Model A & B

Ford Garage

Timing is Everything

But What Is It?

Proper timing is very important for optimized and reliable operation!
The results of improper timing are far less than favorable!

Hell, you might even shoot your own propellor off!
(or blow holes in the tops of your pistons!)


Model A Ford enthusiasts love to talk about timing! If the hood is open or someone has a car running not quite right, then you can bet someone nearby is talking about timing.

The thing I have often noticed is that many people are not quite clear about what timing is, and are sometimes confused by different aspects of timing as it relates to engine operation.

There are two distinct types of 'timing' which affect engine operation and performance. The first is Valve or Camshaft timing, and the second is Ignition or Spark timing.


Valve or Camshaft Timing:

Valve or camshaft timing is the mechanical relationship between the movement of the crankshaft and pistons, and the movement of the camshaft and intake and exhaust valves.

Valve or camshaft timing controls when, and for how long, the air fuel mixture from the carburetor is drawn into the cylinder for compression, as well as when, and for how long, the exhaust is released from the cylinder.

The valve timing is mechanically designed into the engine to control the valve opening and closing points, as well as the rate of movement and lift of the valve(s) relative to crankshaft position and piston travel and position.

The valve movements are controlled by the design of the camshaft lobe profiles.

Valve timing is fixed, and is neither speed dependent nor adjustable on a Model A or B engine while it is running.

The valve timing is controlled by the mesh of the camshaft and crankshaft gears, the profile of the lobes on the camshaft, and the clearances between the valve stems and tappets.


Ignition or Spark Timing:

Ignition or Spark timing is the relationship of the firing of the spark plug relative to the piston and crankshaft (and camshaft) position.

The distributor (or ignition system) also controls the electrical saturation of the ignition coil and the collapse of the electrical field strength, and thus controls the resulting spark energy intensity from the secondary winding of the coil. That is the spark energy that is delivered to the spark plug.

The actual spark timing is controlled by the start and stop of current to the ignition coil, through the primary contact points in the distributor.

The mechanical contact points in the distributor are operated (and timed) by the rotation of the distributor cam inside the distributor body, under the rotor. The distributor and oil pump shafts are both gear-driven off of the engine (valve) camshaft, which in turn is gear-driven off of the engine crankshaft.

The ignition timing is something that is determined by the shape and rotational position of the distributor cam relative to the crankshaft position, as well as the size of the contact point gap compared to the design specified gap.

Changes in contact point gap affect both the coil saturation (dwell time) as well as the actual spark timing (instant of firing).

Wear of the movable contact point arm rubbing block on the distributor cam causes a reduction in gap over time, and retards the effective spark timing (as well as increasing the dwell time).

The contact point gap is statically adjustable in both the Model A and Model B distributors by loosening the clamp screw on the point block and screwing the stationary contact point in or out to change the gap.

In the Model A ignition distributor, the relative rotational position between the distributor contact points and distributor cam can be varied manually while the engine is running (by the adjustment of the spark advance lever on the steering column).

In the Model B ignition distributor, the relative rotational position between the distributor contact points and distributor cam varies automatically with change in engine rpm based on the response of a centrifugal flyweight advance mechanism (shown above) built inside the Model B distributor.

Also, the static spark timing position of the Model B distributor can be manually externally adjusted plus/minus several crankshaft degrees by loosening a screw and rotating the upper contact point plate as shown above.


So how do you know if your engine is timed right?

Watch this entertaining video from Tex Willis to be sure!


Links to more Model A Timing topics on Ford Garage:

  1. Model A, B, & Model 46 Camshaft Gear Timing Cover Differences
  2. Model A & B Camshaft-to-Crankshaft Positioning
  3. Model B Distributor Advance Operation
  4. Model A & B Valve Tappet Adjustment ~ Rule of Nine
  5. Model A & B Camshaft Specifications
  6. Model A & B Bill Stipe's Camshaft Specifications

June 2006