Valve Spring Tester
- Below is an excerpt from a book I'm working on. It describes a simple
valve spring tester and a method of calibration. The illustrated
How-To article in the HVX files shows a slightly different procedure
in which a stove-bolt is used as the height gauge, adjusted to spec
(31mm for stock VW valve springs) by simply turning it to the required
height. The 'bathroom' scale shown in the HVX article is in fact a
certified 'medical grade' scale accurate to one pound in three hundred
with excellent repeatability. As such, I felt no need to include the
calibration procedure in the article.
Cylinder Heads (General)
A Volkswagen cylinder head contains seventy-seven individual
components, the majority of which are capable of rendering the engine
inoperable should it fail. Some of the components, such as the studs
and the head casting itself are static and not subject to friction but
due to the large number of dynamic components and generally poor valve
train lubrication, they make up a significant portion of the engine's
pumping losses. Since the pumping losses represent the engine's
`overhead,' any reduction in the pumping losses appears as an increase
in the engine's output, usually for no increase in fuel consumption.
By focusing on the details of those pumping losses experience has
shown that it is possible to achieve a significant increase the output
of the engine.
Complex by modern-day standards, where an increasing number of engines
are OHC, despite its high parts-count the VW valve train is reasonably
robust thanks to seventy years of use during which the most
failure-prone components have been identified and re-designed to
improve their durability. That is, durability in vehicular terms.
When compared to features found in aircraft engines, Volkswagen heads
are something of a joke. When the displacement of the `1600' (actual
displacement is 1584cc) is increased, as is common when converting the
engine for use in aircraft, durability takes a further hit.
Fortunately, it takes only a modest amount of effort to improve its
durability by an order of magnitude.
Most of the valve-train durability enhancements are covered in the
so-called HVX modifications, previously posted and discussed.
Although rarely seen on engines built for the Kiddie Trade and not
found on ANY of the VW's converted for flight, the HVX mods have
proven their worth through forty years of use in professionally built,
high-output engines. Most recently, the use of thick-film lubricants
enhance durability even further. (Specific `how-to' information for
applying thick-film lubricants to valve train components will be found
in the chapter on Coatings.)
Valve Train (Springs)
Poppet valves are a one-way sort of creature The cam pushes them
open but they are closed by the action of the valve spring. The
spring needs to be strong enough to close the valve tightly enough to
make a leak-free seal but the valve spring merely initiates the
sealing process. The real sealing is accomplished by the tapered
sealing surface of the valve being wedged into the cone of the valve
seat by the enormous pressure of combustion.
Modern-day valve springs are coiled compression springs installed
around the stem of the valve and connected to it by a retainer that is
free to rotate. The retainer is secured to the stem of the valve by a
pair of keepers in the form of a cylindrical wedge which mates with
grooves machined into the stem of the valve.
The strength of the stock VW valve spring is determined by measuring
the amount of force needed to compress the spring to a height of
31.0mm (~1.220"). A number of factors can effect the strength of a
coil spring and like all other VW specs, the tolerance is quite large,
ranging from 117 to 135 pounds.
The valve's spring must be compressed when the valve is opened. The
energy needed to compress the spring is part of the Otto Cycle's
`pumping losses' and anything that helps reduce those losses will
improve the engine's efficiency. For a low rpm engine the lower
valve spring value is more than enough to ensure proper operation and
since the lower value reduces the pumping losses, it also serves to
improve performance. Further enhancement occurs when the strength of
all eight springs is equal or as nearly so as possible. For those
reasons, a standard practice in any properly built engine is to use a
set of springs that have been closely matched.
Matching a set of valve springs to within a pound or so can be quite
difficult if you're drawing upon used parts. Not only are there
different varieties of VW valve spring, each time a VW engine is
stopped at least two valve springs will be compressed. In a vehicle
that is driven daily this is seldom a problem but in an airplane
engine that may sit for weeks between flights, the compressed spring
is liable to weaken. When doing a valve job on a VW engine modified
for flight, it's a good idea to re-test the valve springs.
