Balancing

The bubble shivers in the bullseye of the level as I tap the slider of
the Ohaus triple-beam scale with the eraser-end of a pencil. I'm on
my knees, kneeling before the work-bench in a sweltering shop, both
fans turned off, doors closed, my eye aligned with the scale's
pointer, parallax eliminated by a second alignment mark twelve inches
behind the scale and at exactly the same height, givertake a thousandth
of an inch.

Another tap and the pointer finally aligns. The piston weighs 444.2
grams, making it the lightest of the set, the others tipping the scale
at 446.3, 446.6 and 448.1.

There's 28 grams to an ounce, according to Miss Rose Saghetti, my
Fourth Grade teacher. It's actually 28.3497 but the Fourth Grade was
nearly sixty years ago and an error of 1.2% is allowed. Indeed, the
spread of 3.9 grams across a set of cast 94mm Mahle pistons is allowed,
the VW spec being five grams. (Okay, ten for repair parts.) And if
close enough is good enough, you'd go ahead and slap the thing
together. After all, we're talking about a measly one-seventh of an
ounce, fer crysakes. What's the big deal about all this balancing
stuff?

In the case above, about 1.6 hp, at the rpm you swing a propeller.
More, if you wanna spin it faster. Then too, that means you have to
burn an additional 1.6 horsepower's-worth of gas to balance the
books. That is, if you want to off-set the 'unimportant' 1.6
hp-loss resulting from the imbalanced mass of your jugs. (So what
happens to that 3.2hp? It appears as additional heat, friction and
fuel consumption. None of it appears as torque.)

- - - - - - - - - - - - - - - - - - - -

If you've got a lathe and know how to twirl the knobs, and if
you've built a big-bore stroker or two, you'll already have a set
of heavy copper collars for your 4-jaw chuck, each sized to accept a
particular diameter of piston, along with a matching 'test-plug'
that looks suspiciously like an old piston of that diameter, cut down
to leave just the crown. The test plug has a small but distinct
center-punch. To build a good engine I must precisely align that punch
mark with the lathe's axis of rotation.

Four-jaw goes onto the lathe, the collar goes onto the four-jaw and the
test-plug goes into the collar. You set up your wiggler on the tool
post and adjust the four-jaw until the wiggler isn't wiggling any
more. I've got a 12" lathe and my wiggler happens to be sixteen
inches long with a ratio of 4:1 between the tail stock - - which is the
'true point' I want to match - - to the center-punch on the
test-plug. Blip the switch, the chuck begins to spin and the wiggler
becomes a blur of motion indicating the center of the collar is not
aligned with the center of rotation. So you adjust it, loosening one
jaw and tightening another, nudging the center into truth as you rock
the chuck back & forth with your hand. Once you've got it right, you
know. No need for any measurements because the wiggle vanishes when
the disparity between the alignment of the centers becomes something
less than a thousandth of an inch, more than close enough for the task
at hand. Which is to shave precise amounts of metal from inside the
three heavy pistons.

The wiggler goes back in the tool-box and is replaced by a tool holder.
A curiously shaped cutting bit is mounted in the tool holder. Nothing
fancy, just a hunka 3/8" square tool steel ground to a shape that
allows me to reach inside the skirt of a piston and make a nice clean
cut about a quarter of an inch wide. The depth of the cut is based on
experience, in that advancing the tool so many thousandths of an inch
will remove so many grams of metal.

To make the cut I use a dial indicator clamped to the bed of the lathe.
The saddle - - the thing on which the tool post is mounted - - butts
up against the plunger of the dial-indicator allowing me to measure the
depth of the cut in thousandths of an inch as I gently advance the
carriage. Each jug has to be zero'd of course but the collar puts me
within striking distance and a piece of Zig-Zag cigarette paper used as
a feeler gauge tells me when I'm there. Once I've zero'd -n each
piston, a bit of arithmetic tells me how deep of a cut I need for that
particular piston and the dial indicator tells me when I'm there.

