|Information courtesy of Larry Witter
Classic definition of a Stovebolt----A threaded fastener used in the
assembly of wood and fossil fueled heating devices.
Racing definition of a Stovebolt----Any of a class of inline 6 cylinder,
overhead valve car and truck engines manufactured by Chevrolet and
GMC from the early 1930's to 1962.
So how did these engines get this somewhat derogatory name? It is this
authors theory that the flathead folks took one look at them and saw a
bunch of sheet metal (valve cover, lifter cover and timing cover). How
do you secure sheet metal? With stovebolts.
In the late 1920's the Chevys had little 4 cylinder engines as did most
of their passenger car competitors, mainly Fords. In 1932 Henry Ford
knocked the automotive world on its ear with his venerable V8. In
1934 Chevy responded with an inline 6 but not a great one. It had
about 200 ci displacement, a long 4 in. stroke and only three main
bearings. It was cheap and reliable but not very powerful. The long
stroke and very long crankshaft limited the extent of “souping up” that
could be done and maintain any reliability. Poured babbit rod and main
bearings along with a rather primitive lubricating system were also
prohibitive. For these reasons the performance tuners mostly ignored
the Chevys. Henrys V8s ruled!
In 1937, as the public demanded more power and reliability, Chevy
revamped the basic Stovebolt with a new head layout, four main
bearings, a shortened 3.750" stroke and a generally beefed up block. A
3.5 in. bore gave it a total displacement of 216.5 cu. in. 1948 saw the
introduction of precision insert bearings but the babbitted rods and
weird lubrication system were still in place. In 1950 the Hi-torque truck
engine was introduced in the Powerglide transmission equipped cars. It
was the same basic engine but with a higher block casting, thicker
cylinder walls, 3-9/16" bore and 3-15/16 stroke for a total piston
displacement of 235.5 cu. in. 1953 saw the final iteration of the 235
Stovebolt with precision insert rod and main bearings and full pressure
lubrication system which remained in place until 1963. Watch out
Henry, we’s a-comin’!
to be continued...
Let’s begin by taking a look at the pre - 1953 Chevy engine weak point; the
lubrication system. It consists of 10 - 15 lbs. pressure to the mains, a throttled
pressure to the rockers and a jet - scoop pickup for the rods. This is better
known as the “splash” system. It works ok for engines under 120 horsepower
and crank speeds under 4000 rpm. It is NOT, however, suitable for modified all-
out racing conditions. My first and last attempt at using this system hand-
grenaded in the first turn at Morris.
If you are building a full race engine and using a pre - 1953 block it is advised
that you make some modifications to the lube system. The first step is to
replace the “splash” system with full pressure to the rods. To do this the
crankshaft must be “rifle” drilled from the main journals to the rod end journals
and the main journals must be grooved. Remove the dipper plumbing from pan,
the dippers from the rods and plug the dipper holes in the rod caps. Remove the
pressure regulator from the left side of the block and make appropriate
connections to bring full oil pump pressure into the main oil galley. It is
recommended that a GMC 228 oil pump be used for its greater volume and built
in 60 lb. regulator. Replace the pressed in soft plugs at the ends of the main oil
galley with 3/8" pipe plugs. (The soft plugs won’t stand the pressure).
The best solution to this problem, of course, is to start with a 1953 or later
engine if the rules allow. You get the full pressure lube system, full insert
bearings (rods and mains) and better availability of parts. With this in mind this
writing will be based on these later model
Now lets review what it is we are trying to do. We want to convert the energy in
the fuel we burn to rotational torque and horsepower. Lots of torque and horse
power! To do this we must make our engine pump air. The more air it can pump
the more fuel it can burn. So we have to build the biggest and most efficient air
pump possible within the constraints of the Stovebolt engine geometry. The
size of the engine/air pump is measured in cubic inch displacement (CID) or the
volume displaced by the pistons as they move up and down. To increase this
volume we must lengthen the distance the piston moves (Stroke) and/or
increase the diameter of the pistons (Bore). Also keep in mind that increasing
the CID of the engine is one of the few modifications that will increase the
torque and horsepower over its full R.P.M. range, not just the high end.
As stated above the 1953 Stovebolts have a bore of 3-9/16" and stroke of 3-
15/16" for a total displacement of 235.5 CID. Increasing the stroke (Stroking),
although a viable method on the Ford Flathead, is not recommended for the
Chevy 235. Internal clearances between rod and cylinder base greatly restrict
stroking. It has been done but is quite expensive and limits reliability. The
small gain in CID just isn’t worth it. However, since the CID is proportional to
the mathematical square of the bore, increasing the bore (Boring) yields a much
higher gain in CID than stroking and is highly recommended.
