Article by Mick Collins:
The following is an introduction to a
suggested method for making small piston rings, based on articles by
Prof. Chaddock and Tom Walshaw (Tubal Cain), but I must admit that
I've NOT yet attempted the final machining operation.
In these explanatory notes the
following abbreviations will be used:- D = cylinder bore. t = radial
thickness of ring.
w = axial width of ring. p = wall pressure. Fdd = force required to
close ring, applied at 90 deg's to the gap, and
" " = actual quotations from the articles.
In an article in the Model Engineer
(21/4/67 p.396) Professor Chaddock describes an efficient 5cc Four
Stroke I/C engine, built for an attempt on the model aero plane duration record and he details his method of making piston rings by
"The rings are turned to a diameter D + 0.002" and a
thickness not more than 1/25 or less than 1/30 of the bore diameter.
The modern tendency is to make width equal to or even less than
their thickness so the same rule can be applied to width. The ring
must now be cut with the thinnest possible saw or (preferred method)
broken by holding in the fingers and snapping it. A tiny gag piece,
of width t x 4, is now inserted into the gap to expand the ring
before clamping it, with several others if required, between two
metal plates by a center bolt and heating the whole assembly to
annealing temperature in a gas flame or muffle."
Professor Chaddock does not give the temperature required but says
that the practical test is that if, after cooling, the ring springs
inwards from the t x 4 gap then it is not fully relieved of stress
and the process must be repeated. (but see later) He says:-
"Finishing the rings once they have been "heat
formed" can follow conventional practice; that is, they can be
closed in a tube and clamped between mandrel plates ------ for
finishing the periphery and the sides lapped freehand on stone or
emery paper. Very little, one or two thou' only need to be taken off
in these final operations to bring them dead to size. -- they hold
compression as well or better than a ground and lapped piston and
He doesn't mention any 'working' gap being required and the rings he
actually made were of 0.025" x 0.025" cross section to fit
an 18 mm (0.709") bore.
The essay by Tom Walshaw (Tubal Cain)
in the SMEE Journal (Dec 1992) is largely concerned with designing
rings with the correct wall pressure for their particular
application and he refers to an article by Michael Smart (M.E.
16/8/1974 p.824) in which is explained how a properly fitted ring is
expanded by the pressure behind it to augment the initial wall
Tom's conclusions being that the majority of model engines were
running with excessive wall pressures and he included a table of the
wall pressures he had measured on a selection of commercially made
He then describes a simple method of
measuring it by closing the ring to its bore diameter with a force,
Fdd, applied at diametrically opposite points, 90 deg's from the
gap. Then :-
p = (Fdd x 0.881)/(w x D)
An expression for calculating p is
p = g x E/7.06 x D x (D/t - 1)cubed
Where g = difference between the free and closed gaps, ins. and E =
Young's Modulus, lbf/sq.in.
"Values of E can vary between 12
million to 24 million lbf/sq,in. and reference to the supplier is
advisable. A figure of
18 million is typical for 20-22 ton UTS, and between 15 and 17
million for the 17 ton UTS centrifugally cast iron normally
available for home-made rings."
"To complete this exploration of
the arithmetic, the stress in the ring can be calculated in the
usual way from the bending moment - - - - - and simplified to:-
f = 3 x p x (D/t)squared lbf/sq.in."
After a long dissertation on model
piston ring practice in which Tom draws on every article he could
find in the model engineering press he discusses the criteria for
design and comes to the following conclusions:-
"Wall pressure for models (a)
Steam Plant. It is clear that the wall pressures used in our models
are higher than need be, even with the plain Ramsbottom type; still
more so with modern heat formed rings. For stationary, marine and
road locomotive engines, all of which can be motored for initial
bedding down, 6-8 lbf/sq.in. should be adequate. However, there is a
small problem; the radial thickness is so small with
"classical" gaps that parting off may be difficult.* For
this reason I set a minimum value of "t" at 0.035",
and use a smaller gap when necessary. Fixing on g = D/10 instead of
4 x t simplifies the pressure formula to:-
D/t = cube root of E/(70.6 x p) + 1
but this ratio should not be used for
wall pressures above about 12 lbf/sq.in. without checking the
"fitting" stress. (see later)
For rail locos it may be prudent to use slightly higher pressures.
The low end of the "Chaddock standard" provides about 13
lbf/sq.in. with "E" at 17 million. (i.e. D/t = 30 and g =
4 x t), but this will vary pro rata with the value of E."
