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

Paul Kane

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.

Paul


Paul

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Ted 66

Thanks, Paul. And I appreciate the specs and the pic. I haven't sheared a pin (yet) and will be checking the clearances before re-installing my distributor on the 460 I'm building. I've built several windsors and Clevelands over the years but this is my first big block and your knowledge of this engine is obvious from the posts I've read. So if you don't mind, let me hit you with a con rod question. We've all pretty much read the "blueprinting" manuals out there. Do you think that polishing the beams on a set of new M-6200-A75 rods is necessary for an engine that will probably never see 6000 rpm? It'll be a warmed over easy breather with hydraulic cam and approx 10.5:1 compression in front of a C-6. Thanks again for all the education you provide here. Ted.

Paul Kane

Does polishing the beam help? Yes, somewhat. If this engine is not over 550 hp and will stay below 6000 rpm, then I wouldnb't worry about it unless you are really just into that kind of craftsmanship...nor could it hurt. (Inspect each rod for any flaws that might be of concern (aas you should do with any part from anywhere).

I feel the A75 is the best stock 460 rod Ford made.

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PaulProe

Paul
Can you comment on this in regards to a FE motor?

Ford called for 3.071-3.078 for the gear position on an FE. Are we better to only use this as a guide and physically check each installation for the installed clearance and adjust the gear position accordingly?

I am assuming with the .020-.032 endplay, when the distributor is clamped down, the gear rides on the boss and there should still be about .010 clearance.

Thanks for any input you can offer. I am paranoid about the position of my cam gear. I changed to a steel gear for my roller cam and need to be sure I get it positioned properly.

Thanks

Paul P