CC'ing, CR'ing and BS'ing
Most folks arrive at this point not because they are building an engine but
because they are repairing one. Doing a valve job on a VW engine alters the
chamber volume. Or they've dropped a valve and have had to replace one of the
heads. Replacement or repair of a head alters the chamber volume and chamber
volume is a factor in the compression-ratio equation. Before they can
re-assemble the engine they have to measure the chamber volume and re-compute
the CR to ensure the repair work has not upset the compression ratio.
Measuring chamber volume is easy to do and the procedure is all the real
service manuals for the VW engine. But just to keep all the information in one
place, I'll give you a quick sketch of the procedure.
Using the catheter-type syringe and a sealing disk similar to those shown in
the photos, you should be able to measure the volume of a Volkswagen combustion
chamber to within 2cc. That is, if you measure it a multiple number of times
your results should not vary from the AVERAGE result by more than plus or minus
1cc. (As a general rule, always measure each chamber at least three times.)
Herez how: Install a spark plug in the head and torque to spec. If the valves
are not perfectly fluid-tight, pop them out and put a light smear of Vaseline
on their sealing surfaces. Level the head in both directions then smear a
light coat of Vaseline around the periphery of the sealing disk and drop it
into the chamber. If the sealing disk is 3/8" or thicker it will be heavy
enough to stay in place by itself (the specific gravity of cast acrylic sheet
is only 1.19). For thinner sealing disks you'll need to hold them in place
while filling the chamber or add a few ounces of weight to their edges.
The disk(s) doesn't have to be perfectly round. You can saw the thing out with
a jig saw, smooth the edge with a file and you've kept it to within +/- sixty
thou or so, it will work perfectly well. Nor do you need five holes in the
disk as shown in the factory workshop manual; you can do a quick check with
Using a suitable fluid, fill the chamber and the hole through the sealing disk.
Subtract the volume of the hole from the total. Do both chambers. For OHV
engines I think you'll find ‘wetter' water does a better job than the
traditional kerosene. This is because you need to measure the chamber several
times and average the result and that means you have to remove ALL of the fluid
from the chamber. With a flat-head engine it's pretty easy to simply wipe the
chamber dry but with an OHV the kerosene tends to cling to all the nooks &
crannies. So use water with a drop of wetting agent as your fluid. When it
comes time to dry the chamber, simply dump it out, slosh it with a bit of
alcohol then BLOW it dry. Alcohol is hygroscopic - - it will mix with any
remaining water and once mixed with alcohol it takes only a modest blast of air
to dry even the deep recess around the spark plug's insulator.
A quick check using the syringe shown in the photos will allow you to determine
the volume of your combustion chambers to within 2cc (i.e., plus or minus 1cc).
To compute your compression ratio use the LARGEST volume you measured as the
factor in the CR equation.
That's it. If you're a shade-tree mechanic you're all done. Good luck in the
CC'ing Tools & Equipment
WHY ARE WE DOING THIS?
Truth is, some of you aren't. Although this article was written to explain how
to improve your engine's efficiency by adjusting it's volumetric balance, many
builders are interested only in determining their compression ratio. If that's
all you're here for, drop down to MEASURING CHAMBER VOLUME.
By applying modern-day standards of balancing, of both mass and volume, you can
pick up as many as ten free horsepower from a bone stock VW engine. And I mean
‘free' in the sense that it will use the same amount of gas. That seeming
impossibility reflects the fact the engine must first overcome any internal
imbalance before it can deliver any usable power. That means an imbalance
generating a loss of five horsepower costs an additional five horsepower to
balance the books, resulting in a net loss of ten horsepower. This is a key
factor in the importance of balancing and one most people overlook. That is,
for each imbalance you eliminate you will gain approximately TWICE that amount
Of even more importance to builders of flying Volkswagens is the fact that a
properly balanced engine is more durable and the reduction in wear is even more
pronounced than the improvement in fuel efficiency. That's because most of the
internal imbalances appeared in the form of friction and heat, which could only
be overcome by generating MORE friction and heat. The bottom line is that a
3hp imbalance could cost you up to four times that in additional wear.
Eliminate the imbalances and the engine's wear-factor takes a dramatic drop.
