SAE vs. Metric Sizes
Fraction to Metric

I'm sure you sit around on your day off converting fractions to metric numbers just for fun. No? You don't? I'm shocked! So who cares that American standard tool sizes do not exactly fit metric nuts & bolts? They'll work, right? No, they won't. If you want to work on a non-American bike, you will need to be familiar with metric sizes.

If you are the type of mechanic who uses a butter knife to remove screws because the tool box is all the way over there stop reading. I can't help you.

Standard American tools are often called SAE after the Society of Automotive Engineers who developed the size standards in the early 1920's.  SAE tools are measured in fractions of one inch.

Machines designed outside the US use the metric system for measurements.  Metric tools are measured in millimeters.  Japanese, British, European, etc cycles will have all metric bolts.  Do not use standard sockets on metric bolts.  

If you are new to bikes or just getting started in their repair, the chart below will help estimate of the size tool needed, if you are not familiar with metric sizes.  If a metric bolt head looks like a 5/16" it is probably an 8mm.  Mechanics should invest in a good micrometer for their toolbox.  A model you can switch between Std & metric measurements.  A tape measure will not get you by for long.  Also, they are useful if you need to do a valve adjustment.

This chart lists fractions and their decimal and millimeter equivalents.  The millimeter measurement column is not rounded up or down.  We have only included common sizes here.

Fraction Decimal Millimeter
1/16 .0625 1.588
1/8 .125 3.175
3/16 .1875 4.763
1/4 .250 6.350
5/16 .3125 7.938
3/8 .375 9.525
7/16 .4375 11.113
1/2 .500 12.700
9/16 .5625 14.288
5/8 .625 15.875
11/16 .6875 17.463
3/4 .750 19.050
13/16 .8125 20.638
7/8 .875 22.225
15/16 .9375 23.813
1 1.00 25.4mm

Most common metric sizes.

Return from Fraction to Metric to Need to Know page.

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