So how much bigger can we make the bore? The block can be over- bored 1/8"
to a diameter of 3-11/16" increasing the displacement to 255 CID. However, the
block must be closely inspected for bore alignment with the casting. Detroit
gets a little lax when casting and machining the blocks so if you want to bore
this big, start with a well aligned block. Otherwise you may make the cylinder
walls too thin. Also keep in mind that you have to find pistons of this size to fit
the bores. Good luck!
A good over-bore is .060". This raises the displacement to 244 CID and makes
the engine competitive with the Watertown and Waterloo 260 Flatheads of the
era and still retains reliability. These .060-over pistons are still available as
stock replacement parts and special racing equipment.
It’s time now to talk about the Stovebolts heart and soul ---- The cylinder head.
It is the one assembly that allows the Stovebolt to compete with the Flathead
power-wise even with a smaller CID than the Flathead. The secret? Better
volumetric efficiency (breathing).
We know we must pack as much fuel-air mixture (charge) into the cylinders as
we can and remove the burnt gases (exhaust) efficiently to get power and the
engines ability to breath greatly influences this. Better breathing, more power.
One of the big factors in breathing is the flow path of the intake charge from
the valves into the cylinders. The Stovebolt overhead valve design eliminates
the Flatheads rugged, sharp turbulence-producing 180 degree turn from the
valve port into the cylinders. (See illustration) Turbulence within the
combustion chamber is desirable but NOT in the flow path.
Since many of the Stovebolts power-enhancing tricks directly or indirectly
involve the cylinder head we will take a close look at it. It has three intake
ports and four exhaust ports. Each intake port feeds two cylinders as do two of
the exhaust ports. The other two exhaust ports service the end cylinders (#1 &
The combustion chamber design is a little strange. It is basically flat around the
intake valve but deeply recessed around the exhaust valve. The spark plug is
located in this recess just under the exhaust valve. The intake valve sets at a
slight angle with the head surface and the exhaust valve sets at a greater angle.
This chamber design creates good turbulence and efficient charge burning but is
a little hard to deal with when trying to raise the compression ratio.
The two performance items we can improve with head modifications are the
breathing and the overall compression ratio. We will tackle the breathing first.
Fact: In the stock head, the location of the intake and exhaust valves within
the combustion chamber enhances gas flow through the chamber over the
entire piston power cycle without resorting to super-wild cam timings.
Fact: The stock intake valve heads on the 235 engines are 1.94" in diameter,
second largest in the industry at the time. We can use them just as they are.
Fact: The stock exhaust valves are 1.5" in diameter. Making them larger won’t
gain much. It is, however, recommended that they be replaced with the heavy-
duty nickle steel GMC 270 or 302 exhaust valves for durability. They fit with
Enlarging the ports between intake manifold and the intake valves (porting)
definitely helps the breathing, especially at the higher RPM’s and is
recommended. When doing this porting keep in mind that the wall castings in
the head are quite thin and going beyond the suggested limits is dangerous. I
speak from experience. My first porting job ended with an engine that pumped
coolant water out the exhaust due to heat cracks under four of the six intake
valves. Don’t go too far!
The stock 235 intake valve ports are 1.44" diameter and may be safely
increased to 1.625" by reaming. Pictured is a reamer made from a pipe de-
burring reamer that works quite well for this. Note pilot on the end to keep it
centered with the valve guide.
The three side intake manifold ports on the 235 head may be safely enlarged to
1.5". They go straight in so a drill press with a shell reamer can be used to do
this. A die grinder must be used to smooth up the area where the reamer meets
the “Siamese” branch. Use the grinder to smooth out any rough casting spots.
Remember, both the intake manifold and manifold gaskets must be ported and
aligned with the head ports or you’re going to get turbulence in the flow path.
Also, don’t cut off the valve guides where they extend into the port. The slight
reduction in port turbulence isn’t worth the larger reduction in valve reliability
incurred by doing this.