The benefits of mass-balancing became self-evident as the normal operating
speed of internal combustion engines was increased but the role of volumetric
balance was not fully appreciated until computers came along. Once they had a
super-computer to play with automotive engineers discovered that a lot of what
we thought we knew about the process of combustion was not entirely correct,
that there were transient phenomenons due to volumetric imbalances that we'd
attributed to other causes. This is one of those non-intuitive kinds of
things, not easily explained without a bit of background but the basic reason
has to do with the process of combustion and the fact those seemingly
insignificant differences in volume appear on both sides of the compression
ratio equation. Turns out, relatively small variations in volumetric balance
can produce some relatively large losses of power. Since that time they've
taken exquisite pains to reduce those imbalances. How this is accomplished in
a mass-production environment is quite interesting (at least, to me :-) but the
bottom line is that we can take advantage of what they've learned by simply
paying more attention to the volumetric balance of the engines we build.
Thanks to computerized equipment a modern balance shop can do mass-balancing to
a fraction of a gram at a very reasonable price. But matching the volume of a
pair of heads (i.e., four chambers) to a tenth of a cubic centimeter remains
more art than science. In the racing world it isn't uncommon to pay a thousand
dollars for a pair of full-trick heads, their volumes accurate to 0.1cc across
all four chambers. The cost isn't in the tooling; it doesn't take a lot of
tools for head-work. The money goes to buy the head-man's time. That fact is
in our favor because time is about the only resource of which lo-buck
homebuilders have a surplus. By acquiring a few tools and devoting a bit of
time to your heads, you can bootstrap yourself into a better engine than you
can afford to buy.
At this point a majority of non-mechanic home-builders are sitting there with a
large black question-mark floating in the air over their heads. If adjusting
chamber volume is more art than science, how can they hope to tackle the job
The answer is so simple it will make you smile. ANYTHING you do to reduce the
magnitude of the existing imbalance will improve the efficiency and durability
of your engine. The fact the pro's regularly balance to a tenth of a cubic
centimeter is like running the four minute mile. But we're just a bunch of
week-end joggers so let's not even think about that degree of precision. At
least, for now. Instead, let's look at the basic problem.
You have four chambers. You measure them (the procedure is outlined below) and
come up with four values, probably accurate to within half a cc or so. Using
real numbers now... two heads measured 56.5, 58, 58, and 60cc. Rather than
trying to turn you into a head-flow guru capable of achieving 0.1cc accuracy on
a repeatable basis the question becomes: "Can we reduce that 3.5cc spread to
The answer is a decided ‘Yes!' And it really isn't all that difficult
because you only have to work on THREE of the chambers. That is, you can't
make a chamber any SMALLER so the largest chamber becomes your target-size.
So relax. You CAN do this. And you'll end up with a better engine because of
Then comes the question, how much metal are we actually talking about? Just
what the hell IS a ‘cubic centimeter'? The answer to both: Not much.
If you've been following the Practice Wing project you should have some
high-density urethane foam on hand. Print out the ONE_CC drawing, glue it to a
piece of foam then use a razor blade to make a cube the size of the printed
square. That is, a cube 0.3937" on a side. No foam? Then make the cube out
of wood. Or plastic. Or tool steel (if you can). The goal is to give you a
tangible reference to the volume of a cubic centimeter.
Hold a cubic centimeter in your hand and three things are immediately apparent:
Firstly, it isn't very large. Just a tad over 3/8" on a side. Secondly, you
can appreciate the heroics it takes to achieve repeatable tenth-cc accuracy.
And finally, you can see why a lot of people consider such imbalances to be
insignificant. To the uninitiated, something that small has to be unimportant;
something they can ignore.
Got some sixteenth-inch ply? Okay, howz about some poster board a sixteenth of
an inch thick. Or a piece of .063 aluminum. Three-eighths wide, six inches
long, you're looking at about one cc. Now go back and look at what we're
trying to do. To balance the 56.5cc chamber we need to remove three and a half
cc's of material. Just unshrouding the valves is usually good for that volume.
So how do we do all that? I'll get into that in a minute but it all begins by
measuring the volume of your combustion chambers.
