Set yourself up for success using a dial indicator to index your bellhousing
By Jefferson Bryant
Big horsepower V8? Check. Tremec 6-speed? Check. Hydramax bearing? Check. Sounds like you have it all figured out, ready to go into the car. Guess it is time to just slap all that premium gear together and turn some tires into smoke, it is all made to go together so it should be nuts and bolts from here, right? Not so fast, cowboy. While the parts are made to work together that doesn’t mean that everything is good to go without checking a few things first. You wouldn’t rebuild an engine with a new crank and rods without checking the bearing clearance, right? Well, your manual transmission is the same way, there are couple of items you need to check before you just slam it all together, specifically the bellhousing index.
Every engine and bellhousing are similar, but they are still mass produced, so you need to index the bellhousing to the engine block to ensure that the transmission is concentric with the centerline of the crankshaft. The is some tolerance, .005” in one-direction, any more than this and the input shaft will run at an angle to the crank. Side pressure on the input shaft induces more heat on one side of the bearings, causing premature failure, hard shifting, and it voids your warranty. Too many builders skip this step, but that is a huge mistake. It only takes a few minutes to check the index, and doing so can save you a ton of time and money down the road.
The process requires an assembled engine, bellhousing, and flywheel. You do not need to have the transmission or clutch installed. You will need a magnetic dial indicator (digital or analog), and a wrench to turn the engine over. The hardest part is setting up the dial indicator. The indicator plunger (the part that touches the surface to be measured) should be as close to perpendicular with the opening of the bellhousing as possible to get the most accurate measurement. This can be tricky depending on the style of arm your indicator base, some are easier than others, and tight (shallow) bellhousings are more difficult than deep bells.
Measuring The Housing
First, the bellhousing should be installed to the engine block with any shim plates required. Leaving out the plate will throw off the measurements. You don’t need to install every bolt, but at least 4 bolts covering the ends and center of the housing tightened as you would the final installation. The flywheels should be installed and torqued to spec. Use the old flywheel bolts if you don’t have reusable bolts so you don’t over-stretch the bolts.
Place the indicator base inside the housing on the flywheel. It needs to ride the clutch surface; this is a true flat surface. Lock it down and then adjust the indicator so that the dial is reading the opening for the transmission, with the indicator as perpendicular to the inner bore as possible.
Have a helper spin the engine 1 full rotation while you watch the indicator. Using a marker, note the location of the largest positive number on the gauge. This is the 0-point. Return the indicator to this point and reset the gauge to “0”. Mark this as #1. Rotated the engine 1 full turn and verify the gauge is back to zero.
Rotate the engine 90-degrees. Note this measurement as position 2. Spin another 90, take the reading for position 3, then one more 90-degree rotation, note this measurement as position 4. Complete the rotation to the zero point and verify the reading is still zero. Using this method, the maximum offset is automatically set, regardless of the clocking position. If you only measure at 12, 3, 6, & 9 clocking positions, you may miss the highest offset point.
For example, the 2015 L83 GM V8 shown here had the following measurements with a QuickTime Bellhousing for a TKX: #1 0.00”, #2 -.009”, #3 -.019”, and #4 -.010”.
The readings for 2 and 4 are essentially the same, they cancel each other out. The bellhousing is offset by .019”, this number is divided in half, making the offset radius .0095”, which too far out to be used as is. We need some offset dowel pins for the engine block. The side measurements should clear up on their own once the bell is shifted to close the offset to zero point. The bellhousing can be removed at this point.
Offset pins come in a variety of sizes, if there is not a pin that exactly hits your need, you choose the closest option. For our L83, we selected a .007” offset, which will get the index within the .005” tolerance. The old dowels are removed with a punch and a small hammer. They usually come out fairly easy, but the older the engine, the more difficult they may be. Keep them for future needs.
Some dowels are marked for the offset, some are not. The easiest way to check is to put them on a flat surface and roll them until you see the offset’s peak, marking the position. Install the dowels to the engine in the correct direction, moving the bellhousing center towards the zero point. Offset dowels have flats on one side, this is the side that goes to the bellhousing and allows you to rotate them to dial in the indexing point. The dowels should slip right into the engine, the offset dowels use a bolt to secure them once the bellhousing is in position. Reinstall the bellhousing to the engine, tighten the bolts securing the bell to the block.
Position the dial indicator into the same position, and follow the same process as before, finding the largest offset, reset to zero, then check each 90-degree point. The bellhousing should be within the spec, the L83 shown in our example came in at .003” offset on the 1-4 axis, and the 2-3 axis came within .001”.
Once you have verified the bellhousing is centered to the crank within spec, you can tighten the locking bolt in the center of each dowel, and you are all done. You can now remove the bellhousing and complete the clutch and transmission installation.