CRG Research Report
CAMARO Manual Transmission Bellhousing Alignment
ARE YOU ALIGNED?
© 2007-2011, Camaro Research Group
Primary Author -
Reviewed by the CRG
Last Edit: 11-Feb-2007
Previous Edit: 08-Feb-2007
Original Release: 08-Feb-2007
If you own a manual transmission Camaro, ask yourself this question,
have I ever checked the bellhousing alignment? In most cases the answer
is probably no. Many of us who restore Camaros install the clutch assembly,
pilot bearing and bolt the bellhousing up to the engine block without
even giving it a second thought. However, there is a very high probability
that a stock (or aftermarket) bellhousing installed with stock engine dowel
pins will not be in tolerance. How will this affect your car? Very simple - if the
bellhousing is not concentric with the crankshaft within 0.005 inch,
serious damage can occur to the pilot bearing in your crankshaft, and/or,
the input shaft of your transmission. An alignment issue like this can
also cause vibrations in your Camaro while driving under normal conditions
as well as hard shifting.
Shown below is a close-up shot of a stock Chevrolet dowel pin installed in
the engine block, right side.
Stock Chevrolet Engine Block Dowel Pin
As an example of the damage that can result from misalignment, the photo below shows
a seized throw-out bearing stuck on the transmission input shaft bearing retainer.
In this case, the bellhousing
to block alignment was out of tolerance by 0.014 inch. This misalignment created
excessive stress on the pilot tip of the input shaft, which eventually fatigued,
broke off, and allowed the transmission input shaft to wobble extensively around in the
crankshaft. This resulted in tremendous heat sufficient to cause the
throw-out bearing to weld itself to the bearing retainer. When this happens
the vehicle is not driveable at all. Note the broken ear on throw-out bearing
from trying to remove the transmission.
Seized Throw-Out Bearing Due to Misaligned Bellhousing
How and why does this happen? Chevrolet installs two alignment dowel pins
into the block, locating one on each side where the engine block mates
to the bellhousing. These dowel pins locate the bellhousing relative to the
engine block, and the bellhousing - in turn - locates the front bearing retainer
and input shaft of the transmission.
For more than 99 percent of all production vehicles these
components will mate together at the stock dowel pin position and function well
enough for street applications. But in the world of performance cars, such
as Z28, L30, L48 or an SS396 Camaro, misalignment can lead to serious issues,
and can cause severe transmission problems. Every year Chevrolet Motor Division
casts and machines hundreds of thousands of engine blocks, as well as bellhousings.
Statistical variation in geometric tolerances will result in features of
these parts that vary in true position from nominal. If an engine block
is at the top of a tolerance range and the matching bellhousing is at the
bottom of a tolerance range, a serious misalignment issue between the two
parts can occur.
Chevrolet is willing to balance the risk of a small number of
warranty issues for stock applications against the greater cost of 100% correction.
Most people install the bellhousing on the dowels and install the bellhousing mounting
bolts without checking the factory alignment. But we now know that GM does not
do a sufficient alignment to assure full coverage of the tolerance band, even for stock
conditions, and the factory alignment certainly does not cover high-performance
needs. So it is left to you to check this during your rebuild.
How does one check their bellhousing alignment, and what does "concentric"
mean? If an engine component is concentric with another part such as an
engine crankshaft, this means that the centerline of the crankshaft is in
perfect alignment with the centerline of the input shaft of your
transmission. This also means that the input shaft of your transmission
will ride perfectly centered in the pilot bearing of the crankshaft.
A Common Problem, and the Solution
Is this truly a common problem? During the writing of this article, and in
support of it, Camaro Hi-Performance inspected three different engine blocks
to check block-to-bellhousing alignment tolerances, using the same bellhousing
as a control for all three evaluations. The blocks evaluated were two Chevrolet
small block 3970010 castings and one Chevrolet small block 3914678 (all three coded as built for a
manual transmission application). The results were disturbing. The same bellhousing produced
three different out-of-tolerance concentricity measurements when it was mated to the
three blocks. The concentricity for the 678 block, measured from the crank centerline,
was 0.019 inch from nominal. The first 010 block was 0.016 inch out of
nominal. And the second 010 block was out about 0.008 inch from nominal. Only the
last block was usable as-is for normal street use because it was only 0.003 inch over
the maximum desired value. And usable for street use means driving with no abuse to the drive
train, high rpm shifts, etc. For a performance application even the best of these three blocks
would have required correction to the bellhousing alignment. The other two engine blocks,
however, urgently needed correction to bring them into the tolerance band required to avoid
the drivetrain alignment issues discussed above.
How is this correction made? By use of offset alignment dowel pins. Many variations and brands
are available. Two of the popular brands of offset pins are shown below.
Offset alignment dowels can be purchased at any parts store, speed shop, or
performance center. They come in different alignment offsets starting at 0.003
inch all the way up to 0.021 inch. How do they work? One version has a slotted
end at the end of the dowel for a flat blade screw driver. Once installed in the
block, the dowels can be rotated in the engine block until you locate the correct
offset to bring the bellhousing into tolerance.
Typical Aftermarket Offset Dowel Pins for Correction of Misalignment
|Moroso Dowel Pin Set||Lakewood Dowel Pin Set
How to Check Bellhousing Alignment
But before you will know which size of offset dowel pin you need, or if you even
need one, you will have to tackle the issue of how to measure the concentricity
between the engine block and the bellhousing.