Unless you are building an all-out lakes racer porting of the exhaust ports won’t
gain you much. The ports are not straight making the job difficult and I don’t
consider it worth the effort. Of course manifold and gasket alignment must be
Now lets talk about the compression ratio (CR). It is defined as the ratio of the
total cylinder volume at the bottom of the piston stroke to the volume at the
top of the stroke. The volume at the bottom of the stroke is the CID of the
cylinder plus the volume of the combustion chamber and at the top of the Ok,
we’ve defined it. So what does it mean in simple terms? If, for example, our
engine has a CR of 8:1 we will be compressing the intake fuel charge in the
cylinder into an area 8 times smaller than the cylinder area. This of course
raises the pressure of the charge so that as it burns it exerts its energy on the
piston to produce power. The higher we raise the CR the higher the pressure
and the higher the horsepower. What’s nice is this power boost, like that of CID
increases, occurs over the full RPM range of the engine, not just the high RPM
As we can see, to increase the CR we must reduce the size of the combustion
chamber. Basically we have three ways of doing this on the Stovebolt ------
Milling, filling and special pistons. We will look at each method separately and
in combination with each other. Keep in mind that just boring the cylinders .
060" over will raise the CR from its stock 6.75:1 to 6.95:1.
Milling of the head surface, an effective trick on flatheads, is a little more
complicated on the Stovebolt. The intake valve seats have to be recessed back
the same amount as the milling. Now shims must be added under the valve
springs to restore the correct valve spring tension. The valve rocker shaft
brackets should also be shimmed up the same amount as the milling. The
Stovebolt head may be safely milled 0.125". This will, with the .060" over-bore,
yield a CR of 8.4:1. For racing we should try to raise the CR to at least 9:1 with
10:1 as an upper limit, dependant on the fuel we are using. So how do we get
the CR up there on the Chevy?
Before the advent of custom made racing pistons the only other way to reduce
the volume of the combustion chamber was by filling. Filling, as the name
implies, is the adding of metal to the combustion chamber. It involves heating
the stripped head to a cherry red (about 1500 degrees F.) and acetylene welding
cast iron to the recessed area of the chamber while it is hot. After cooling, the
fill is ground smooth and the head resurfaced because the heating has certainly
Filling is not highly recommended because it is a complicated process, weakens
the head and tends to create “hot spots” that can cause damaging detonation
(pinging). If done carefully, the total CR is kept under 10:1 and high octane fuel
or alcohol is used it will work.
An excellent alternative to head filling is the use of special domed racing pistons.
In the late 1940's Frank McGurk of McGurk Engineering in Gardena, Calif.
developed and marketed a light weight aluminum racing piston for the Chevys.
It featured a specially shaped dome (“Dog Turd” as my buddy Jeff calls it). This
dome raises the CR to 8.25:1 in a stock 3-9/16" bore with no other head
modifications. The piston was also available in a
3-5/8" size to accommodate a .060" over bore. The dome is shaped to enhance
turbulence and breathing in the combustion chamber with no interference with
the valves. With these pistons and a .125" head mill we get our CR of 10:1
without the problems associated with filling.
Now comes the problem. These pistons are now out of production as over-the-
counter items and have been for some time. McGurk Engineering has been
gobbled up by Wayne Mfg. of Petaluma, Calf. and all attempts to question
Wayne about the pistons have failed. If you are lucky enough to find a usable set
of originals at swap meets etc. grab ‘em! If you don’t want them someone will
including me. Of course companies like Ross or Venolia will make you a set but
be prepared to sell the farm to pay for them. Hey, it ain’t cheap any more.
Before leaving the subject of heads lets take a brief look at a legendary classic.
In 1939 a fella named Wayne F. Horning took a look at the stock Chevy 6-
banger cylinder head and thought “This dumb thing don’t breath right”. He
formed his own company, Western Mechanical Development Co. in L.A Calif.,
and started development of a unique new design called a 12-Port. It was a cast
iron head with individual intake and exhaust ports for each cylinder - Intake
ports on the left side and exhausts on the right. It had a flattened oval
combustion chamber with the intake and exhaust valves located vertically in
the center of the chamber. Compression was varied by using different piston
crowns with special Venolia pistons.
By 1946 Horning was ready to produce and market his new baby but needed
some financial support. Enter one Mr. Harry Warner. Horning and Warner
formed the Wayne Mfg. Co. in 1947 and began production of the now famous
Wayne 12-Port head. As often happens and for what ever reasons the
Horning/Warner partnership was dissolved in 1950 with Warner buying out
Horning’s interest in the Wayne Mfg. Co. Horning continued to build Chevy
racing engines but none with the 12-port head while Warner continued
production of the Wayne 12-Port. Both he and Horning moved into development
of 12-port heads for GMC truck engines and production of the Chevy cast iron
12-ports seems to have died by 1951.
Estimates vary on the actual number of Chevy Wayne cast iron 12-ports made
but it was probably less than 20 for which the Flathead folks were extremely
thankful. Of coarse the chances of ever finding one of these heads are way less
than locating an Ardun conversion for a Flatty. Wayne Mfg. is still producing an
aluminum 12-port for the Chevy 235/261 blocks but the cost is out of this
Original Wayne Cast Iron Head
Lets take a look at what opens and closes those nice big valves we have in the
head; the OverHead Valve (OHV) train. Lets review this.