Overall, balancing a set of pistons is about as difficult as making a
good pot of coffee. And while it may sound hi-tech my particular
method isn't all that precise. My lathe is almost as old as I am and
despite having rebuilt the thing a few years ago its repeatability
isn't that good.

I chuck the jugs, do the math, twirl the knobs and when I'm all done I
end up with a spread of two-tenths across the three jugs: 444.1, 444.2
and 444.3. (The original 444.2 sits aloof atop the big red carton,
lording it over his fatter cousins.) I take a die grinder to the heavy
jug and bring it down to 444.2 and studiously ignore the 444.1gm
piston. I've reduced the imbalance from 3.9 grams to 0.1grams and
decide that's good enough. I know from experience that I can spend an
hour or more chasing that last tenth of a gram and I've already spent
an hour on this batch of jugs and have two more sets to balance before
supper time.

- - - - - - - - - - - -

Although balanced, the pistons are not ready for assembly. The crowns
will get a zirconium-based ceramic-metallic coating that serves as a
heat barrier. The insides of the pistons get coated with a thermal
dispersant and the skirts get a coating of moly-based solid lubricant
to combat scuffing. The process will increase their mass by a couple
of grams but their balance usually remains unchanged. If adjustment is
needed, it's done with a die grinder, taking a tad of metal from the
balancing pads inside the skirts.

The balancing and the Thermal Barrier Coatings are 'unimportant'
details of course - - no one builds engines that way, other than me and
a few other fools you'll meet at the finish line. But having built
several hundred engines in my life I've found the aggregation of such
unimportant details to be difference between a smooth-running,
long-wearing reliable engine and the other kind. And a point most tend
to forget is that it isn't the fastest car (or plane) that wins,
it's the one who finishes first.

- - - - - - - - - - - - - -

Which is jus' swell if you happen to have a shop full of tools and
pick your teeth with a micrometer.

The truth is, even if you don't have all that stuff - - even if
you've never built an engine in your life - - you can still build
yourself a better engine than you can buy. The reason for that
apparent conundrum is that while you may not be able to achieve a tenth
of a gram spread across a set of four jugs, using nothing more
sophisticated than a Dremel tool and a reasonably accurate gram-scale
you can sure as hell do better than four grams.

Although the imbalance in this case was a scant four grams I've seen
a spread of sixteen grams from Mahle and more than twice that - - more
than an ounce of imbalance - - in sets of jugs from other makers. The
bigger the imbalance, the greater the losses... and the shorter the
service-life of the engine. If you only reduce the imbalance by half
you'll still have a better engine.

So you buy one of those electronic scales, the kind that are only
accurate to two grams. Then you remove the rings, wash the pistons,
dry them good and weigh them. The lightest piston becomes your gauge,
the one whose weight you want to match. You chuck a coarse rotary file
bit into your Dremel or hobby tool or even into your quarter-inch
drill, pick up the heaviest jug and start removing metal from the
balancing pads or ribs. No mystery there because the jug will already
have been 'balanced' at the factory and you can see where they
removed some metal. Take your metal from the same areas.
Periodically, you weigh the thing. When you get close to your goal you
wash the piston in solvent to get rid of all the tiny metal particles
you've been throwing around, and weigh it again. When you get to
within about a gram of your goal, stop. You've done well enough.
Your set of pistons is now better balanced than it was.

- - - - - - - - - - - - - -

Across a set of four, the individual piston pins and the set of rings
will usually weigh within a few tenths of each other. Any major
imbalance is usually the fault of the piston pin and can be adjusted by
a bit of judicious grinding on the interior of the pin. An alternative
is to return the pins to their bores and weigh them with the pistons,
adjusting their combined mass by removing metal only from the piston
(which isn't always possible, hence the need to know how to lighten a
pin).