For years, racers have tried to perfect this measurement task, but the traditional method
is tedious at best. It takes a lot of patience and the right tools to do the job
correctly. The traditional method measures the position of the bellhousing opening
relative to the crankshaft, and requires a dial indicator, a magnetic base (for
mounting to the crankshaft) and adjusting rods to mount the indicator into the base.
You will then be faced with the fact that the bellhousing hole through which the
transmission inserts is not a true machined surface; the dial indicator cannot
ride on the bellhousing surface without the indicator needle bouncing around. This,
of course, tends to yield inaccurate readings. For someone who is a machinist or has
a machinist's background, there is no doubt that this method can be accurately applied,
but it takes years of experience to perfect. For the novice, doing an alignment check
this way can take hours and when you're finally finished the alignment might still
not be within specifications. There is a much better and easier way to check your
alignment, which is explained further below.
Shown below is a typical one-inch travel dial indicator mounted to an adjustment
rod and magnetic base, capable of reading to 0.001 inch or better. This tooling setup
is needed to do an alignment check by the traditional method. A correct bellhousing
runout check by this method requires someone with experience as a mechanical technician
In the photograph below, the magnetic base of the indicator stand is now mounted
to the crankshaft, and the indicator rod is positioned on the inside surface of the
bellhousing hole diameter. The crankshaft is then rotated (and the indicator rotates
relative to the bellhousing at the same time) using a ratchet and socket on the
harmonic balancer bolt. During this rotation, if the bellhousing hole is perfectly
concentric with the crankshaft, the dial indicator needle will not move. If the indicator
reading moves, and is biased to any clocking position by more than 0.005 inches, there
is an alignment problem and offset dowel pins are required. Of course, the delicate
issue in accurately making this measurement is that the surface roughness of the
bellhousing hole can be several thousandths of an inch, which has to be ignored when
assessing the bias.
Typical Dial Indicator on Rod/Magnetic Base
Determining Bellhousing Concentricity Using a Crankshaft-Mounted Dial Indicator
Making This Measurement Job Easy
Thanks to a company called Browell
Bellhousing, a tool is now available
to make this job easy and repeatably accurate, even for the relative novice.
This tool looks similar to a bowling pin, as shown in the next two photos, and bolts
to the flywheel flange of the crankshaft.
Once the tool is installed, mount the bellhousing to the block, as shown in the next three photos.
Browell Alignment Tool
Included with the bowling pin tool is a ring that slips over the large end of the bowling pin. The
outer diameter of the ring is 4.6825 inch, approximately 0.0025 inch smaller than
the hole in the bellhousing where the transmission throw-out bearing retainer
installs. If the ring slips into the bellhousing hole without difficulty, you have
good alignment that is within 0.003 inch. The next three photos show the ring sliding onto
the bowling pin.
Browell Alignment Tool Pin Prior to Fitting Ring
|View 1 - Pin Mounted to Crankshaft
||View 2 - Down crankshaft centerline
||View 3 - View from above
If the ring will not insert into the
hole, there is an alignment issue. In simple terms, this gauge acts like a go/no-go
gauge. By shining a drop light into the back side of the bellhousing, you'll be
able to see a gap in the hole area when the ring is pressed up against the
bellhousing. Think of the hole in the bellhousing as a clock. If there is light
in the upper left side quadrant of the transmission hole, say at the 10 o'clock position,
that means you want to move the bellhousing toward the directly opposite 4 o'clock position,
which you accomplish by rotating the alignment dowel pins towards the 4 o'clock position.
Or if there is light at the bottom of the hole at 6 o'clock, that means you want to rotate
the dowel pins towards the opposite, 12 o'clock, position. The next photo shows the ring
fully engaged in the bellhousing hole, ready for examination with the light.
Browell Alignment Tool, Inserting Ring Onto Pin
|Starting point on pin
||Mid-way down pin
||Ring fitting into bellhousing hole
Once you have correct alignment, apply either red or green Loctite to the dowel
pins so they will not turn in the engine block, and install the bellhousing.
Browell Alignment Tool, Examination of Ring Fit
Related Business: Replacing the Pilot Bushing with a Pilot Bearing
Chevrolet installed a solid bronze pilot bushing from the factory when these cars
were new. Our experience indicates this bronze pilot bushing allowed for misalignment
between the engine block and bellhousing. The bushings are very soft and wear easily
and the transmission will still function under normal conditions. This was adequate
for easy street driving, but in time, with wear from misalignment, the input shaft
of your transmission will be wobbling around within a worn pilot bushing.
It is strongly recommended that anyone who is restoring a manual shift Camaro
install a needle bearing pilot bearing at the same time that they perform the
bellhousing dowel pin alignment. This bearing is available from Chevrolet
under part number 14061685. Cost is about $18.00. Installing this pilot bearing,
together with the mandatory proper bellhousing alignment, will add tremendous
integrity, balance and smoothness to your driveline. Shown below are the stock GM pilot
bushing (left) and the GM replacement needle bearing pilot bearing (right).
For serious racers or Camaro hot rodders, Jerico Racing Products has a ultra heavy duty pilot
roller bearing. This design inserts into the larger end of the crankshaft moving it deeper
into the pilot of the input shaft for added strength, as shown below.
Pilot Bushing versus Pilot Bearing
Jerico Roller Pilot Bearing
The Browell bellhousing tool is available from Browell
Bellhousing upon inquiry to anyone in the general public. All major applications
are available: Chevrolet, Pontiac, Ford, etc. Cost at the time of this writing is