So do I. I make my living doing technical writing and editing. That's what this is. There's nothing about this TSB that suggests they're trying to cover up a failure or an error or any other kind of oops.
That's not a "truth", it's a guess, and I don't think it's a correct one. On your side you've got your imagination and your interpretation of what you think you see in that TSB. On my side? Well, Chrysler didn't go back to the long-tip pre-'75 rotor, and the aftermarket rotors from every brand copied the running change and went to the shorter tip. And yet...somehow...Chrysler products didn't start dropping to the roadside like houseflies under a bug zapper. Even when they're just owned by people who put in whatever rotor is available, not by geeks who try to learn all about the effects of rotor tip length.
Any other part of the ignition system.
Yes, and at that time Chrysler's quality control was in the toilet, and parts failures were a whole lot more common than they are now. Ignition modules, pickup coils, ignition coils, spark plug wires, spark plugs. Sorry, dude, but the TSB explanation is more cogent and reasonable than the scenario in your imagination.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Bringing table forward to this point and added gaps for Wide Width Rotor & Echlin cap #10 combo.

http://tinyurl.com/hj64bff

Gaps are tight because the above rotor was extended almost .010 longer than a MO3000 so that the Echlin Cap # 10 could be machined with gaps similar to the 60's Gold Standard. The rotor itself is slightly wider than the 60's double width terminal cap.

Note that the Gaps do not go under .016......what might be the minimum gap ever wanted. Backing into that assumption based on the following:

1) When placing large eccentric caps on multiple new and old distributors it was observed that the rotor might clear the terminal (when clearances were less than .010) but if a side load was applied to the bottom gear then the rotor would hit the terminal. Said another way if a distributor has enough bushing play the distributor shaft can angle slightly in the bushing causing the rotor to "eat up" all the small clearance.

2) When machining some of the cap terminals to clear the extended rotor I found that after getting all to clear if I applied side pressure to the jig the rotor would hit. So I took off a few more thousands to get the rotor to clear the terminals when the side pressure was exerted. Then went back and measured clearances and got .016 several times......essentially I was backing into a minimum clearance that would work if the distributor shaft was going thru an orbit when side pressure might exist such as applied by the cam, by inertia of the rotor itself, or points pressing on rubbing block or all together.

Essentially backing into the idea of a .016 minimum clearance to satisfy new and worn distributor shaft to bushing tolerances.

Anyone can do this with their own spare distributor to validate if their cap will work with their MO3000 or extended rotor........simply apply a sided load to the gear pressing away from the rotor tip while rotating. If you hear rubbing then reject the cap as excessively eccentric?

Slant Six Distributor Cap Terminal To Rotor Tip Gaps
Table shows results with Exchlin MO3000 Rotor (uln)
CAP MODEL CAP# MAX MIN ECC
Echlin MO 40 1 .030 .013 .008
Echlin MO 40 2 .038 .010 .014
Echlin MO 40 3 .041 .012 .014
Echlin MO 40 4 .022 .017 .002
Echlin MO 40 5 .034 .014 .010
Echlin MO 40 6 .038 .008 .015
Echlin MO 40 7 .040 .015 .012
Echlin MO 40 8 .032 .015 .008
Echlin MO 40 9 .041 .009 .016
Echlin MO 40 10 .030 .025 .003
STND CH-410 1 .030 .008 .011
STND CH-403X 1 .040 .015 .012
AIRTX 5D1022A 1 .030 .005 .012
REMANUF Echlin 9 .018 .012 .001
UNITED CC611X 1 .040 .006 .017
Echl #10 Wide Rtr .022 .016 .003

1960's & Early 70's Typical Gaps

ECH MO 6/1838516 1 .028 .013 .007
2642986/1838516 1 .027 .010 .008

For Different Rotors measured:
Echlin MO 13 add .057 to all gaps
CARQUEST D147P add .056 to all gaps
MOPAR 1838516 subtract .008 to all gaps (interference fit on Echlin #6 & Standard #1)

All dimensions in inches

Secondary Voltages
THE DATA CASE GAP SEC VOLT
The Worst Gap .114 22,000
Medium Gap .025 9,000
60's Dbl. Width .028 2,200
Machined Gap .018 5,000
Wide Rotor Tip .022 1700

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

Looked threw both 75 and 76 tsb books. no further info on rotors.Still looking.

