Conversions
One inch is exactly 25.4 mm. Everything below follows from that, so these tables are exact rather than rounded off a chart.
Metric sizes read as almost-inch
A 50 mm rod measures 1.9685". On a rule that is 2 inches, and a 2 inch kit will not seal it. If a reading sits a few thousandths under a round inch figure, check the machine before you order.
Metric sizes and their inch near-misses
The bore and rod sizes that actually turn up on metric cylinders, with the fraction a rule would give you and how far off it is. Anything in the last column over about 5 thousandths is the difference between a kit that seals and one that does not.
| Metric | Exact inches | Nearest 1/16 | As decimal | Off by (thou) |
|---|---|---|---|---|
| 16 mm | 0.6299" | 10/16" | 0.6250" | 4.9 |
| 20 mm | 0.7874" | 13/16" | 0.8125" | 25.1 |
| 25 mm | 0.9843" | 16/16" | 1.0000" | 15.7 |
| 28 mm | 1.1024" | 18/16" | 1.1250" | 22.6 |
| 30 mm | 1.1811" | 19/16" | 1.1875" | 6.4 |
| 32 mm | 1.2598" | 20/16" | 1.2500" | 9.8 |
| 35 mm | 1.3780" | 22/16" | 1.3750" | 3.0 |
| 40 mm | 1.5748" | 25/16" | 1.5625" | 12.3 |
| 45 mm | 1.7717" | 28/16" | 1.7500" | 21.7 |
| 50 mm | 1.9685" | 31/16" | 1.9375" | 31.0 |
| 55 mm | 2.1654" | 35/16" | 2.1875" | 22.1 |
| 60 mm | 2.3622" | 38/16" | 2.3750" | 12.8 |
| 63 mm | 2.4803" | 40/16" | 2.5000" | 19.7 |
| 70 mm | 2.7559" | 44/16" | 2.7500" | 5.9 |
| 75 mm | 2.9528" | 47/16" | 2.9375" | 15.3 |
| 80 mm | 3.1496" | 50/16" | 3.1250" | 24.6 |
| 90 mm | 3.5433" | 57/16" | 3.5625" | 19.2 |
| 100 mm | 3.9370" | 63/16" | 3.9375" | 0.5 |
| 110 mm | 4.3307" | 69/16" | 4.3125" | 18.2 |
| 125 mm | 4.9213" | 79/16" | 4.9375" | 16.2 |
Fractions to decimals and millimetres
Every 64th of an inch. Seal sizes are quoted in fractions on inch catalogues and in millimetres on metric ones, and this is the bridge.
| Fraction | Decimal | mm |
|---|---|---|
| 1/64 | 0.015625 | 0.397 |
| 1/32 | 0.03125 | 0.794 |
| 3/64 | 0.046875 | 1.191 |
| 1/16 | 0.0625 | 1.587 |
| 5/64 | 0.078125 | 1.984 |
| 3/32 | 0.09375 | 2.381 |
| 7/64 | 0.109375 | 2.778 |
| 1/8 | 0.125 | 3.175 |
| 9/64 | 0.140625 | 3.572 |
| 5/32 | 0.15625 | 3.969 |
| 11/64 | 0.171875 | 4.366 |
| 3/16 | 0.1875 | 4.762 |
| 13/64 | 0.203125 | 5.159 |
| 7/32 | 0.21875 | 5.556 |
| 15/64 | 0.234375 | 5.953 |
| 1/4 | 0.25 | 6.350 |
| 17/64 | 0.265625 | 6.747 |
| 9/32 | 0.28125 | 7.144 |
| 19/64 | 0.296875 | 7.541 |
| 5/16 | 0.3125 | 7.938 |
| 21/64 | 0.328125 | 8.334 |
| 11/32 | 0.34375 | 8.731 |
| 23/64 | 0.359375 | 9.128 |
| 3/8 | 0.375 | 9.525 |
| 25/64 | 0.390625 | 9.922 |
| 13/32 | 0.40625 | 10.319 |
| 27/64 | 0.421875 | 10.716 |
| 7/16 | 0.4375 | 11.112 |
| 29/64 | 0.453125 | 11.509 |
| 15/32 | 0.46875 | 11.906 |
| 31/64 | 0.484375 | 12.303 |
| 1/2 | 0.5 | 12.700 |
| Fraction | Decimal | mm |
|---|---|---|
| 33/64 | 0.515625 | 13.097 |
| 17/32 | 0.53125 | 13.494 |
| 35/64 | 0.546875 | 13.891 |
| 9/16 | 0.5625 | 14.287 |
| 37/64 | 0.578125 | 14.684 |
| 19/32 | 0.59375 | 15.081 |
| 39/64 | 0.609375 | 15.478 |
| 5/8 | 0.625 | 15.875 |
| 41/64 | 0.640625 | 16.272 |
| 21/32 | 0.65625 | 16.669 |
| 43/64 | 0.671875 | 17.066 |
| 11/16 | 0.6875 | 17.462 |
| 45/64 | 0.703125 | 17.859 |
| 23/32 | 0.71875 | 18.256 |
| 47/64 | 0.734375 | 18.653 |
| 3/4 | 0.75 | 19.050 |
| 49/64 | 0.765625 | 19.447 |
| 25/32 | 0.78125 | 19.844 |
| 51/64 | 0.796875 | 20.241 |
| 13/16 | 0.8125 | 20.637 |
| 53/64 | 0.828125 | 21.034 |
| 27/32 | 0.84375 | 21.431 |
| 55/64 | 0.859375 | 21.828 |
| 7/8 | 0.875 | 22.225 |
| 57/64 | 0.890625 | 22.622 |
| 29/32 | 0.90625 | 23.019 |
| 59/64 | 0.921875 | 23.416 |
| 15/16 | 0.9375 | 23.813 |
| 61/64 | 0.953125 | 24.209 |
| 31/32 | 0.96875 | 24.606 |
| 63/64 | 0.984375 | 25.003 |
| 1/1 | 1 | 25.400 |
Rules of thumb
- 1 mm is just under .040". A millimetre out is ten times the tolerance a seal cares about.
- To go from millimetres to inches, divide by 25.4. The other way, multiply.
- A "thou" is a thousandth of an inch, 0.0254 mm. Seal glands are specified in thou.
- Kubota and Komatsu are metric throughout. Bobcat mixes metric and inch on the same machine, so measure both ends.
See also: measuring a cylinder and gland dimensions.