Originally Posted by Ted 66
Is the downward thrust enough to consider machining the block for a small, enclosed thrust (roller or needle) bearing? Could this modification have a positive impact on ignition stability? Has anyone tried it? Curious. Ted
I don't know of anyone that has done this, and personally I don't see the need. The factory design, with the block thrust boss as machined, seems to hold up for a hundred thousand miles in passenger car vehicles and will still look good enough for continued service.
Distributor spark scatter is not generated from the downward thrust but is more so the byproduct of the oil pump's internal gerotor mechanism loading/unloading as it pumps oil and slightly twists the hexagonal driveshaft (which is connected to the bottom of the distributor). The modified Melling oil pumps that we offer on our website actually have modifications to address this issue and thereby help the stabilize distributor timing issue that you are noting.
The most important aspect of the distrbutor thrust boss/oil pump drive system is setting the proper clearance specs; then everything works well and wears properly.
There is a distributor shaft end play spec
of .020" and an installed clearance spec
of less than .010" between the bottom of the dizzy gear and the block's dizzy gear thrust boss (the pad that the dizzy gear rides on once the distributor is installed). Few people understand Ford's reasoning behind the obscure specification and therefore dismiss it.
The reason for these spec is because as the distributor is turned by the cam gear, it is thrust downward against the dizzy's thrust boss in the block.
If the shaft end play and boss clearance are out of spec (too much boss clearance/not enough end play), the dizzy gear does not ride on the boss but instead is floating in mid air all the while being thrust downward by the turning of the gear.
Eventually, the dizzy gear roll pin shears downward
because the spec's were not set by properly measuring/installing the dizzy gear and/or shimming the underside of the dizzy gear to take up the slack between the boss and gear. Here is the spec, please read:
To say that the Ford engines randomly shear pins rotationally is a fallacy which is assumed by those that are not familiar with the detailed workings and necessary specifications of these engines. Their fix is usually to drill oversize and put in a bigger pin though the gear.
The joke about such an approach is that this is a band-aid fix that may or may not end the pin shearing, because more than likely the root of the problem has not been corrected.
Here is a picture of the dizzy gear thrust boss in the block:
If you look in the mirror's reflection in the above photo, you will see just the lightest wear (from the bottom of the dizzy gear) against the top of the boss. This is exactly how the wear pattern should look. If you have more wear, the block gets chewed up; if you have NO wear, the gear is floating and you are shearing pins.