My understanding is the early 1990 DIS ignition modules did not have control circuitry which limited the maximum voltage signal generated by the crankshaft magnetic pickup at high engine RPMs. With the magnetic pickups the LT5 use the faster the crankshaft turns higher the voltage output will be.
To limit the output voltage from becoming to high for the DIS electronic components they installed shims to increase the air gap between the magnetic sensor tip and crankshaft slotted reluctor wheel.
Later DIS ignition modules incorporated circuitry which automatically limited the voltage the crankshaft sensor could generate to a safe level therefore installing shims was no longer required.
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