- - - - - - - - - - - - - -

After-market VW pistons & cylinders come complete with rings, making
each P&C a unique set. That means you have to make sure the same rings
go back onto the same piston and into the same jug. Because you've got
to take them apart. As received, the bores have not been cleaned and
rings are often clotted with Cosmoline. So the first thing you do when
you receive a carton of pistons & cylinders is to apply 'work
marks.' That's a fancy name for numbering them, one through four.
But you need to mark them in a manner that will not be obscured by
paint (on the jugs) nor coatings (on the pistons). I use a file or die
grinder to cut notches in the top fin of the cylinder, over on the flat
side. The same number is put inside the skirt of the matching piston
using indentations (gently!) or an electric scriber. Since coating the
crown of the piston will obscure the arrow indicating the off-set, I
orient the piston so that my marks can be used to show the off-set at
assembly time.

To keep track of your rings and the pin, you put them in baggies with a
note showing the work-number of their piston. Since the geometry of
the rings tends to vary I often include a sketch showing the
orientation of the top rings. Now you can take them apart with the
reasonable assurance things will be returned to their proper place at
assembly time.

- - - - - - - - - - - - - - -

I'm not set up for dynamic balancing. The crankshaft and any
concentric part attached to it is sent to a balancing shop where it
gets spun-up & balanced as a complete assembly. But static balancing
is different. It doesn't take much to do pistons & rods. In fact,
the way gas prices are going it would cost me almost as much to pick up
& deliver the parts as it does to have them done by a professional
balancer. (You'll need a support fixture for the rods since you must
also adjust their center of mass, a chore usually referred to as
'big-end vs little-end' balancing.)

If you aren't into Thermal Barrier Coatings there's shops that are,
some of which are familiar with aircooled Volkswagens.

- - - - - - - - - - - - -

The hand-maiden of mass balance is volumetric balance and the reward is
equally profound. Adjust the chambers of your heads to within a
fraction of a cubic centimeter and the specific impulse - - the amount
of power produced by each cylinder - - will be more uniform. Because
when it isn't - - when one jug is producing less power than the
others - - it represents a pumping loss that must be made up from the
output of the other cylinders before any usable torque can appear in
the crankshaft. This situation is similar to the losses that result
from mass-imbalance because the power needed to overcome the imbalance
literally doubles the loss.

Fortunately, volumetric balancing is just as easy as balancing your
pistons and the same principle applies, in that any improvement will
result in a better engine.

-R.S.Hoover

wrote
... (A great article.)

Wow.

Could you explain more about the thermal dispersant that goes
inside of the pistons?

--

FF

Mr. Hoover,
I very much enjoy your posts. Thank you.

Dan Horton

wrote:

Could you explain more about the thermal dispersant that goes
inside of the pistons?
----------------------------------------------------------------------

I'm using TechLine coatings. I suggest you go to the source:

http://www.techlinecoatings.com

The code for the thermal dispersant is 'TLTD.'

Cast aluminum heads for the Type I VW engine show a 5% to 7%
improvement in heat transfer (depending on the core temp) when they are
blasted with coarse media and treated with TLTD. (I also apply it to
the push-rod tubes, valve covers and the floor of the valve gallery.)
It has to be baked on and unless you're into coatings (or want to
devote a couple of years to it) it's best to send your parts to one of
the shops certified by TechLine.

Although coatings are slightly miraculous don't expect miracles from
using just TLTD. It works best in conjunction with a thermal barrier
coating on the other side of whatever you've applied it to. The TBC
reduces the amount of heat that get into the part; the TLTD helps get
rid of any that does. Net result is that you can keep the core
temperature of the combustion process hotter, longer, meaning you'll
see a higher BMEP (and more torque because of it) but without
exceeding the engines inherent thermal limitations.

Alas, there's no such thing as a free lunch :-) The waste heat that
didn't go into your heads & pistons has to go somewhere... which is
into your exhaust and the oil coming from the valve galleries. The HVX
mods (see the archives of the AirVW Group) address the first issue
while insulating the stacks (Black Satin, Cerama-Chrome, etc.) takes
care of the second.

-R.S.Hoover