The Driven Man/American Powertrain Bellhousing Alignment Measurement and Correction
Background: Modern transmissions use tightly tolerated taper roller bearings to support the input shaft. These bearings are far superior to the ball bearing assemblies of older transmissions; however their exacting specifications require that the input shaft has almost perfect linear engagement to the pilot bearing in the crank shaft. Improper centering of the transmission to the crank can cause premature wear on the input shaft bearings, creating noise, vibration and the eventual failure of internal parts of the transmission.
Due to manufacturing abnormalities in traditional and modern bell housings and scatter-shields it is necessary to measure the accuracy of your bell housing as it relates to the crank shaft. This will insure that the input shaft, located by the bore hole in the back of the bell, lines up directly with the crank.
NOTE: IT IS IMPORTANT THAT YOU MAKE THIS MEASUREMENT. FAILURE TO CORRECT A MISALIGNED BELL HOUSING WILL VOID YOUR TRANSMISSION WARRANTY.
A video of this procedure can be viewed here:
You will need:
- A magnetic base dial indicator with a .001” resolution (.0005” preferred)
- Large ratchet handle, breaker bar or flywheel tool to turn crank
- Appropriate tools for tightening bell housing bolts
- Permanent black marker (Sharpie)
You may need:
- Appropriate tools for removing dowel pins
- Offset dowel pins to correct out of spec condition
- With the flywheel installed if possible, place the bell housing on the back of the engine and fasten with at least four bolts, insuring that the bell is seated against the block.
- Place magnetic indicator on the flywheel (or crank hub if flywheel is not installed). IT DOES NOT NEED TO BE CENTERED IN THE CRANK. Set the measuring finger on the inside of the bore of the bell housing (see photo) at the 12-o’clock position.
- Zero the indicator dial at 12 o’clock. With a permanent black marker, mark a zero on the back of the bell at 12 o’clock.
- Turn the crank until the measuring finger is at the 3 o’clock position. Record the measurement with the marker. Make sure you record if the result is positive (to the right of zero) or negative (to the left of zero).
- Repeat step four at the 6 o’clock and 9 o’clock positions, recording your results. Then return the indicator to 12 o’clock and make sure it measure zero. If the indicator does not measure zero at 12 o’clock then the base has moved during the procedure and you will have to repeat steps 3 through 5.
How to interpret the numbers:
The acceptable tolerances for Tremec transmissions is .005” of total runout. The following formula will tell you how much runout you have.
Reading at 12 o’clock – Reading at 6 o’clock = Vertical Axis Run Out
Reading at 3 o’clock – Reading at 9 o’clock = Horizontal Axis Run Out
1. 12 o’clock = 0
6 o’clock = -.004
Then your Vertical Runout is 0 – -.004 = .004” (within tolerance)
2. 3 o’clock = .009”
9 o’clock = .006”
Then your Horizontal Runout is .009 – .006 = .003” (within tolerance)
3. 3 o’clock = -.008
9 o’clock = .013
Then your Horizontal Runout is -.008 – .013 = .021” (.016” out of tolerance)
Interpreting The Numbers:
- If you are out of tolerance (> .005” +/-) on only one axis, then you will correct directly along that axis. The vertical axis will be corrected by a direct north or south movement of the bell housing, the horizontal by a direct east or west movement.
- If you are out of tolerance on both axes, then you will be correcting along a diagonal. For example, if you’re bell housing is sitting .008” to the south (down) and .009” to the east (right) of center then you will correct toward the northwest to move the bellhousing up and to the left.
If your two axis measurements are less than .005”, as shown in example one and two, then your bell housing is within tolerance and you can proceed to install your transmission.
If the number is greater than .005” (+/-) then you must use the following procedure to correct the runout.
- Remove indicator and bell housing and set aside for later use.
- Remove dowel pins using one of the following methods:
- Some engines have punch holes behind the pins. If you have this style of engine you can simply use a pin punch and a hammer to remove the old pins.
- Grasp with vise style pliers and slowly turn pin while pulling away from the block.
- Our favorite method for removing ornery pins involves the use of an aerosol spray freezing agent available at most parts stores. Simply place vice style pliers on the pin, heat the block around the pin area, spray the pin with the freezing agent and pull. The pin will shrink away from the heated block and usually come away very easily.
- Once the dowel pins are removed you will need to install “offset” dowel pins in the block. Offset dowel pins come in .007”, .014” and .021” sizes and correct for twice their value. Therefore, a .007” dowel pin will correct for as much as .014” of total runout. These pins are available from us, or from your local speed shop or racing supply retailer.
- Remove the dowel pins from their package and using the permanent marker or bright nail polish, mark the high side of each pin. This will help you align the pins when you press them into the block.
NOTE: THE MOST ACCURATE WAY TO FIND THE CORRECT POSITION FOR THE DOWEL PINS IS TO DETERMINE THE POSITION WHERE THE BELL HOUSING BORE IS CLOSEST TO THE CENTER OF THE CRANK AND POINT THE HIGH SIDE OF THE OFFSET PINS AT THAT POSITION. YOU CAN FIND THIS POSITION WITH THE DIAL INDICATOR. SEE ILLUSTRATION:
!!ALWAYS POINT HIGHPOINT ON PINS IN THE SAME DIRECTION!!
- Once you have marked the high side, tap the pins into the block using a brass hammer, dredge or hardwood block to avoid mushrooming the pin. A light petroleum based lubricant should be used. Approximate the direction of correction as described above. In the example above we would put a .007” dowel pin in, with the high side pointing northwest.
- Once installed, place the bell housing back on the block and repeat the runout procedure. If you are still out of tolerance you can turn the new pins to fine tune their position. Most dowel pins have a slot or Allen head hole to make it easy to make fine adjustments after installation.
- Once you have reached a runout condition of less than .005” total runout, record your four measurements on this document and keep for warranty purposes.
12 o’clock ______________
Vertical Runout __________
Horizontal Runout ___________
Thank you for taking this important step in the installation process. If you have any questions or need help interpreting your results you can call us anytime at 931.646.4836.