MEASURING CHAMBER VOLUME
If you've never cc'd your chambers, relax; it isn't difficult nor do you need a
lot of equipment.
My first exposure to cc'ing heads was in the mid-1950's. I bolted a piece of
plex to the freshly milled head of a Ford V-8, leveled the thing up with wedges
and used a turkey baster to fill each chamber with kerosene. After doing both
heads I plugged the SMALLEST volume into the compression-ratio equation and
came up with a CR of about 11:1, assuming I used a sixty-thou gasket. Life was
good, assuming I could swipe enough avgas for a couple of runs through the
Determining the volume of the combustion chambers is a chore common to all
engines and the basic procedure is the same for all. The procedure requires:
1. Some means of holding the head level.
2. Some means of sealing the space to be measured.
3. A method of accurately measuring the amount of fluid needed to fill the
Working with nothing more than a 1/4" drill motor and a couple of carbon steel
rotary files, cleaning up the marks with a strip of sandpaper spiraled around
the split end of a quarter-inch dowel, you can expect to produce a cc-job
accurate to about 0.5cc across all four heads. Indeed, the usual problem when
a novice does his first set of heads is going too far, opening up the smaller
jugs to a volume larger than the target-jug. Which means they have to take a
bit out of the big chamber, which usually leads the them chasing the volume
back & forth until they get sick of it.
SAME PRINCIPLE, DIFFERENT PROCEDURE
The secret to hitting your chamber volume dead-on is exactly the same as for
hand-cutting a piece of aluminum to a precise dimension in that you don't shoot
for the exact dimension. You always give yourself a clean-up allowance.
Cutting aluminum by hand you snip or saw the thing about forty-thou over-size
then dress the edge with a file. You do much the same when cc'ing your
chambers in that you use the rotary files to get close to the finished size
then begin smoothing things up with abrasives, checking the volume
I mention this now because the next subject has to do with the precision of
your cc'ing job and the fact balancing your chambers to a fraction of a cc
might take you a couple of weeks (!) if you've never done it before, whereas
balancing them to 1cc (across all four) should take only a few hours. If you
don't want to spend the time then there's no need to spend the money for an
accurate liquid dispenser, such as the one shown in the photos, which is
graduated to 0.1cc. Turkey baster or the big syringe, you can work down to
1.0cc with fair accuracy. And there's a BIG difference in price between a
surplus syringe and a certified-accurate, lab-grade burette.
All engines get their chambers cc'd, not just Volkswagens. The cc'ing of
combustion chambers is illustrated and talked about in the various manuals on
engine building. The three key elements mentioned above reflect the basic core
knowledge for any cc'ing job. Then comes a host of details; the How-To stuff,
most of which is engine-specific but some of the common factors address the
accuracy of the finished job, the degree of precision, how long it takes and
various convenience factors. As with most other aspects of proper engine
fabrication, cc'ing your heads calls for a keen attention to detail. Most of
The point is that you really don't need much to do a quick check.
Lots of after-market retailers will sell you a ‘cc kit.' It usually consists
of a plastic syringe like the one shown in the pictures (see the Fly5kfiles
archive) plus a plastic disk with a hole in the middle.
What's it cost? Hard to believe but some outfits want as much as thirty bucks
for a cc kit.
See the syringe in the photos? It comes from a veterinary supply house. New,
quantity one, you're looking at three or four dollars. But syringes have a
certain shelf life and once they're past it, the supply house often flogs the
stuff off as surplus, out-dated or whatever. Buy them a dozen at a time, the
syringe shown cost about half a dollar.
The plastic sealing disk is any sort of flat plastic, eighth of an inch thick
or more. Doesn't have to be clear. The sealing disks I use for 77mm jugs are
made from a hunk of red Plexiglas I dug out of the scrap box at a plastics
retailer. In a pinch I've even cast my own using polyester resin on a sheet of
waxed glass. It only has to be flat on one side and just clear enough so you
can chase the bubbles. Chasing the bubbles is one of those little details no
one mentions :-) The bitter truth is that some bubbles don't like to be chased
and you can spend the best years of your life trying to coax the thing over to
The sealing disk in the typical kit has just a single hole in the middle.
Cheap, easy to make and so forth. But harder to use than one with more holes.