Added secondary voltage results to above table.......1700 volts for extended wide width rotor.

Time to start reliability check for the wide width rotor with .016 to .022 gaps to cap terminal......specifically when does the cap center electrode start fracturing.

Already have several fracturing data points for past conditions:
a) 5000 volts - 20,000 miles
b) 8000 volts- 7000 miles
c) 10000 volts - 10,000 miles (plug gaps at .045)
d) 25000 volts - 1000 miles

Next would be 1700 volts - TBD
Hopefully this will take some time.

Might be important info to those that drive their cars frequently.

Will also quickly check out another of the wider width rotor tips Dan had in his list to see if I can make the rotor lighter and easier to build.

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

Same here and same here. I know I've seen this! At this point I'm thinking it was either in another book by Stockel and Stockel (Auto Service Fundamentals), in another Petersen book (Petersen's Big Book of Auto Repair—this one's the most likely candidate, but I don't have it) or in one of the less-popular DIY magazines like Mechanix Illustrated.

Donpal: COOL!

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

A 65 Cuda owner provided data for above chart giving secondary voltage (8000) and observed fracturing of cap center electrode at 7000 miles so chart was updated.

PM me with any of your similar secondary voltage/cap electrode observations so there is enough data points to build a curve that predicts cap replacement time frame for your slant......that way you can know if this is going to be a problem that is going to be a long term one for your yearly mileage.

Secondary Voltage can even be measured by an old analog Sears Penske Auto Analyzer at swap meets or on Ebay if you don't have access to a diagnostic Sun Tester or newer digital measuring tool like TA100 SmarTach.

http://tinyurl.com/SecondaryVoltageTestors

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

An easier rotor to make using a FD117 rotor tip among SlantSixDan suggestions involves cutting to length and drilling a hole.

Below picture steps involve:
1) comparing MO3000 tip to FD117
2) marking FD117 tip for cutting
3) center punching FD117 tip for drilling
4) trial fitting modified FD117 tip in MO3000 rotor
5) using soldering iron to flow MO3000 rotor plastic into FD117 grooves
6) marking distributor gear to use as indicator for when machining distributor cap gaps

http://tinyurl.com/h3gc79n

I can provide more detail on each little part of the machining if there is an interest.

The Cap:
Next step is to try machining a poor cap with excess eccentricity into a decent one to fit the above rotor (which is about .010 longer than a standard MO3000 rotor tip). The idea is to do it without a jig as most might attempt. Then check it with the jig to see how good it actually turned out.

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

YES, PLEASE!
You are doing some very great work here, Don. Thank you for doing it and thank you more for documenting and sharing it. I look forward to seeing your work on the caps!

Would it be possible to use a modified distributor body as a sort of lathe with a cutting tool in place of the rotor so distributor caps could be clamped on and machined quickly and easily?

"Would it be possible to use a modified distributor body as a sort of lathe with a cutting tool in place of the rotor so distributor caps could be clamped on and machined quickly and easily?"

Concept wise ...YES

Unlikely to work though (without large mods) due to excess tolerance on distributor shaft which would bounce at least .010 to .015 causing random dimensions. If the shaft was removed and double bearings installed then it would be possible to correct for the sloppy distributor/shaft arrangement. The feed mechanism for the tool would be provided for by feeding the distributor shaft thru the two bearings.

Good that type thinking is in progress!!

This also begs the question; why isn't the factory machining working?
It's not sloppy tooling because the gaps measured point to an eccentricity problem and not sloppy/loose machining equipment. One possibility is the cap is clamped to it's bottom surface and the inner surface ID provides for locating the cap center. But when we fit the caps onto our distributors the cap seats on the inside surface ring.....perhaps the plastic molding process hasn't got the tolerance needed to match the two surfaces parallel to each other. (bottom surface versus inside seating surface). Down at the bottom of the cap there is likely no eccentricity but a few inches away where the terminals are there is eccentricity. If the factory changed the clamping surface from the bottom of the cap to the inside seat where our distributors clamp .....all would be fixed?