Ideally, a new engine or a rebuilt head should include a set of new
valve springs but with the number of registered air cooled Volkswagens
in steady decline, it has become increasingly difficult to locate
quality parts. It isn't uncommon to find after-market VW valve
springs which are not square, in that the ends of the spring are not
perpendicular to their axis. Such springs do not provide a
symmetrical force when compressed and should not be used, an item
mentioned in the factory service manual. You will also find new
springs wound of lighter gauge wire than stock springs and which fail
to provide the required strength when compressed. Springs longer than
stock are also fairly common, often needing excessive pressure to be
compressed to the specified height. Such junk is often advertised as
`racing' equipment, clearly meant for mechanically naive youngsters.
Volkswagen valve springs are progressively-wound, with the coils being
closer together at the bottom than the top. Some after-market springs
are NOT progressively-wound. It pays to inspect all after-market VW
parts BEFORE you buy.
Twenty years ago I would never put used valve springs in an engine.
Nowadays, used stock springs are often better than new, after-market
stuff. If a used spring isn't rusty and shows no signs of fretting or
jamming, I'll go ahead and test them.
New or used, it is extremely risky to use any valve spring without
Valve spring testers are commonly available but even the least
expensive model is several hundred dollars if purchased new.
Fortunately, a common bathroom scale may be used to make your own
spring tester. Unfortunately, inexpensive bathroom scales are not
very accurate. Accuracy - - at least enough for the task at hand --
is assured by calibrating the scale with a mass of known weight, such
as your own body, immediately prior to use. That of course assumes
you know your own weight to within a pound. Balance-beam type scales
tend to be more accurate than low-cost spring-type scales. To
calibrate the valve-spring's scale simply weigh yourself on a
balance-beam scale then adjust the bathroom scale to read the same
If you do not have access to a balance-beam type scale you'll have to
create a test-mass of known weight. Having a specific gravity of
1.00, water is the handiest mass but you'd need at least fifteen
gallons to verify the accuracy of your scale and the container would
introduce some amount of error.
Lead is a very handy mass, having a specific gravity 11.34 times that
of water and if you have a graduated beaker (which is easy enough to
make) it's quite easy to determine the volume of a given lump of lead.
Unfortunately, pure lead is rather rare stuff and since other metals
often make up half the mass of wheel weights and other common lead
alloys, it is impossible to calculate the weight of such alloys based
If you have an accurate scale, such a laboratory type, you can of
course weigh a sample of melted wheel weights, plumber's solder or
other lead alloy, determine it's specific gravity and apply that to
the mass as a whole.
When you are forced to create your own calibration mass without access
to a precision scale you'll probably find plain old fashioned mild
steel to be the best choice. This is because the amount of carbon and
trace elements is typically less than 1%, allowing you to use a
specific gravity of 7.93 or about 495 pounds per cubic foot ( about
4.4833 ounces per cubic inch ).
Since mild steel comes in standard sizes, even when purchased as scrap
you can determine it's weight with good accuracy by simply measuring
the piece, calculating its volume and applying the figures above.
Then too, many scrap yards now use electronic scales accurate to a
fraction of a pound, allowing you to simply buy a test-mass of the
appropriate weight. Of course, being able to calculate the weight is
a handy means of keeping them honest. (Hint: Weigh yourself on the
junkyard's scales. Everyone does :-)
- - - - - - - - - - - - - - -
At one time it was common for EAA chapters to maintain a tool crib and
test-mass for use by its members. The test-mass was usually pigs of
lead-alloy cast in convenient sizes from five to twenty-five pounds,
clearly stamped with their weight after being accurately weighed. The
fact EAA headquarters no longer puts any emphasis on such basic needs
is good evidence of their growing disinterest in supporting
- - - - - - - - - - - - - - -
(I use a mill-end of 6" steel bar as my test mass. It is about 16"
long and weighs 128 lb, 4-3/4 oz).
