Spanner Size Conversion Chart
Imperial AF to Metric · Closest socket sizes · Thread standard guide · Printable PDF
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Imperial to Metric Spanner Size Converter
Imperial (inches AF)
Closest socket
13mm
Exact: 12.70 mm
| Imperial | Exact mm | Closest Socket |
|---|---|---|
| 1/16" | 1.59 mm | 1.5 mm |
| 5/64" | 1.98 mm | 2 mm |
| 3/32" | 2.38 mm | 2.5 mm |
| 7/64" | 2.78 mm | 3 mm |
| 1/8" | 3.18 mm | 3 mm |
| 9/64" | 3.57 mm | 3.5 mm |
| 5/32" | 3.97 mm | 4 mm |
| 3/16" | 4.76 mm | 5 mm |
| 7/32" | 5.56 mm | 5.5 mm |
| 1/4" | 6.35 mm | 6 mm |
| 9/32" | 7.14 mm | 7 mm |
| 5/16" | 7.94 mm | 8 mm |
| 11/32" | 8.73 mm | 9 mm |
| 3/8" | 9.53 mm | 10 mm |
| 13/32" | 10.32 mm | 10 mm |
| 7/16" | 11.11 mm | 11 mm |
| 15/32" | 11.91 mm | 12 mm |
| 1/2" | 12.70 mm | 13 mm |
| 17/32" | 13.49 mm | 13 mm |
| 9/16" | 14.29 mm | 14 mm |
| 19/32" | 15.08 mm | 15 mm |
| 5/8" | 15.88 mm | 16 mm |
| 21/32" | 16.67 mm | 17 mm |
| 11/16" | 17.46 mm | 17 mm |
| 23/32" | 18.26 mm | 18 mm |
| 3/4" | 19.05 mm | 19 mm |
| 25/32" | 19.84 mm | 20 mm |
| 13/16" | 20.64 mm | 21 mm |
| 27/32" | 21.43 mm | 21 mm |
| 7/8" | 22.23 mm | 22 mm |
| 29/32" | 23.02 mm | 23 mm |
| 15/16" | 23.81 mm | 24 mm |
| 31/32" | 24.61 mm | 25 mm |
| 1" | 25.40 mm | 25 mm |
| 1-1/16" | 26.99 mm | 27 mm |
| 1-1/8" | 28.58 mm | 29 mm |
| 1-3/16" | 30.16 mm | 30 mm |
| 1-1/4" | 31.75 mm | 32 mm |
| 1-5/16" | 33.34 mm | 33 mm |
| 1-3/8" | 34.93 mm | 35 mm |
| 1-7/16" | 36.51 mm | 37 mm |
| 1-1/2" | 38.10 mm | 38 mm |
| 1-9/16" | 39.69 mm | 40 mm |
| 1-5/8" | 41.28 mm | 41 mm |
| 1-11/16" | 42.86 mm | 43 mm |
| 1-3/4" | 44.45 mm | 44 mm |
| 1-13/16" | 46.04 mm | 46 mm |
| 1-7/8" | 47.63 mm | 48 mm |
| 1-15/16" | 49.21 mm | 50 mm |
| 2" | 50.80 mm | 51 mm |
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Most Common
Workshop Tips
3/4" ≈ 19 mm — near perfect
The 3/4" / 19 mm pairing is 0.05 mm out — effectively interchangeable in everyday use.
Use 6-point sockets
On corroded or worn imperial heads, 6-point sockets grip better than 12-point and are less likely to round the flats.
What is AF?
AF = Across Flats. The distance between opposite flat faces of a bolt head — what the spanner grips. Not the same as bolt shank diameter.
Print the PDF
Laminate it and keep it in your toolbox lid or on the workshop wall — saves hunting for the right size mid-job.
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Which Thread Standard Is My Tractor? — UK Farm Machinery by Era
The most common question when working on older machinery is whether the bolts are metric, imperial AF, Whitworth, or something else entirely. The answer depends almost entirely on when the machine was built and where. This guide covers the main eras of UK farm machinery manufacture.
Common Makes & Models
Ferguson TE20, early Fordson Major, David Brown Cropmaster, older Marshall tractors
Spanner to Use
Whitworth AF — NOT the same as imperial AF or metric
Whitworth spanner sizes refer to bolt shank diameter, not across-flats. A "1/2 Whitworth" spanner is larger than a "1/2 AF" spanner. Keep a Whitworth set if working on pre-1960 machinery.
Common Makes & Models
Massey Ferguson 135/165, Ford 3000/4000/5000, David Brown 880/990, Nuffield Universal
Spanner to Use
Imperial AF — 3/8", 1/2", 9/16", 5/8", 3/4" most common
This era is the most confusing. Engine and transmission bolts may be BSF; body and implement attachment points may be imperial AF. Some MF models introduced metric fasteners on later production runs.
