41 mm	1-5/8"	Large structural bolts
M30	46 mm	1-13/16"	Very large — axle and chassis bolts

Standard ISO 4014/4017 hex bolt head sizes. Some manufacturers use non-standard head sizes — always check the actual head measurement if in doubt.

Free Tool

Metric Imperial Conversion Chart

Full searchable conversion table from 1/16" to 2" with exact mm equivalents, nearest socket sizes and PDF download.

Open Chart →

Nearest Metric Spanner for Every AF Imperial Size

For emergencies — when you need to use a metric spanner on an imperial bolt. The "Fit" column tells you how safe it is. Loose means risk of rounding the head on a tight bolt — use with caution and only in emergencies.

Always use the correct size where possible. Using a mismatched spanner on a corroded or over-tightened bolt will round the head. This table is for emergencies only — not as a substitute for having the right spanner set.

AF Imperial	Exact mm	Nearest Metric	Difference	Fit
1/4"	6.35 mm	6 mm	+0.35mm	Loose
5/16"	7.94 mm	8 mm	-0.06mm	Good
3/8"	9.53 mm	10 mm	-0.47mm	Good
7/16"	11.11 mm	11 mm	+0.11mm	Good
1/2"	12.70 mm	13 mm	-0.30mm	Good
9/16"	14.29 mm	14 mm	+0.29mm	Loose
5/8"	15.88 mm	16 mm	-0.12mm	Good
11/16"	17.46 mm	17 mm	+0.46mm	Loose
3/4"	19.05 mm	19 mm	+0.05mm	Good
7/8"	22.23 mm	22 mm	+0.23mm	Good
15/16"	23.81 mm	24 mm	-0.19mm	Good
1"	25.40 mm	25 mm	+0.40mm	Loose
1-1/16"	26.99 mm	27 mm	-0.01mm	Good
1-1/4"	31.75 mm	32 mm	-0.25mm	Good
1-5/16"	33.34 mm	33 mm	+0.34mm	Loose
1-13/16"	46.04 mm	46 mm	+0.04mm	Good
1-7/8"	47.63 mm	48 mm	-0.37mm	Good

Combined metric and AF spanner set

The single most useful purchase for a farm workshop that runs mixed old and modern machinery. Covers both standards without separate sets. View on Amazon →

Affiliate link — we may earn a small commission at no extra cost to you.

Whitworth (BSW) Spanner Sizes

For pre-1960 British farm machinery — Ferguson TE20, Fordson Major, David Brown Cropmaster and similar. Whitworth sizes the spanner by bolt shank diameter, not head size. A dedicated Whitworth set is essential.

Never confuse Whitworth with AF. A "1/2 Whitworth" spanner has jaws of 21.0mm — much larger than "1/2 AF" (12.70mm). Using the wrong standard will round bolt heads on your vintage machinery.

BSW Size	Head (mm)	Head (inches)	AF Equiv	Metric Equiv	Common Use
1/4"W	11.2 mm	7/16"	7/16" AF	11 mm	Common on older British implements
5/16"W	13.9 mm	9/16"	9/16" AF	14 mm	Engine and gearbox fasteners
3/8"W	16.5 mm	11/16"	11/16" AF	17 mm	Very common on TE20, early Fordson
7/16"W	18.9 mm	3/4"	3/4" AF	19 mm	Structural bolts, engine mounts
1/2"W	21.0 mm	13/16"	13/16" AF	21 mm	Common on pre-1960 farm machinery
9/16"W	23.6 mm	15/16"	15/16" AF	24 mm	Axle and heavy structural bolts
5/8"W	26.8 mm	1-1/16"	1-1/16" AF	27 mm	Large structural applications
3/4"W	31.8 mm	1-1/4"	1-1/4" AF	32 mm	Heaviest pre-metric fastenings

BSW = British Standard Whitworth. The AF and metric equivalents shown are the nearest sizes — they will not fit as precisely as the correct Whitworth spanner. For regular work on pre-1960 machinery, always use a proper Whitworth set.

Common Questions — Direct Answers

The most frequently searched spanner size questions — answered directly.

What is 10mm in imperial?

