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Corvette Distributor Rebuild - Rebuilding The Tach-Drive Distributor

Restoring Performance In A Delco-Remy Window Cap Distributor

Moses Ludel Dec 2, 2009
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The postwar General Motors OHV V-8s were difficult to tune. Ignition distributors mounted at the firewall end of the engine, and the breaker points and condenser were difficult to replace. In the late '40s and 1950s, professionals turned to distributor strobe machines and worked on the ignition with the distributor removed from the engine.

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As a solution to these tuning difficulties, the '56 Cadillac introduced Delco-Remy's innovative "window cap" distributor. The newly designed ignition afforded ready access for breaker point adjustment. Dwell angle settings and point gap changes could be made with the cap in place and the engine running. This eliminated the cumbersome, repetitive removal of the distributor cap while performing a tune-up in the engine bay. Other G.M. divisions adopted the window cap distributor, and by 1958, only the dual four-barrel carbureted, dual-point Corvette engines retained a non-window cap distributor. This '57-'61 #1110891 application is smaller in diameter at the cap to permit installation of two WCFB carburetors.

From 1958-'74, single-point Corvette distributors have window caps. (Most '58-'65 mechanically fuel-injected engines feature a unique, dual-point window cap distributor. Some later applications use a transistorized distributor.) The window in the distributor cap provides access to a set of redesigned points that use a horizontally positioned breaker-point adjuster screw. Window caps simplify and refine the tuning of breaker points.

Breaker point Delco-Remy distributors were well crafted and durable. Engineering innovations, like a window cap distributor, were aimed at ease of service. In addition to the window cap, the breaker points have a pivoting feature that enables external adjustment of the point gap. Using a dwell meter, the point gap is set without needing to remove the distributor cap several times. These point sets can be adjusted with the engine running!

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Unknown to many tuners, the window cap distributor's breaker points can often be adjusted without a dwell meter. In the earlier service data, one approach called for adjusting the points roughly, enough to start the engine. Gap the points with a feeler gauge or by eye then install the cap. Start the engine. With an Allen (hex) wrench inserted into the adjuster screw, turn the screw clockwise until the engine begins to misfire. Back the screw out 180 degrees. This brings dwell within specification in most cases.

Trackside, this fast breaker point adjusting method proved a boon. It also works nicely when the points fail while cruising through a deserted neighborhood at night. If you carry an extra set in your road kit, you can install points and adjust them with nothing more than an Allen wrench. You can confirm dwell with a meter once your Corvette is back in a secure environment-with access to more tools.

Traditionally, these point sets were packaged with the points preset, often close enough to start the engine. Slots at each end of the point set allow easy installation of the points without removing either hold-down screw. The starting gap is approximately 0.016 inch, which can be eyeballed if a feeler gauge is not available. As soon as the engine starts, adjust the points with a dwell meter or use the "turn-in-to-roughness then back out 180-degrees" technique. Accurate dwell angle must be set with a dwell meter.

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Tuning must be performed each 12,000-15,000 miles, even with the long life point sets available. Some owners opt for installing an electronic breakerless conversion, and this is not difficult with the kits available. The choice is simply owner preference. Most kits permit restoring the distributor to stock if necessary.

Tach-Drive Distributor Rebuild
At higher mileage, a C-1 through 1974 window cap distributor likely needs rebuilding. For restoration or rebuilding purposes, a tachometer drive distributor has some additional wear points. The distributor illustrated is distinguishable by its iron housing and no drive for the fuel injection. The 1111493 distributor tag number indicates a '69-'70 350 horsepower 350 V-8 application. This design is typical of '62-'74 non-transistorized, non-fuel injection units.

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.

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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.

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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.



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