Distributor Drive End Play

[Scott Novak]

Distributor Drive End Play

You want the drive dog to engage the distributor drive gear fully, but with a small amount of clearance.

As the engine temperature increases towards operating temperature, the aluminum distributor shaft housing is growing and moving the distributor drive dog towards the distributor drive gear and reducing the clearance.

At the same time the magnesium engine case is also growing and moving the distributor shaft housing and drive dog away from the distributor drive gear and increasing it's clearance.

As thermal expansion coefficients of magnesium and aluminum are fairly similar at the operating temperature of the VW engine, I suspect that the two expansions negate each other and will result in very little change in clearance between the distributor drive gear and the distributor drive dog, as the engine reaches operating temperature.

In his book "How to rebuild your VW Air Cooled Engine", Tom Wilson recommended a 0.020" minimum clearance between the distributor drive gear and the distributor drive dog. However, as I don't believe that this clearance changes much as the engine temperature increases, a smaller clearance would probably be adequate.

Also realize that this clearance is dependant on the actual distributor that you are using. Every distributor will have a slightly different length. The recommended 0.020" will probably accommodate the variations in distributor length from one distributor to another.

If you have reduced this clearance below the recommended 0.020", you can't just slap another distributor in the engine without checking and adjusting the clearance.

Gene Berg had mentioned that he reduced this distributor drive gear clearance to reduce timing scatter, but I don't see where he mentioned how much. John Connolly mentioned that he used a distributor drive gear clearance of 0.005".

Ultimately the clearance should be checked while the engine is cold and also when the engine is hot to be sure that you have sufficient clearance.

I've noticed that nearly every used stock steel distributor clamp that I've seen has been over tightened and bent. The bent clamp will raise the distributor upward slightly. So you may want to check your distributor clamp to make sure that it's flat and not bent, as this will make it more difficult to set the clearance between the distributor drive gear and the distributor drive dog.

Has anyone ever measured both the room temperature clearance and running temperature clearance to see if it changes?

What are your opinions about the minimum clearance needed?

Have any of you noticed a reduction in timing scatter by using reduced clearances?

What methods do you use to measure the distributor drive clearance when the engine is already assembled?

Scottt Novak

[raygreenwood]

I think I have a few threads on this in teh 411/412 forum and maybe the type 4rum. The stock dizzy had way too much slop..especially on the fuel injected models.

I find that .020 is about the max. you would want..considering you also have radial play for the dog to slide side to side on the roll pin. That is actually more important for axial alignment differences.

What you really want to do during assembly is find out how well the dizzy fits into the drive slot when the drie assembly is under torque.
Do this by inserting the dizzy...and teh drive gear and the crank on its bearings in a case half. Then turning the crank in the correct drive direction to take up the slack....and see that the rise of the drive gear comes right up to the face of the drive dog on the dizzy. Between teh spring and say.....010-.015...you have plenty of slack.

Just aso make sure that if you change shims between drive dog and dizzy body....you heat test the assembly to abot 325 in the oven to make sure that you have enough slack. Ray