A fluctuating dwell angle reading is the first clue that a distributor needs rebuilding. If the distributor shaft bushings have wear in excess of two degrees variation on the meter, the distributor needs a complete rebuild. Shaft and bushing play can be tested with the distributor still in the engine. Remove the cap and check the movement of the shaft. If a dial indicator is not available, insert a feeler gauge into the point gap with the rubbing block on the high point of the cam lobe. Pull and push the shaft toward and away from the rubbing block. Note the amount of change in the point gap. Excess movement is a clear sign of bushing wear. If in doubt, remove and disassemble the distributor. Often, after cleaning away varnish and debris, the shaft play will be even greater.
If a rebuild is necessary, mark the location of the distributor base and the intake manifold. Cap removed, index the rotor by placing the crankshaft damper at the TDC mark for #1 cylinder. Mark the distributor housing where the rotor points toward #1 spark lead. This will enable easier installation of the distributor and provide a firing position for starting the engine. Once the engine starts, you can verify timing with a light, following the procedure in your factory workshop manual.
Though not complicated, distributor rebuilding requires care, detail, and cleanliness. The distributor's driveshaft spins the oil pump, which must function flawlessly. Debris that falls into the engine can contaminate the crankcase. If abrasive, debris can circulate and cause internal parts damage. Use a shop vacuum around the distributor's base and the intake manifold before removing or installing the distributor. Cover the distributor opening with a clean rag to prevent debris or hardware from dropping into the engine when the distributor is out.
The ignition distributor, including its advance mechanisms, plays a crucial role in the performance and reliability of a Corvette engine. Adequate spark and timing accuracy determine the ease of startups, acceleration, cruise performance and passing power. When wear exceeds simple tune-up needs like new spark plugs, points, condenser, rotor and cap, a distributor rebuild can restore power and provide years of reliable service.