Measure/Install Trigger Wheel

A crank trigger upgrade allows precise ignition timing control and compatibility with modern EFI systems like MegaSquirt. This guide covers how to measure your crankshaft pulley to determine the right DIYAutoTune.com trigger wheel for your engine, and then walks through installation of the trigger wheel itself.

Step 1: Measuring Your Crank Pulley

Before ordering parts, you need to measure your pulley to ensure a proper fit. Depending on whether you have a flat or dished pulley, the process differs slightly.

Flat Pulley Measurements

For flat pulleys, only three measurements are required:

• Outer diameter of the crank pulley

• Center bolt diameter

• Length of the center bolt you’ll need

Dished Pulley Measurements

Dished pulleys require two additional measurements:

• Distance from the front of the pulley to the mounting face

• Maximum diameter that will clear the pulley bolts or shoulder

DIYAutoTune trigger wheel spacers (0.25″ thick) can be stacked as needed to ensure the wheel is supported by spacers rather than riding on the pulley edge.

Tip: Use calipers for accurate measurements. The depth probe on a caliper can help determine the required crank bolt length.

Note: All DIYAutoTune trigger wheels and spacers ship with a 1/2″ center hole. You’ll need to drill this out to match your crank bolt size.

Step 2: Choosing the Right Trigger Wheel Size

The trigger wheel must extend beyond the edge of your crank pulley so the VR sensor can “see” the missing tooth.

• 4, 6, or 8 cylinder engines typically use our standard 36-1 trigger wheel

• The wheel’s missing tooth must be positioned a set number of teeth ahead of the sensor at Top Dead Center (TDC)

Step 3: Installing the Trigger Wheel

Once measurements are complete and the correct trigger wheel is selected, you’re ready to install

Installation Steps for Trigger Wheels

1. Set Engine at TDC
   Rotate the engine so that cylinder #1 is at top dead center (TDC). This ensures the missing tooth will be indexed correctly during installation.

2. Choose Mounting Method
   You can mount the trigger wheel in one of two ways:

   • Center Bolt Method: Drill out the center hole of the trigger wheel to match your crank bolt (e.g., ¾”). Use a longer crank bolt to stack the trigger wheel and spacers on top of the pulley.

   • Direct Pulley Mount Method: Drill and tap a minimum of three evenly spaced holes in the face of your crank pulley. Secure the trigger wheel directly to the pulley with bolts. For balance-critical engines, consider using at least four bolts or placing bolts evenly around the wheel.

3. Spacer Considerations

   • If using the center bolt method, stack components in this order: lock washer → trigger wheel → spacers → pulley → crank bolt.

   • If using the direct mount method, use spacers if needed to ensure the trigger wheel sits flat and clears any pulley lips or bolt heads.

4. Secure Hardware

   • For the center bolt method, torque the crank bolt to the manufacturer’s specification.

   • For the direct mount method, snug all mounting bolts evenly to prevent warping, then apply final torque.

5. Verify Missing Tooth Position
   Ensure the missing tooth is positioned correctly relative to the VR sensor for your engine configuration:

   • 4-cylinder: 9 teeth ahead of sensor at TDC

   • 6-cylinder: 6 teeth ahead

   • 8-cylinder: 5 teeth ahead

   • 12-cylinder: 8 teeth ahead

6. Final Checks

   • Spin the crank by hand to confirm there is no interference.

   • Verify alignment between the missing tooth and the VR sensor bracket.

   • Once satisfied, finish by fabricating or installing a secure VR sensor bracket.

Step 4: Sensor Bracket & Final Setup

• Fabricate a bracket to hold the VR sensor so it aligns with the trigger wheel teeth.

• Ensure proper clearance and alignment to avoid tooth strike.

• Paint or powder coat the trigger wheel and spacers to prevent rust.