Volkswagen's valve-spring specification calls for a compression of 117
to 135 pounds at a height of 31mm. I made a tube of this length,
accurately square on each end. The tube stands atop the bathroom
scale on a little pallet to distribute its weight. The scale itself
sits on a plywood base to which a fulcrum has been attached. The
spring being tested is stood inside the tube and a lever is used to
compress it. When the lever touches the tubing I know the spring has
been compressed to a height of 1.220" (ie, 31mm). And I know
precisely WHEN that happens because I've rigged the lever to turn on
an LED when it touches the tube. The LED is taped to the dial of the
bathroom scale; all I have to do is keep my eye on the dial. When the
light comes on I read the dial and jot down the weight on a stick-up.
To eliminate human error each spring is tested at least three times.
Any obvious flyers are thrown out and the testing is repeated until I
have a cluster of similar values.
I try to do forty or fifty valve springs at a time. The first step is
to clean them and inspect each spring visually for scratches or
pitting anything that might serve as a stress-riser. They are then
gauged for total length, then for squareness, both tests done on a
surface plate allowing me to do a handful of springs at a time. Alas,
when dealing with new, after-market springs those two tests may reduce
the batch by half.
Any springs that pass the initial tests are then tested for
compression height. They are then sorted according to their stick-ums
and made up into matching sets, coated with preservative and put aside
until needed. It isn't the Bureau of Standards but it's better than
guess-work, which is what you have if you don't test your springs.
In making up a set of springs for a low rpm engine I want the lowest
strength and the narrowest range. Of the two, I think matching the
range is the most important factor. If I can't make up a set within a
pound or two of a given strength, I'll generally keep looking.
- - - - - - - - - -
Many fail to appreciate the importance of `balance' in an engine. The
reason professionals put so much emphasis on balancing is because the
engine must use power to overcome any imbalance before any usable
power can appear at the crankshaft. That means any imbalance is
effectively multiplied by two. Using springs of equal strength is
part of the balancing process.
- - - - - - - - - -
If you are building just one engine you should try to find someone who
has a valve-spring tester. Baring that, you should cobble up your own
using a bathroom scale.
So what happens if you simply buy a new set of springs and throw them
in? Hopefully, not a lot. There is a chance the set may contain a
spring having a radically different value but with a tolerance of 18
pounds, the odds are the engine is going to run. Sorta :-)
I should also mention that I don't know of a single non-professional
engine-builder who tests their valve springs. This is another of
those details they deem `unimportant.' And when addressed in
isolation, perhaps it is. But a professional engine builder addresses
ALL of those `unimportant' details, picking up a little torque here,
better fuel consumption there, optimizing each unimportant detail for
better efficiency, more power, cooler running and slower wear. No
single one of those unimportant details results in a dramatic change.
But add them all together and it isn't uncommon for a professionally
built engine to produce up to 25% more power than a poorly built
engine of exactly the same displacement. And to last twice as long as
- Here's one for cheap:
I'll bet you can think of other uses for a 0-300 lb load cell.
And you can get stock VW valvesprings from bobsvintagevw cheap ($1 each plus shipping)
He also has cam gears in the various sizes that Bob talks about, but you can't get no more.
- --- In AirVW@yahoogroups.com, "flybynightkarmarepair" <ryoung@l...> wrote:
> Here's one for cheap:
In the article on building a do-it-yourself valve spring tester I
provided catalog photos of K-Line's products including an inexpensive
hydraulic load cell similar to the one Ryan has shown. (See their web
site, which I believe is: www.klineind.com )
Unfortunately these inexpensive testors aren't very practical. The
gauge is typically 2" in diameter or less, having fifty divisions
across 270 degrees of the dial-face covering 300psi. The 6psi
divisions (on the 300lb version) are typically less than 1/8" apart
and the needle is about half that. No matter how carefully you make
your measurements, with that degree of precision the best you can do
is about +/-3lb. Even then, you'll only get that degree of accuracy
if you come up with a method of measuring the compression height,
which should be accurate to within +/- .001"
One way to accomplish this is to use a simple screw-jack (I used the
threaded ram from an old C-clamp), a suitable frame, and an
inexpensive 6" beam-type caliper having a digital read-out (HF has
such things for <$20). Put all that together and you're still left
with a pressure gauge that isn't very precise.