*It isn't - see method.
"(b) I.C. Engines. For the
classical horizontal gas/petrol engine model I have substituted
rings at 6 1/2 lbf/sq.in. with no problems, but where the designer
has called for two rings I have used three. This pressure lies
outside the "Chaddock standard" (t = D/33-35) so that,
again ring gaps of D/10 are used. Many published designs use rings
which are too wide, and I suggest w = 0.03 x B as a guide, with a
minimum of 0.04".
For all other I.C. engines it is no
surprise to find that adherence to Prof. Chaddock's rules will give
satisfaction, though at the high end (D/25) it is important to check
stresses, as the stresses are higher than the usually available 17
ton iron can carry.
For the benefit of those not familiar with these rules the figures
Radial thickness, t = D/25 to D/30;
ring gap, g = 4 x t.
If "E" is 17 million lbf/sq.in.
these rules offer wall pressures from 28 down to 13 lbf/sq.in. The
wall pressure will, of course, vary with the value of E in direct
proportion. The lower the wall pressure the less friction, of
For very high performance engines it
goes without saying that experiment is always necessary, and
although it may add to the time and cost a single-cylinder
prototype, arranged for measuring both oil consumption and blow-by
(as well as output and fuel consumption) is well worth while. Even
here I would not expect to find more than about 25 lbf/sq.in. to be
necessary and that only when a single pressure ring is supported by
a stepped scraper between the top ring and the main oil
(a) Running gap" After an illustrated explanation of the
comparative insignificance of this gap, Tom recommends:-
"The minimum gap - for steam or
I.C. - should be 0.002" and a guide might well be an installed
gap equal to
0.001" + 0.001"/inch of cylinder bore."
"(b) Fitting gap. This is the
dimension G when the ring is sprung over the OD of the piston when
fitting. If this is excessive the ring may be overstressed, but it
is the dimension G - g which is significant, so that, as already
remarked, the risk of overstressing increases as the free gap is
reduced. Unfortunately "the books" all seem to assume that
the ring clasps the piston closely when fitting but this is not the
case, and stresses based on this assumption will be too low.
Geometric analysis is almost impossible, as the ring does not assume
the shape of a pair of half-circles, and in any case the actual
direction of the loads holding the ring apart are indeterminate.
Experiments with a number of rings of various D/t ratios and values
of D show that G varies from 6.6t to 7.5t. If g = 4 as under the
Chaddock rules, then the ring will not be overstressed when
installed - provided the working stress is safe, of course. As a
very rough approximation, the installing stress can be estimated by
fi = fw x (7 x t - g)/g Where fi =
installing stress, fw = max. working stress, t and g as before.
This estimate is by no means exact,
but a check on the "risk" can be made by comparing the
value of "g" before and after a trial fitting. If there is
a marked and permanent increase in "g" then the ring is
very near the limit."
The original "Ramsbottom"
rings were plain circles from which a gap was cut so that the closed
ring fitted the cylinder with no more than a working gap. It was realized
that such a ring would not fit properly, and so could not
exert an even pressure, even after hours of running. The necessary
shape to achieve this was known, of course, and requires the free
ring to have a radius at any point which varies as the sine of the
angle of the section from a point directly opposite from the gap.
Lanchester devised a machine which would turn rings to this shape,
but it is doubtful whether any model engineer would undertake to
make one. However, with modern NC machines the process is much
easier, and many large engine rings are so manufactured."
Tom then describes how the inside of
a plain ring can be peened to produce the required effect - and
considers that it is too difficult with a small ring.
He next considers a tapered ring, produced by boring the ID
eccentric to the OD, but rules this out as, to get the correct
characteristics, it requires "t" to be reduced to zero at
Then "A near perfect ring can be
achieved by "heat forming", a process which is adaptable
to high volume production, and which can be used for very small
rings indeed. Here, a circular ring of uniform thickness is cut with
a very small gap. It is then forced into or onto a shaped former,
and stress relieved so that, when cooled, the ring is of the correct
form to provide both true circularity and a uniform wall pressure
when fitted into the cylinder, though in almost all cases a final
machining operation is carried out to "skim" the O.D. to
allow for the inevitable tolerances. The shape of the former is, of
course, the obverse of the shape used by Lanchester years ago.
It is an adaptation of this method which was described by Prof.
