How -To Rebuild Autolite/Motorcraft Distributors

Take note of the centrifugal advance weights and springs before removal. This is something you don't have to do either because the shaft assembly will come out intact. We're disassembling the mechanical advance because we want you to see how it works. It's a good idea to disassemble this mechanism for clean up and lubrication with high-temperature grease. The C4AF distributor has a different mechanical advance design than C5AF-and-newer distributors, which have smaller weights and a different type of rotor cam, just to name one difference.

Gently remove the distributor shaft. Use WD-40 on the shaft, then gently twist to prevent damage to the shaft and bushings.

Reassembly has begun and the mechanical advance is in place. Note a heavy spring and light spring, which allow seamless transition from vacuum advance to mechanical advance. Advance adjustment is achieved by gently bending the spring arms in or out to increase or decrease spring tension. Increasing tension slows the rate of advance so it happens at a higher rpm. Decrease tension and mechanical advance happens sooner at a lower rpm.

The rotor/cam button has been reinstalled, which is easier than removal. You just have to work the C-clip in there until it seats. Use a high-temperature lubricant between the rotor/cam and shaft for smooth operation. If the rotor/cam sticks, you can forget the spark advance you need at higher rpm. It must operate freely.

We rebuilt our Autolite breaker plate using bushings from a new Motorcraft plate. This way, you get the steel pivot and retainer for reliability. The only part that wears out is the nylon rub bushings between plates. Replace them and you have a new breaker plate. Note that the best modification you can make to a classic Mustang is the PerTronix Ignitor II, properly adjusted with the 0.030-inch thickness gauge provided in the kit. Do not forget to connect the stray ground wire, securing it where the condenser was. This wire grounds the floating breaker plate and the PerTronix Ignitor II module.

MCE Engines' Marvin McAfee knows how to dial in distributors. Have your distributor curved by a professional, or if budget or logistics is an issue, you can curve a distributor in your engine. You will need a timing light, a dwell meter, and good street tuning smarts. We've got more on this in the "Precision Tuning" sidebar.

Aftermarket vacuum advance units are easy to adjust via the vacuum port with an Allen wrench. Turn clockwise to slow advance rate and counterclockwise to speed up advance rate. Adjustment clockwise increases spring tension inside, which requires more vacuum to get a response. Counterclockwise reduces spring tension, causing spark advance to occur with less vacuum.

This is an original Ford vacuum advance unit that uses shims to adjust advance rate. Adding shims increases spring tension and slows advance rate. Removing shims decreases spring tension, causing spark advance to come on sooner. The shim in this image is the flat washer between the spring and screw-on fitting.

Set initial spark timing with the vacuum advance disconnected and plugged. Initial spark timing without vacuum is anywhere from 6 to 12 degrees BTDC at idle speed. Although your Ford shop manual will call for a specific setting, initially set timing based on idle quality. Make sure you're on the carburetor's idle circuit at true idle when you set ignition timing.

Harmonic balancers are marked in degrees, centered at TDC (top dead center) and marked in 10-degree increments before top dead center (BTDC).

This Fluidampr balancer is also marked to 10 degrees after top dead center (ATDC). These are the marks your timing light (strobe) will capture each time the No. 1 spark plug fires. As you move the distributor clockwise and counterclockwise, these marks move with the timing.

Jeff Latimer of JGM Performance Engineering uses a timing light on an FE big-block. The timing light's strobe flashes in synch with the No. 1 spark plug, which freezes timing increments on the harmonic balancer. As Jeff moves the distributor, these marks move toward advanced or retarded spark timing.

This is how a piston looks around 10-15 degrees before top dead center. With initial timing set at 10-15 degrees BTDC, the mixture fires before the piston reaches top dead center. There's a reason for this. Contrary to popular belief, the fuel mixture does not explode in the chamber. It reacts in more of a quick fire that flashes across the top of the piston, making heat energy that acts on the piston. Because it takes time for fuel to ignite and make heat energy, we need to light it before the piston reaches top dead center. By the time the piston reaches top dead center, it's ready to move downward, turning the crankshaft and making rotary motion.

Here's a C5AF distributor, for 1965, with the revised mechanical advance. Note the "10L" marking, which indicates half the total number of degrees of timing. Expect to see 13L, 15L, and 18L as well. Only one side of the distributor cam provides the function, which means you can turn this cam 180 degrees to achieve the opposite number. So if you're on the 10L side, you get 20 degrees of advance. If you see 15L, you get 30 degrees of advance. And yes, at 18L, you get 36 degrees. Spring tension determines when mechanical advance begins to happen and how quickly it happens.