3/8" AF is the nearest equivalent (9.53mm). A 10mm spanner is 0.47mm larger — workable on a 3/8" AF bolt but can slip if the head is worn.

What is 14mm as AF?

9/16" AF (14.29mm) is the nearest equivalent. 14mm is very close — 0.29mm under — and works well in practice on 9/16" AF bolts.

What is 34mm in imperial?

1-5/16" AF (33.34mm) is the nearest equivalent. A 34mm spanner is 0.66mm larger than 1-5/16" — too loose to use safely on a good bolt head. Use the correct imperial size.

What is 46mm in imperial?

1-13/16" AF (46.04mm) is virtually identical. The M30 bolt head is 46mm and corresponds almost exactly to 1-13/16" AF — this is why 46mm sockets appear in quality AF socket sets.

What is 47mm in imperial?

1-7/8" AF (47.63mm) is the nearest — about 0.6mm larger than 47mm. A 47mm socket is slightly tight on a 1-7/8" AF head. A 48mm or the correct 1-7/8" AF is the right answer.

What is 18mm equivalent in AF?

There is no common AF size close to 18mm. The nearest are 11/16" AF (17.46mm) and 3/4" AF (19.05mm). An 18mm spanner is too loose on 11/16" AF and too tight for 3/4" AF. 18mm is a metric-specific size.

What is 11/16" in mm?

11/16" AF = 17.46mm. The nearest metric is 17mm — 0.46mm under, which is borderline. Use a proper 17mm spanner on 11/16" AF bolts where possible to avoid rounding the head.

What is M12 spanner size?

An M12 bolt has a 19mm hex head. You need a 19mm spanner or socket — which is also very close to 3/4" AF (19.05mm). On older machinery an M12 bolt may have been replaced with an imperial equivalent using a 3/4" AF head.

What fits where a 16mm spanner is missing?

5/8" AF (15.88mm) is the nearest imperial equivalent to 16mm — only 0.12mm smaller. A 5/8" AF spanner will work on a 16mm bolt head in an emergency, though for regular use get the correct 16mm.

BSP Hydraulic Fittings — Different Again

BSP (British Standard Pipe) hydraulic fittings use their own hex sizes that do not correspond to bolt head standards. A 1/2" BSP fitting requires a 27mm spanner — not 12.70mm (1/2" AF). This catches people out constantly.

1/4" BSP

20 mm

3/8" BSP

24 mm

1/2" BSP

27 mm

3/4" BSP

32 mm

Full BSP Spanner Size Chart →

Workshop essentials

Combination Spanner Set — Metric & AF

Combined metric and AF set covering both standards — essential for older farm machinery.

Whitworth BSW Spanner Set

BSW/BSF Whitworth spanner set — required for pre-1960s British machinery including TE20.

Socket Set — Metric & Imperial

1/2" drive socket set covering both metric and AF imperial sizes.

Torque Wrench 1/2" Drive

Accurate torque wrench for tightening bolts to specification — prevents rounding and over-tightening.

Bolt Extractor Set

Reverse-spiral sockets for removing rounded or seized bolt heads.

Thread Gauge Set

Imperial and metric thread pitch gauge — identify BSW, BSF, UNF and metric threads.

Affiliate disclosure: AgriOps participates in the Amazon EU Associates Programme. Links are affiliate links — we may earn a small commission at no extra cost to you.

Frequently Asked Questions

Related Workshop Tools & Guides

Metric Imperial Conversion Chart

Full searchable table from 1/16" to 2" with PDF download

BSP Spanner Sizes Chart

Correct spanner for every BSP hydraulic fitting size

BSP Thread Size Chart

BSP nominal size vs actual OD and TPI reference

Torque Settings Calculator

Correct torque for metric and imperial fasteners

Tapping Drill Size Chart

Correct drill bit before tapping any thread

BSP Tap Drill Calculator

Tap drill sizes for BSP threads

All spanner and bolt head sizes are based on ISO and BSW standards. Some manufacturers use non-standard head sizes — always measure the actual head before ordering a spanner. Whitworth figures are based on BS 1083. Nearest metric equivalents are for emergency use only — always use the correct spanner for regular work to avoid rounding bolt heads.