By comparison, the 1-lb divisions on the 8" dial of the
'Health-o-Meter' (brandname) bathroom scale are quite easy to read.
Of course, this assumes you already have such a scale but as I point
out in the article, the pallet & lever arrangement allows you to use
ANY floor-type bathroom scale, including digital types.
As a machinist, I could have presented you with any number of gee-whiz
designs capable of producing laboratory-grade accuracy and precision
and virtually worthless to 99% of you. Instead, I've offered a tester
that, despite it's ungainly looks, can be built by anyone and is
capable of using any bathroom scale that happens to be handy. The
bathroom scale arrangement looks clunky as hell but for the realtively
low spring pressures found in the VW engine it has proven both
accurate and precise.
A couple of closing notes:
Whatever method you use, you'll end up with a better engine because of
it. But don't expect to achieve precise & accurate results without
expending a bit of effort or money. If you'll invest some time in
learning, you will find you can make superbly accurate tools using
commonly available items. (Using only hardware-store grade
components, I've ground a number of mirrors for Newtonian telescopes
whose curvature was accurate to within 1/8 wavelength of blue-green
light... which is slightly smaller than a gnat's ass :-)
Nowadays the most common valve spring testers simply plug into your
computer. Once calibrated their precision & accuracy is impressive,
as is their price. But they are very fast, take absolutely no skill
at all and provide a printed report for each spring tested.
Finally, if you just throw the engine together, it'll still run. But
until you experience a properly built engine -- one in which the
keenest attention has been paid to the smallest detail -- you simply
won't believe the difference.
PS -- Like greed and avarice, the meaning of accuracy and precison are
often confused. With marksmanship, accuracy means hitting the target;
precision has to do with where the hole appears in the paper. Testing
your valve springs -- tossing any spring that is too weak or too
strong -- guarantees you'll hit the target. Matching eight springs to
within a pound or so guarantees you'll hit the X-ring in the center of
- Just to give folks a few more ideas:
If you're having trouble imagining "a pallet and lever arrangement", think Tortilla Press:
Obviously the "gap" should be the proper height for the compressed spring, at least at the
center where the spring is. And you don't need oak for a piece of probably one-shot
Or maybe you need to think like a bottle capper:
Other schemes will suggest themelves. Or you could send Bob a letter, and get ALL the
Me, I plan to use my ChinCom arbor press, which I use for riveting spars, and a million
If you're having trouble imagining a "suitable frame", for making a press/spring/scale
sandwich, look down the page at the one used for flanging dies:
I've got something similar pegged together out of dimensional lumber, since I don't weld,
and didn't have a bedframe handy to stove bolt together.
Soon, I'll have enough springs to get a decent population, and sort out two sets of 8
matched ones. Why two sets? I plan on having a spare set of heads, so when it's time for
a valve job, I can keep on flying - if I ever get there in the first place.
William Wynne posted something very telling over at http://flycorvair.com today:
"Interestingly, you probably don't recognize the above builders as day to day names on the
Net. I've found that many of the most productive builders keep a low profile. Many of the
most prolific contributors to discussion groups are building at a comparatively modest
Ryan "Extremely Modest Pace" Young
--- In AirVW@yahoogroups.com, "robertshoover" <veeduber@i...> wrote:
> --- In AirVW@yahoogroups.com, "flybynightkarmarepair" <ryoung@l...> wrote:
> > Here's one for cheap:
> > http://tinyurl.com/bct3c
> Unfortunately these inexpensive testors aren't very practical.
> One way to accomplish this is to use a simple screw-jack (I used the
> threaded ram from an old C-clamp), a suitable frame
> the pallet & lever arrangement allows you to use
> ANY floor-type bathroom scale, including digital types.