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On a nice sunny day in July, Judi and I were
leisurely heading up PCH on a SoCal caravan destined for
some back roads, a BBQ at the Vivas’ and an overnight
stay in Santa Maria. A few miles north of Santa Barbara,
I heard a clunk and checked my gauges to discover that
my tachometer wasn’t working. It appeared that I had
thrown a fan belt. Our Solid Axle group got off at the
next exit and to my surprise my harmonic balancer was
resting on the lower radiator shroud, still held in
place by the fan belt. It was amazing that it didn’t do
any major under hood damage or, worse yet, damage to the
cars behind me. We called AAA for a tow and
an exciting ride back to the barn.
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When our cars left the factory the harmonic
balancer was pushed onto the crankshaft with an interference fit
and had no provisions for a backup bolt! This bolt was added
later in the mid 60’s. After the balancer has been off and on a
few times over the years, the interference fit erodes to a snug
fit that leads to … well: Bing … Bang!!! Pushing the balancer
back onto the crankshaft is relatively easy, but drilling and
tapping a straight 7/16 thread into a hardened crankshaft
with the engine in the car is challenging. (see fig above)
A few things to remember while drilling and tapping
the end of a crankshaft without removing the engine:
The
crankshafts are carbonized to a depth of about .010,
so the first few minutes of drilling and tapping are
the most critical.
Buy new quality tungsten drills and 7/16-20
taps.
The one thing you don’t want to do is break
a drill and especially not a tap!
You can never use enough tapping oil.
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I had a drilling fixture made to my sketch by a local
machinist. (see fig above) The initial drill size in the
fixture was 1/4” dia. After I successfully drilled the 1st
¼” dia. pilot hole, I drilled the fixture out using a drill
press to 5/16’s dia and finally out to the .391 dia. tap
drill size (see the figures below for the resulting parts).
All work was performed from the bottom of the car by only
removing the center section of the lower radiator shrouding
(60-62 remove drivers side). (see fig below)
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An inexpensive Harbor Freight angle drill motor (not
their drills) was used. Note that the drill handle points to
drivers side and slightly up when working from underneath.
Remove and discard the old Woodruff key. Remove
shaft seal from timing cover.
Use lots of cutting oil, drill about 1/8 deep at
a time (I used tape on the drill as a depth gauge),
constantly remove the drill fixture, inspect, seal
around crank opening and blow out drillings. Re-oil
and install tool until tap drill is 1-¼”deep. Drill
out fixture to 5/16” and repeat oiling/drilling …
then .391 tap drill to a depth of 1-¼”.
Re-drill the fixture to the next size above 7/16
and use the fixture as a guide for the tap. The
first two turns of the tap are the nervous ones.
I was able to get 1-1/8” of 7/16-20 full
thread depth. (see fig)
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I used the replacement washer from Chevrolet but because
C1’s used a .16 spacer behind the balancer, I purchased a
longer grade #8, 7/16-20 x 2 ½” bolt from a local specialty
hardware store instead of the Chevy 2 ¼” bolt.
Replace seal in timing chain cover.
Add Spedi Sleeve (Fel Pro #16202 or equiv) to reconstruct
balancer-sealing surface if necessary. Use a new Woodruff
key.
Once the crankshaft is threaded, tap the balancer gently
onto the crank to get it started and draw it into place
making sure that you have at least 3/8” thread engagement at
all times … don’t strip your new threads. (see fig )
Fig 6 shows the balancer in place with the new
bolt, lock washer and retaining washer.
I have since modified the drilling tool with drill
bushings for ¼, 5/16 and .391 drills so that it can be
reused (see fig ).
Please contact me with any questions or
observations: costales@west.net
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