More holes, you don't have to chase the bubbles so far but more importantly,
with more holes there's a better chance you'll be working with a truly LEVEL
head. And that's worth mentioning because a lot of after-market heads,
especially rebuilts, the sealing surface is NOT parallel to the valve gallery
gasket rail, which is what you use to sit the head on.
So you sit the head on the gasket rail (after taken the studs & valve stems
into account) and use your machinist's level or framing level or whatever on
the chamber-side of the head to use wedges and so forth to get everything
perfectly level, end-to-end and fore & aft.
That's no guarantee the CHAMBER is level. But if your sealing disk has a
pattern of holes as shown in the drawing (and in the VW factory workshop
manual), as the chamber fills the level of the fluid in the various holes tells
you if the chamber is level.
You subtract the volume of the holes, by the way. Figure it out ahead of time,
scribe it right onto the sealing disk. You can use the same equation as for
V1, just substitute hole diameter for bore and the thickness of the plastic for
stroke. Then multiply that by however many holes you have and be sure to
always fill the hole to precisely the same level.
Buy a cc kit, it usually comes with only one disk for each size of cylinder.
Indeed, some outfits only sell you ONE disk, period, screwing you to the cash
register if you want two disks or a set of a different size. Which is another
example of how VW after-market retailers screw the kiddies, because to do the
job properly you really should do both chambers at the same time... and that
means two disks of each size.
Making disks is pretty simple, assuming you understand plastic. Drills for use
with plastic looks like a needle :-) We usta call them ‘canopy bits' or
‘Plex bits.' Included angle of something like 30 degrees. If you don't have
a set of bits for plastic and don't know how to sharpen a drill bit, you'll
have to use a hand-drill. Even then, you're liable to crack it when the bit
breaks through the other side. That's when it grabs the flutes. Plastic bit,
you needle a pilot hole then flip it over, finish it from the other side.
Slow speed, always. Kerosene makes a good lubricant for thicker stock.
Although the sealing disk drawing I've posted shows dimensions to thousandths
of an inch that's just the math. You can cut one out with a jig saw and it'll
work just fine. That's because the sealing LIP inside the combustion chamber
is at least a tenth of an inch wide. So long as the disk is close to a true
circle and makes good contact with that lip, it will work okay. (Too big?
Then sand the edge.)
Once you understand the purpose of the sealing disk you'll see them all around
you, just waiting to be cut out. See that translucent plastic over the
waterproof light in the bathroom? That'll work. You can still see the bubble.
Got some curvy scraps from an a blown canopy? That'll work. Just put it on a
heavy plate of polished aluminum, pop it in the oven set at about 300 and let
it flow out flat.
Want to keep your sealing disks nice & pretty? Got some old 5-1/4" floppy
disks? Cut one open, throw away the disk, store your sealing disks in the
To make your chamber water-tight you smear a little Vaseline on the valve seats
and a light wipe around the outer-most edge of the plastic disk. Then you fill
the syringe to a precise level and use it to fill the chamber. Subtract what's
left in the syringe from whatever you started with, then subtract the volume of
the hole(s) in the plate and that's the volume of your combustion chamber,
accurate to however accurate YOU are and as precise as the divisions on your
- - - - - - - - - - - - - - -
On the other hand... there's lots of lab equipment scaled to 0.1cc increments
but most of it lacks the volume to fill a chamber. VW combustion chamber volume
may be as small as 40cc or as large as 80cc, depending the size of your valves
and the displacement of the engine. A burette large enough to hold one
chamber's-worth of juice AND accurately marked to 0.1cc precision, you're
talking about an expensive piece of goods. The high price of the Good Stuff
comes as a shock to guys building just a single engine, often causing them to
settle for half a loaf, using a turkey baster of whatever.
So use a marble or two. And a smaller but more precisely marked burette.
Got a marble? There's a free one in every can of rattle-can paint. (That's
the rattle :-) Or you can buy a bag of them at the dime store. Or use steel
ball bearings (but remember they rust). I like ball bearings because of their
uniformity. Marbles vary by quite a bit.
So what's the volume of your marble? Half fill your burette, bring the
meniscus to the line, make a note, then drop your marble into the burette.
