The 45D was a Lucas redesign/replacement in 1975. Both fit and work. The 25D, over the years, used more than a hundred different vacuum advance units, so they can be tailored to your car's needs more effectively. The 25D also can use different or modified advance weights to alter the advance curve (2 different size weights were made by Lucas), but in the 45D you really can't modify the weights and only 1 option exists. 

When the centrifugal weights operate in the 25D, they are unobstructed and their motion is independent, while the weights in a 45D interlock with the point cam's base, and tend to bind. This is the design flaw with aftermarket copies: the shapes of the stamped parts are wrong and they have a more serious binding issue, affecting the repeatability of the advance curve. Repeatability is what its all about - every time you're at 2500 rpm, the timing needs to be the same, not +/- 5 degrees (just one example, as ALL rpms are important!) 

There have been rumors that the larger distributor cap on the 45D actually offers more resistance to cross-firing from cylinder to cylinder. The truth behind this is that in a 4 cylinder, there really is NO cross firing problem to start with. You have 90 degrees between the terminals, which is a HUGE space to play with. In 8 cylinder cars with small distributor caps (think 1930's) it is possible to fire to the next cap terminal if its not set up properly, with a significantly smaller distance between cap terminals (45 degrees apart and physically closer to the center of the distributor). This is a non-issue in any Lucas design. 

The advance assembly in a 25D is held together with a 2BA screw (under the rotor), while the 45D advance assembly is held together with a plastic clip. Aftermarket copies are held together with a combo screw - both phillips and slotted head together. 

Both distributors have shafts with the same hardness, advance weight with the same hardness, great overall durability. In summary, the 25D is more tunable and can be tailored for any application quite easily versus the 45D which isn't as easy to tune for some applications. Out of the box, the 45Ds that are currently sold are ONLY good for use with ported vacuum, as they all have the same vacuum unit with too much advance for manifold vacuum.

By Jeff Schlemmer