The heart of the valve train is the camshaft which is rotated by a 2:1 reduction
gear driven off the crankshaft. The lobes of the camshaft impart lateral motion to
the lifters or tappets. This motion is transferred to the top of the cylinder head
by the pushrods which actuate the rocker arms which in turn open and close the
valves. The valve springs ensure a good valve seal and that the valves closely
follow the cam lobe motion through the full RPM range of the engine.
Since the camshaft is essentially the “brain” that tells the valves when to open
and close, we can drastically alter the power characteristics of the engine by
changing the geometry of the cams lobes. To get more power, more fuel/air
charge must be taken in, burned and exhausted from the combustion chambers.
This is done by increasing valve lift and/or duration or in simple terms opening
them farther and sooner and closing them later. Sounds simple but it gets a little
more complicated than this. Increasing the valve lift but not the timing will
increase the performance without a noticeable change in the power band width
and RPM point especially when multiple carburetion is used. Altering the valve
timing is a whole different story.
The increase in power gained by increasing cam timing and duration will occur
at higher and higher RPM’s; the more radical the cam the faster we have to spin
the engine to get that power increase. That’s the good news. The bad news is the
power at the lower RPM’s
falls off at almost that same rate. A nice rumpy cam sounds great at idle but be
prepared to spin that sucker to get the ponies.
The stock Stovebolt engine is inherently a good low and mid RPM range torquer,
due largely to its efficient breathing ability. Wild and wooly cam timings are not
as critical as those needed for the asthmatic Flathead. Valve lifts of .440" and
timing durations in the 250 to 270 degree range will give a good power band in
the 2000 to 5000 RPM range. It’ll still have a little gallop at idle but that nice
usable low end torque won’t suffer too much. My philosophy is keep the RPM’s
under 5000 and use that low end toque. The engine will live a lot longer. This
old iron is getting a lot harder to find so lets not blow it up.
One item the Flathead builder doesn’t have to contend with but we Stovebolt
guys do is the push rod and rocker arm assembly. The added weight and friction
of these parts as well as push rod flexing must be considered and dealt with.
Radical cam grinds, heavier valve springs and higher engine RPM’s can cause
the stock solid push rods to flex and bend
and of course fail. Loud clatters and lots of smoke will let you know when this
happens. Definitely replace the solid push rods with later model or better yet
after-market tubular ones. They are lighter and stronger and can be purchased in
custom lengths. (More about this later). The stock rocker arms may be used if
they are in good shape. Trash any that are loose on the rocker shaft or
excessively worn on the valve stem face.
Another critical item is the angle of the rocker arm’s valve stem face with the
top of the valve stem. This face is radiused and must be centered with the top of
the valve stem at the valve’s half open point. This angle will normally not change
unless the valves have been recess a lot or replaced with ones with longer stems.
If so, shorten the stems or shim the rocker shaft stands to set the angle.
Sometimes when the head has been milled, valves replaced or recessed, block
been redeck etc. you may find that you can’t set the valve lash with the
adjusters on the rockers. The easiest fix for this is to purchase a set of the afore
mentioned custom length push rods.
Always use new lifters when installing a new cam. The best bet is to use the
lifters supplied or recommended by your cam grinder or supplier. In most cases
the standard flat tappet lifters will work fine with most performance cams. I do
not recommend using hydriodic lifters due to their tendency to “pump up” at
the higher RPM’s. To me it’s like putting a restrictor plate on the engine.
With some radical cam grinds the “toe”of the cam lob will catch the edge of the
lifter instead of sliding under it. Two solutions for this problem are the
Mushroom and the Radius lifters. Jeff Ackerman has very aptly described these
in his Flathead section of Flat Tech so I won’t do it here. A third but rather
expensive solution is the Roller Tappet cam. With it we can have more radical
cam timings, less friction and longer cam life. It is a rather simple installation in
the Stovebolts; less so in a Flathead. Two suppliers of these style cams were
Frank McGurk and Chet Herbert. Good luck finding one.
Chet Herbert Roller
Cam and Lifter Set-Up.