Make another note then do the match. (You may shout ‘Eureka!' if you wish
By adding objects of known volume to your combustion chamber you can use a
smaller dispenser marked to 0.1cc graduations. Just be sure to keep the
numbers straight. (KEEP GOOD NOTES!).
When trying to achieve the smallest possible difference across all four jugs,
statistics are your friend. Make all of your measurements a number of times
and then average them. This helps to reduce the human error, a factor that can
be deadly for the first-timer. Without experience you can't appreciate the
significance of seemingly minor differences. Do everything several times,
throw out the high and the low then averaging the remainder tends to reduce any
errors of observation.
Work with good light. Don't be shy about using a reading glass to inspect the
meniscus in the sealing plate holes and your burette. When taking a series of
measures on the same chamber(s) always be sure to start with it perfectly DRY.
That means you'll need an air blast to blow out the spark plug (which should be
torqued to spec on a new washer).
Once the sealing disk is in place, it don't wanna leave :-) Use a dental tool
or bend a tiny hook on the end of a piece of wire to lift it out using one of
the holes. Pour the water out of the chambers, wipe them down with a clean
towel, blow the plug dry. Before blowing them out I swap the chambers with
lacquer thinner or MEK to remove any oily residue that might prevent the fluid
from fully wetting the surface.
Traditionally, the liquid used for cc'ing heads was kerosene. (I donno... it
just was.) Working with VW's I've heard guys advocate the use of everything
from anti-freeze to ATF and big dummy that I am, I tried them all.
Hell of a mess.
Light oil of any kind is a contaminant in the shop and can be a fire hazard.
And glycol, Prestone or what-have-you is poison. In the mid-70's I started
using ‘wetter' water for cc'ing and found it gave more consistent results and
a lot less mess. How do you make water wetter? Originally, I added a couple
drops of ‘Photo Flow,' darkroom stuff that prevents water spots on your
negatives. But for cc'ing heads, a couple of drops of liquid detergent per
gallon of water has about the same effect. A drop or two of food coloring will
make it easier to see in the burette. Try to work in a room-temperature
environment. The temperature of your measuring fluid and the heads should
ALWAYS be the same. (Keep the liquid in the shop with the heads. You'll need
to do rough spot-checks as the works progresses. I'll have more to say about
that in a minute.)
I normally use a burette similar to the one in the photos. You'll notice that
it's attached to a standard that bolts to the base, which is a plywood box
filled with concrete. To attach the burette to the standard, fancy name for a
scrap of plywood, I twisted some welding rod around a bolt a little smaller in
diameter than the burette so the resulting hair-pin-looking piece would grip
the burette. To fasten it to the standard I drilled a pair of holes, poked the
legs of the hair pins through the holes, bent them over and covered the bent
ends with a slather of Bondo. (You can see all this in the pictures.)
The base is heavy to prevent it from tipping over and breaking the burette. It
is leveled with shims or screws then the burette is OVER-filled using a pitcher
and funnel. To dispense the liquid I use a length of surgical rubber tubing
(drug stores carry it) fitted with the glass tip from an eye-dropper. Flow is
controlled by a clamp on the rubber hose. Once the burette is over-filled I
bring it to the line by releasing the clamp and returning the fluid to the
pitcher or bucket. To fill the chamber I bring the fluid level to the base of
the sealing disk then chase out any bubbles by tapping on the disk with my
fingernail or the handle of a dental tool. Once all the bubbles are accounted
for I fill the central hole a drop at a time until all of the holes are at
precisely the same. Then I read the burette, hunkering up or down to eliminate
Measuring a chamber takes me less time than it took you to read the paragraph
above. Using two disks allows me to do both chambers with one set-up.
Then I do it over again.
To determine the finished volume, as when I'm down to the point of making
fractional cc adjustments, I measure each chamber as many times as I think I
have to. Sometimes I'll get three identical measurements in a row on both
chambers. And sometimes I'll do it half a dozen times and get a different
measurement each time, usually when I'm tired or whatever. (People wonder why
I don't answer the phone or throw rocks at them when they arrive unannounced.
Now you know :-)
That's how I do it when I'm balancing the volume of the heads, trying to
achieve 1cc or better across all four. When I'm just WORKING on the heads, I
generally use a syringe. It's only accurate to 2cc but it's a lot faster and
when you are first opening up the heads 2cc accuracy is more than enough.