Common Makes & Models
MF 290/590, Ford 6600/7600, JCB 3C, Case IH early models, John Deere 1030–2130 (US spec = UNF)
Spanner to Use
Mix of imperial AF and metric — check each fastener
Manufacturers moved to metric at different rates. John Deere US-manufactured tractors retained UNF threads well into the 1980s. European-manufactured models (including JD Mannheim) moved to metric earlier. Hydraulic fittings often remained imperial AF throughout this period.
Common Makes & Models
All modern tractors — MF, New Holland, JCB, Case IH, Claas, Fendt, Valtra, Deutz-Fahr, modern John Deere
Spanner to Use
Metric throughout — M8 (13 mm), M10 (17 mm), M12 (19 mm) most common
Modern machinery is fully metric. The only exceptions are some hydraulic fittings (BSP or JIC remain common) and replacement parts on machines assembled from older tooling.
AF, Whitworth, UNF and UNC — What's the Difference?
These are four different systems that all use imperial measurements but are not interchangeable. Getting them confused is one of the most common causes of stripped threads and broken bolts on older farm machinery.
AF — Across Flats
AF describes the spanner jaw width — the distance between opposite flat faces on a bolt head or nut. Imperial AF sizes are in fractions of an inch. This is NOT the bolt diameter — a "1/2 AF" bolt head sits on a shank that is typically 5/16" or 3/8" in diameter. Modern metric spanners are also measured across flats, just in millimetres. AF spanners are what most people mean when they say "imperial spanners".
BSW / BSF — Whitworth
British Standard Whitworth (BSW) and British Standard Fine (BSF) are older British thread standards common on machinery made before the mid-1960s. Crucially, Whitworth spanner sizes refer to the bolt shank diameter — not the across-flats dimension. A "1/2 Whitworth" spanner is noticeably larger than a "1/2 AF" spanner and they are not interchangeable. Whitworth bolts can be identified by their rounded thread form. You need a dedicated Whitworth spanner set for pre-1960 British machinery.
UNF — Unified National Fine
UNF is an American thread standard that uses imperial AF bolt heads but with a different thread pitch from BSF. UNF threads are common on US-manufactured equipment including older John Deere tractors, some Case IH, and American-spec implements. The bolt heads use the same AF spanner sizes as imperial bolts, so you will not notice the difference with a spanner — but a UNF bolt will not thread into a BSF or metric hole of the same nominal diameter. A thread gauge is the only reliable way to distinguish UNF from BSF.
UNC — Unified National Coarse
UNC is the coarser-pitch version of the Unified thread system, also common on American machinery. UNC has fewer threads per inch than UNF for the same bolt diameter — making it faster to assemble and less prone to cross-threading, but slightly weaker in tension. Like UNF, UNC uses imperial AF head sizes but will not interchange with BSW, BSF or metric threads. On older US farm equipment you may find both UNF and UNC used on the same machine depending on the application.
Practical rule of thumb
Pre-1960s British machinery?
Get a Whitworth set
1960s–1980s UK tractor?
Imperial AF — this chart
Older American equipment?
Imperial AF + thread gauge
Dealing with Rounded or Seized Imperial Bolts
Rounded bolt heads and seized fasteners are inevitable on older farm machinery. Using a slightly-too-large metric socket on an imperial bolt is the most common cause. Here is the correct approach, in order.
Penetrating oil first — always
Apply a quality penetrating oil (Plus Gas, WD-40 Specialist, or Kroil) to the threads and leave for as long as possible — ideally overnight. Heat from a gas torch applied to the surrounding metal (not the bolt) expands the parent metal and breaks the corrosion bond. Never apply heat to zinc-plated or cadmium-plated bolts as the fumes are toxic.
Bolt extractor sockets
Bolt extractor sockets have reverse-spiral or left-hand fluted interiors that bite harder into the rounded head as you apply turning force. They work best on heads with some flats remaining. Fit the extractor with a breaker bar — not an impact wrench — to avoid shattering the extractor on the bolt head.
Mole grips or pipe wrench on the shank
If there is enough of the bolt shank exposed, locking pliers (mole grips) or a pipe wrench applied to the shank can grip better than a socket on a rounded head. This works particularly well on longer bolts where the head has rounded but the shank is accessible.
Left-hand drill bits for flush-broken bolts
If the bolt is sheared flush with the surface, a left-hand spiral drill bit will often extract it — as the drill engages the metal, the reverse rotation frequently unscrews the broken stud before drilling through it. Start with a small pilot hole and work up in diameter. Use cutting oil to keep the bit cool.
Drill out and re-tap as a last resort
Centre-punch the broken bolt accurately, drill through with a standard drill bit leaving a thin wall of bolt material, then use a screw extractor or pick out the remaining thread. Re-tap the hole to the original size, or slightly larger if the thread is damaged, using a tap with cutting oil. On through-holes a Heli-Coil thread insert gives a permanent repair.
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