I'll put a detailed write up for suggestions in getting the FD117 tip onto the rotor in the right location and mounted securely....all with the thought of enabling machining of the worst cap into something with decent gaps using a 1.5 inch wheel on a Dremel......without the need for a jig.

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

Your evidence (NOS '60s caps being much less eccentric than recent-production caps) and my experience in the auto parts industry suggests today's distributor caps are lousy for the same reason today's sealed-beam headlamps are lousy: there's no money in 'em, so the tooling is old, decrepit, worn out (new tooling would never pay itself back) and being operated as fast and cheaply as possible.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

See attached link for requested detailed FD117 Rotor Tip & MO3000 Rotor fabrication instructions:

http://tinyurl.com/Build-FD117-MO3000-Instruction

Any suggestions for improvements?

ps: If printing suggest setting your margins to narrow and number of pages to 3.

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

Hmmmm. I wonder how difficult it might be to 3D print a distributor cap. And could one halt the program mid-stream to insert brass terminals?

Over my head, but speculating.

Roger

"Hmmmm. I wonder how difficult it might be to 3D print a distributor cap. And could one halt the program mid-stream to insert brass terminals? "
Here is an article on where you are are likely headed.....it's a complex approach to a simple existing problem......turn to page 177 and beyond:

http://tinyurl.com/5AxisDistributorCapMilling

That concept basically tries to take a complex shape and get the tools to straighten out issues using high tech digitizing and CAD modeling.

The internet also has a 1954 patent on how to position the terminals without machining!!!!

But our problem maybe is this simple; to understand make yourself a machine designer for a minute:
1) Picture thousands of caps going down a conveyor belt headed toward the boring bar; they are resting on their bottom plastic surface.
2) When the belt stops at the boring location several clamps come up from below and plunge 1/8 inch into the cap bottom,pivot and center the cap on the boring bar center.
3) a smooth underlying machined surface moves up to the cap bottom and places the cap at the right height before an upper clamping bar presses the cap firmly into the machined surface
4) the mint perfect boring bar strokes enough to machine the terminals in a near perfect circle with the correct diameter. All tolerances are measured nearly perfect within .001 ID and concentricity. The surface finish matches a rough surface generally acceptable for a quick plunge and the duty cycle......arcing.

So your so proud that in less than a second you have a thing of beauty.....which it is from your measuring vantage point.....the bottom surface and it's center.

Then someone sells the cap and the new owner clamps it onto the distributor with slightly long rotor and gets instantaneous poor results......the rotor hits a terminal on one side of the cap and big gaps occur on the diagonal opposite side.

???

Unfortunately the eccentricity occurred when the new owner moved the reference point from the bottom of the distributor cap to an inside surface of the distributor cap.

So even if the machining was perfect......it was relative to the bottom of the cap which the final owner ignored when snapping the distributor cap onto the distributor body.

After doing some searches on google for how they machine distributor caps and looking thru the various patents it appears that the importance of machining relative to the inside seating surface of the cap (to the distributor body) isn't a highly discussed topic.

What would happen if that fine machine surface that came up from below was a little smaller in diameter and rested on the inside shelf of the distributor?.....probably a cap way better than we needed??

So you the machine designer come up with the simple fix of altering the retractable bottom machine surface outer diameter and tell the machine shop owner of the improvement needed. But the machine shop owner states "that's the way it used to be but only a few type of distributor caps could be machined...the new approach allows for machining of thousands of different caps.....forget it".

Suggest if there is any interest that a little surfing might be helpful to understand what goes on.....but won't fix the problem which is the main goal.

Plus we are lousy machine designers as a lot so it's best to sticking to correcting the caps to be concentric to our distributor shaft rotor travel path.

Next up ....how to attempt that effort using a spare distributor, your eyes, ears, sense of feel, extra long rotor, and a dremel with a 1.5 inch diameter cutoff wheel......no jigs or fancy measuring devices.

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

Hey, keen! The pics even look like a \6 distributor cap.
Link plz
Exactly.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.