- - - - - - - - - - - - - - - - - - - - - - - - - - -
When building a big-bore engine, cc'ing your heads begins long before you get
around to calculating your compression ratio. In fact, it starts when you
first acquire the heads. If they're new, with valves already installed, just
install a spark plug, grease them up and do a rough measure of volume.
Otherwise, rough in the valves then install a plug. Level them up, select the
appropriate sealing disk (if you haven't opened them up they'll be some other
size) and use a baster or syringe to get some idea of their as-cast volume.
It's handy to write that volume directly onto the head using a crayon or wax
pencil. The heads should already have been documented and assigned a work
number and a documentation package (sounds complicated; actually, each head is
just a page in a notebook.) A lot of folks think that's overkill for just one
engine but during the course of the work you must have some means of
identifying the chambers and even one engine has four of those suckers in two
identical heads. And if it's a really GOOD engine, you'll make up a set of
spare heads, identical to the first set. Having a spare set of heads on-hand
allows you to swap heads when necessary and do the valve work at your
When you open up your heads for bigger jugs you are trying compress the swept
volume of the larger jug into the original compression space. There's no doubt
the compression ratio will go UP. The big question, is how much? If the only
thing that changed was the swept volume and the deck clearance was kept the
same, going from 1600cc to 1834 is going to raise your CR by about one point.
But that's the myth that leads to blown engines because NOTHING remains
unchanged. When you open up the heads even the slightest skim cut will reduce
the chamber volume. Another part of the myth is that most guys don't know what
their original compression ratio was to begin with. Later model engines ran
7.5 or 7.7 (the Export crate engine from the Puebla plant is 7.7) depending on
the heads but if it's been overhauled a few times you could be using heads that
have been flycut. Bottom line: It isn't uncommon to see 1834's with a CR of
9:1 or more.
Opening up your heads for bigger jugs offers the opportunity to unshround the
valves. Unshrouding the valves results in a profound increase in flow-rate for
the same amount of lift. The procedure is covered in Bill Fisher's "How to
Hot-rod Volkswagen Engines" but I'll try to insert a couple of photos to show
you what I'm talking about.
To unshroud your valves you have to remove metal from the chamber. Any time
you remove metal from the chamber you need to cc the chambers to make sure they
all end up the same size. This is normally done several times as the work
Okay, so you're using a syringe and a sealing plate with one hole and you've
never done it before and you think my talk about attention to detail is all
bullshit so you squirt them just once and come up four measurements ranging
from 55cc to 60cc. Now what do you do.
What you do is go back and do it again :-)
When you're satisfied that your measurements are as accurate as they're going
to get, the next step is to decide on a strategy for INCREASING the volume of
the small chambers to bring them closer to the volume of the largest chamber.
By this time you should have some idea as to the physical size of one cubic
centimeter and your measurements have told you how much each of the three
smallest chambers must be opened up to match the volume of the largest. The
tricky bit is WHERE to remove the metal from.
Basic starting point is unshroud the valves using rotary files. Then do a
quick syringe-check. You want to STOP with the filing when you get to with 1
cc of your goal.
If unshrouding the valves does not give you enough volume then you begin
‘laying-down' the edge of the combustion chamber. Here again, start with
your rotary files. Try to visualize the amount of metal you want to remove.
Remove NEARLY that amount then CHECK IT. When you get to within 1cc, stop
using rotary files and shift to using abrasive rolls.
The principle behind smoothing things with abrasives is to simply remove the
tool marks from previous steps. That means using a tool (or abrasive) that
will leave SMALLER marks. So you start with coarse sandpaper and when you've
removed all of the marks of the rotary files you'll be left with the marks made
by the sandpaper. So you shift to a FINER grade of sandpaper and do it all
over again. I generally use 80, 120 & 220. After 220 I shift to felt hobs and
A key point here is that I never try to hit the target volume dead-on, I only
try to get CLOSE to it and always on the high side. A second point is the need
to periodically check the chamber volume as the work progresses... and to check
it with an increasing degree of accuracy. That is, a syringe or turkey ba