Subject: o-rings metric vs english
Author: stephen brenton
Date: Jul 20, 2001, 7:46 AM
Post ID: 1707577428
As one would expect there are industry standards for o-rings and as it
turns out they seem to recognize the need to satisfy metric needs as
well as english. When it comes to the size of an o-ring it is
sufficient to completely describe an o-ring with ID (inside diameter) of
the ring and CS (cross section diameter of the cord). When we use these
sizes we are actually using nominal dimensions, similar to saying a
framing stud in building construction is a 2 x 4.
O-ring cord is only manufactured in a limited number of standard cross
section diameters. I'm sure there are special cases which require a
size outside of the standards, but most designers and engineers try to
use what is available off-the-shelf.
The standard metric cross sections are 2, 2-1/2, 3, 3-1/2, 4, 6 and 8mm.
The actual cross sectional diameter for a 2mm o-ring and a 1/16" o-ring
is .070 +/- .007 inch.
The list below shows sizes that we will commonly encounter in working
with motorcycles:
Nominal CS Actual CS Nominal metric
1/16" .070 +/-.007" 2mm
3/32" .093 +/-.007" 2-1/2mm
.103 +/-.007" 3mm
1/8" .125 +/-.007" 3-1/2mm
.139 +/-.007 4mm
What does all this mean?
Well if I have an o-ring that measures about .070" it could be a 1/16"
or a 2mm (nominal) o-ring. From CS alone I can't distinguish it as a
metric or english.
Inside diameter follows the same pattern. O-rings have a nominal ID
which is "close" for description purposes but different from the
"actual" ID. For example a 3/8"(nominal) o-ring has an actual ID of
.364" or 9.25mm. There are tolerances applicable to this actual ID
which adds even more complexity because there are different "classes",
similar to threads on a bolt. A class I o-ring of this size (3/8) has a
tolerance of +/-.005"(.13mm); a class II has tolerance of
+/-.007"(.18mm).
A 3/8 nominal ID o-ring would be a reasonable choice to seal a 10mm
cylinder stud. Maybe there is a 10mm nominal ID metric o-ring
available, but it's not listed in the supplier catalogs I have. So if I
choose to use a 3/8 o-ring then the only decision is the CS size. Good
choices are 1/16 or 3/32, which coincide with 2 or 2-1/2mm.
I'm going with 3/8ID x 1/16CS.
I'll let you know how it works out.
stephen in sc.