Flywheels and Clutches
Clutch and Flywheel Technical
From 1955 to 1962 Chevrolet had one flywheel size. It had 168 teeth on the ring gear. The 168 tooth ring gear measures approximately 14 1/8" in diameter. Starting in 1963 Chevrolet started using 153 tooth flywheels, alongside the 168 tooth flywheels.
There are two reasons to determine which flywheel you have or which one to buy. The first is the starter to be used and the second the bellhousing size.
Most (but not all) Chevrolet engine blocks have three taped holes for use with both a 168 tooth flywheel starter and a 153 tooth flywheel, but it is best to check, the threaded holes, before buying your clutch, flywheel and bellhousing. 153 tooth starter mount holes are both perpendicular to the engine crank. 168 tooth starter mount holes are diagonal to each other.
Most new clutch discs are marked "Flywheel Side", but some aren't. And used ones may have the markings worn off.
Pressure Plate Side
The flywheel center can not accommodate this
The flatter of the two sides goes toward the
flywheel. This disc has a raised center to insure
there is no interference with the flywheel bolts.
Loose Pressure Plate or Incorrect Disc Orientation
This diaphragm style pressure plate "fingers" stick
out from the center, when it is loose or the disc
is installed backwards.
Tightened Pressure Plate
On this clutch disc and pressure plate system the
"fingers" retract all the way into the center when
the clutch disc is oriented correctly and the pressure
plate is tight.
Clutch Disc Sizes
Casting #3988999N is a 168 tooth flywheel drilled for a 10 1/2" clutch.
Casting #3788733 is a 168 tooth flywheel drilled for a 11" clutch.