Chaddock ----------- "
"Unfortunately there seems to
have been a misunderstanding of the nature of the process by some,
including Mr Trimble and Mr Tulloch. First, the process is NOT a
copy of that used in industry and cannot form the
"perfect" ring. Whilst it relies on the relationship in
expression (2), in that the wedge exerts the "tangential
force" there referred to, the process does not and cannot
produce exactly the correct shape, This force may induce the correct
bending moment in the ring, but it does not reproduce the correct
deflection, for the tangential force introduces an additional
compressive stress. This is small but has a devastating effect on
the shape of the rings adjacent to the gap. " etc (see
"There is no way of correcting
the stresses by altering the nature of the wedge (e.g. by applying
the force at an angle instead of tangentially) and the fact that the
wedge may fall out after clamping up the parcel of rings makes no
difference. nor is there much point in "fitting" the wedge
to the angle of the gap. However, As Prof. Chaddock realized, the
effect can be mitigated by carrying out a final skimming operation
on the O.D. after heat treatment. This ensures true circularity and
reduces the deviation from the uniform pressure condition to
negligible proportions. This final machining operation is essential.
However it need be no more than a skim, and the removal of
0.001" of metal should suffice for a 1" ring and pro rata
for larger sizes. For models true circularity is more important than
a uniform pressure."
It appears there has also been some
misunderstanding about the stress relieving process, possibly
because the original article also referred to "annealing".
Most subsequent writers have quoted a "good red heat",
though Mr Trimble quotes an actual figure of 800degs/c. as does Mr
Tulloch. This is a mistake, 800degs/c. or "good red " lies
above the critical temperature and a metallurgical change will
occur. The Brinell hardness will be reduced as will the U.T.S. and
the value ore and some grain growth will occur - just the wrong
requirements for a piston ring. If the material is an alloyed iron
the results may be even more serious. There is the further fact that
scaling may be caused. It is not unlikely that the use of this high
temperature has resulted in users going to stiffer rings than were
needed, simply because the heat treatment caused a reduction in the
value of "E" with consequent loss of wall pressure.
The "correct" temperature
is 480 - 520degs/c. with slow heating, the temperature being held
for 1 hour per inch of thickness but with at least 10 minutes for
very thin rings. The stack may be air cooled from this temperature,
though no harm seems to arise from oil quenching. The metal has no color
at this temperature, but Messrs Levermore, 24 Endeavour Way,
London, SW19 8UH are importers of "Markall Thermomelt"
crayons. A mark with one of these will turn glossy at the indicated
temperature and they are available from 100degs/c. up to 1200degs/c.
Alternatively, very little degradation of properties will result
from heating to 550 - 600degs/c. when the metal will be just visible
in a dim light, but on no account must the temperature be allowed to
rise any higher. (The critical temperature is 720degs/c.) It is
preferable to use the lower temperature for the full time rather
than to try to speed things up by going higher. Incidentally,
scaling at these temperatures is minimal - it will come off with
Tom also devotes a couple of pages to
oil control and scraper rings before concluding that a stepped
scraper with a wall pressure of between 20 and 30lbs/sq.in. and with
adequate oil escape holes should be adequate - and that provided the
plugs don't oil up, it is wiser to tolerate a high oil consumption
in a model. (Dennis Chaddock doesn't use any on his engine.)
Rings of bronze are mentioned fairly
often, generally for use in gunmetal cylinders. I have had little
experience of these for models, but have no doubt that they can be
satisfactory. However, certain points should be born in mind. First,
the working stress must be kept below the yield (or 0.1% proof)
figure - typically about 20tonf/sq.in. - especially during
installation. Young's Modulus is of the same order as for iron - 15
million lbf/sq.in. Second, the stress relieving temperature lies
very close to the annealing temperature, and great care is needed
when heat forming. On no account must 350degs/c. be exceeded and a
temperature of 300degs/c. should be aimed at. This is the
temperature at which bright steel turns blue - a useful guide!"
Tom's final conclusions are that for steam engines and for the
classical model I.C. gas/petrol engines, which can be run in on the
bench 6 - 8 lbf/sq.in. should be adequate. Locomotive rings may need
higher pressures to speed bedding down, but no more than 12 lbf/sq.in.
Lower pressures should be possible with properly formed rings, with
consequent freer running. The workhorse type of I.C. engine may need
16 - 18 lbf/sq.in., unless the duty is severe, but even then the
"Chaddock rules" should not be exceeded; 20 - 22lbf/sq.in.
is recommended as a maximum, but experiment may be needed at speeds
over 12,000 rpm.