A sometimes overlooked but very important item in the valve train are the valve
They must be heavy enough to make the valves follow the cam profile without
“floating” at the higher engine RPM’s and seat the valves tightly for good heat
transfer but not so heavy they produce excessive wear. Again, the cam supplier
should supply or recommend the correct spring setup as well as the correct
installed height of the springs. This height is usually set with shims under the
One method of getting more valve lift and a slight increase in duration is the use
of after-market High Lift rocker arms. These are usually installed only on the
intake valves and should be used only with a factory stock camshaft. They work
well in conjunction with a bolt-on carburetion increase for the street but don’t
do much for a racing engine. I have never used them and feel that the money
spent on them would be better applied to the purchase of a performance
One of my many pipe dreams is to own a state-of-the-art cam grinding machine,
an unlimited stack of cam billets, a dynamometer and a lot of time to
experiment. This is what we need to find the perfect camshaft for our particular
application. Fortunately some of the early performance pioneers such as Frank
McGurk, Chet Herbert, Ed Iskenderian and Harvey Crane did just that.
Unfortunately most their specs and original cams and lifters have evaporated
like methanol fumes. If you aren’t lucky enough to lay your hands on one of the
original cam/lifter sets, outfits like Howard, Iskenderian or Crane can grind you
one but again, be prepared to walk around with a flat wallet for a while.
A WORD OF CAUTION : The factory stock cam timing gear is made of fiber.
ALWAYS replace this with the readily available, over the counter aluminum one.
WORD OF CAUTION 2: If not already done by the supplier, the front nose of the
camshaft must be drilled and tapped for a 9/16" bolt. After the timing gear has
been pressed on the camshaft, install a 9/16" grade 8 bolt and large flat washer in
this hole (Use Loctite and/or lock washer) to keep the timing gear from
“walking” off the cam. The timing gear cover will have to be “adjusted” to clear
this bolt and washer. I use a large ball peen hammer to do this.
Okay, the pistons are moving up and down and producing lateral power. We
need rotational power. Hello crankshaft.
The Stovebolt crank is no different in its function than the Flathead or any
other automotive piston engine. Compared to the Flathead it is physically quite
different however. It is longer, heavier and in most cases made of poorer quality
steel than the Flatty. This is compensated for by its four main bearings and
Starting from the front the rod journals are numbered 1 to 6 and are paired in
three groups of two each. Each group is offset by 120 degrees around the center
of rotation, the pairs being 1&6, 2&5, 3&4. Heavy counter weights are cast
integrally with the crank to offset the rod end weight and reduce harmonic
vibrations through the RPM range. Basic stuff.
The connecting rods look like Model A Fords but with fixed insert bearings on
the crank end. The piston end is semi floating in that the wrist pin is clamped in
the rod end and floats in the piston. Here Ford had a better idea for the later V8's.
So what can be done to the crank to increase performance. In my way of
thinking not much. To increase acceleration, weight can be machined off the
counter weights but this can lead to balancing problems. Grooving of the
journals will increase oil flow to the rods but is not needed if grooved insert
bearings are used.
The stock connecting rods with clamped wrist pins are very rugged and work
well in stock or mildly modified engines. They are designed to work with cast
iron pistons with wrist pin bushings. In a high revving engine with alloy pistons
excessive wear can occur in the piston wrist pin bosses. Also what happens
when the clamp bolt fails. BOOM!
A rather costly but effective fix is to replace the Chevy rods with GMC 270 or
302 rods. These are heavier and have a full floating wrist pin. The crank journals
are the same diameter as the Chevy but the wrist pin diameters are larger. The
real problem is they are 0.19" longer than the Chevy. Now we need a set of
custom made pistons with the wrist pins offset to get the pistons back down in
the block. If you are building an all out power house engine you will probably be
getting custom pistons for it anyway so why not have them made for the GMC
rods. I personally have never had the Chevy rods fail but I know some folks who
As the crank rotates it is subjected to forces along its length that are trying to
destroy it, mainly the power pulses from the pistons, the off- center spinning
weight of the rod ends and journals, and the loading forces from the rear wheels.
Due to that nasty law of physics called harmonics these forces can also become
magnified at certain RPMs to the point they can break the crank. This problem
becomes greater with increases in crank length and cylinder spacing. To reduce
these forces a Harmonic Balancer is installed on the front end of the crank and is
an absolute must on any inline engine. I recommend the purchase and
installation of a high quality after-market fluid damper on any modified
Stovebolt. At least use a brand new stock one. NEVER a junkyard one! Lastly,
drill and tap the end of the crank for a bolt and large washer to keep the pressed
on damper from coming off.
Final word. BALANCE! Balance the entire rotating system. Pistons, rods, crank,
flywheel, damper, everything. Not just statically but dynamically. The money
spent on this is worth twice that spent on bolt on speed equipment in power and