Finally, and to forestall any
questions, I must add that Tom did not quote any 'worked examples'
for rings designed to fit these conclusions.
Suggested Method for making Small
First, finish the cylinder bore - to diameter D.
Decide the cross section of ring
required and, if you are using a conventional, non-demountable,
piston then finish it, making the ring grooves deep enough to give
0.004" clearance behind the rings.
Make the steel sleeve for the
machining fixture, bored to D+0.002" and approx D long.
Chuck a length of centrifugally cast
iron and drill it to within 1/16" of the inside diameter of the
rings for sufficient depth. (see next)
Turn outside diameter to D+0.002" (use sleeve as a gauge) for a
length equal to:- (w + parting tool width) x number of rings
required plus a few spares.
Use a narrow parting tool to make a series of grooves. Depth of
grooves to be exactly equal to t + 0.001". Spacing to be
exactly w + parting tool width + 0.001" (lapping allowance).
Now use a sharp fine boring tool to open up the hole to inside ring
diameter. As you approach this, reduce the cut to 0.001" and
'lean' on the tool to prevent it cutting 'on the way out'. When you
reach the final cut you will be rewarded by a little bunch of rings
on the neck of the tool.
Use a Swiss file to break the sharp
corners inside the rings so that they will move freely to the bottom
of their grooves.
Make a tiny nick on the inside of each ring with a very fine
triangular needle file and break it between finger and thumb, or by
using the thumb to press it down onto a piece of wire on a flat
surface. Very carefully dress the broken surfaces with a Swiss file
(No 6 cut)
Make the wedge to hold the gaps open.
Stack the wedged rings around a bolt,
clamp them between two steel plates and heat evenly to 550 - 600
degs/c (900 - 1,000 degs/F) , i.e. just visible in the dark, NO
HOTTER! Hold at this temperature for 10 mins and then allow to cool
- rings should not have scaled. (I've done this successfully on an
electric cooker hot-plate, which can be set to the correct
temperature first and then the rings laid on it and left for 30
Next make the machining
Pack the rings into the sleeve and
slide the sleeve on to the rod. Fit the disc and clamp rings
securely with the nut. Slide off the sleeve and use the engine
cylinder as a gauge to take a one thou' cut off the rings and reduce
their diameter to D.
Finally, lap the sides of the rings
to fit their grooves. They should be perfectly free but with a
clearance of only 0.001" or less.
This drawing is copied from the one accompanying Tom's article.
Prof. Chaddock's drawing showed the D - 0.002" dimension as D,
i.e. the required ring diameter, and also only a single ring clamped
I've successfully made, and used,
C.I. rings down to 1/4" x 0.010" x 0.010" (accepting
25% breakages) using this method but must repeat that I've not
attempted the final skimming operation. Examination of the ring on
my Stuart 10, after very heavy use, shows a perfectly even polish,
but near microscopic examination of the two 0.016" square rings
on my miniature marine engine, after only a few hours running, shows
minute high spots at the gaps with a few degrees of darker metal
extending round from them. Nevertheless, compression/performance is
excellent with very low friction - several thousand rpm at 15 psi..
Since writing the foregoing, I must
admit that I've had serious misgivings about my ability to machine
the last thou' from one of my miniature rings. No problem with a
tool post grinder or rings where 't' is over 30 thou's but for
really small rings, and with tools of dubious accuracy, I would
prefer a 'safer' method.
I have therefore taken the liberty of amending the Chaddock/Walshaw
fixture to make the Ring I/D minus 0.010" diameter into a
spigot, fitting closely into a register machined in the clamping
disc: an arrangement that I would find easier to make with the
required accuracy, and I would also keep to Prof. Chaddock's
illustration and mount the rings singly.
The requirement for extreme accuracy when chucking the fixture could
be overcome by making it the last object to be machined before
machining the rings - and leaving it in the chuck.
However my own preferred method would be to make the fixture from
silver steel, harden it before use and (purists need read no
further!) take off the last thou' with a fine diamond file; using
the fixture as a pair of 'filing buttons' and with the lathe running
at about 200 rpm.
It would still require caution, these files would cut the narrow
ring very rapidly but with the much larger area of the fixture to
act as both a witness and a check, it shouldn't be too difficult to
stop at exactly the right size.
The above article is used on this
site with the permission of Mick Collins at:- firstname.lastname@example.org