Rotor Tip Change you propose sounds OK if you are sure to check clearances as 20% of the MO3000 that I measured had interference clearance issues and your proposed rotor will likely have the same odds of dragging on a terminal.

The purpose of rotor tip extending is to give you an opportunity to machine out cap terminal eccentricity. You do not need a jig to determine which distributor cap posts to machine. Take a spare distributor and mount your cap and rotor after marking the gear as to the direction the rotor is pointed. This allows you to know which terminal the rotor is pointed at any time. Slowly spin the distributor gear and look for a hit.

The trick is to put a side load on the gear pulling the gear toward you while the rotor tip is pointed away from you. Side pressure on the gear will essentially take out distributor bushing "wiggle" just like a cam might exert a side pressure on the distributor gear. I have found this technique builds in a minimum .015" clearance between the tightest distributor cap terminal post.

Without a jig you can't determine all the clearances but at least you avoid stripping a distributor gear due to a dragging rotor tip hitting one or more cap terminals.

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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/

Thanks for the info DonPal , i have a spare distributor so i will use your method .

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DRIVE IT,DON'T HIDE IT

I elongated the hole in the wide brass tip with a dremel like tool and a fine drill bit to give me the required distance .



Before i set the distance i extended the tip a bit longer than the MO-3000 but i was getting a slight hit on one terminal , this was using the method DonPal described .I then set it at the same length as the MO-3000 and had no terminal contact.



Beforehand slanty was running really well with the MO-3000 and the boost but now seems crisper and to be burning better , if you know what i mean .
Especially when boost come on .

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DRIVE IT,DON'T HIDE IT

Is the arc of the wide tip correct?

Does it touch in the middle or the ends?

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Print the picture and swing a divider from the point of contact at the rotor center electrode contact to the tip of the new rotor tip to answer that question or build a jig to measure clearances.

I found the MO3000 standard tip to have the tightest clearance at it's edges because it is sawed off straight and is next to an arc distributor terminal. Although any rotor tip arc beats the pants off an MO3000 standard tip the FD117 tip arc is almost a dead match to the slant distributor cap terminals.....probably because there isn't much difference in the V8 & slant cap OD?.

Of course if you machine the cap terminals then you can shape the terminal post to match the rotor tip very closely.

The end result might be best evaluated by measuring the change in coil wire secondary voltage ( over a range of rpms) when making a change......a drop in coil wire voltage would indicate that gaps in the distributor cap were reduced during the swap over the entire rpm range...... enough to extend life of the ignition components and get improved spark at the plug.

Based on a couple dozen tests it's looking like 2000 to 4000 volts is the best to expect in the coil wire for points or HEI ignition with the standard spark plug gap of .035.

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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/

Purchased two MO40 caps from Napa together. The boxes were different with one having the old Echlin design and the other with a relatively new Echlin design. The older box had a typical tan cap with brass/copper colored electrodes. The newer box had a black cap with aluminum colored electrodes.
The tan cap was measured and had the same type of typical eccentricity in the machining with gaps ranging from .015 to .045” when using the MO3000 rotor.
The black cap was also measured for gaps with the extended MO3000 rotor and had only small deviations in the gaps from .020 to .025”……a pleasant surprise.

Using a slightly extended wide arc rotor should allow use of this cap without any need for machining and have gaps in the desired .015 to .020” range. I’ll measure the coil voltage for that combination but expect great results with voltages below 3000 volts.
Draw your own conclusions but Napa may have changed suppliers and getting different quality results? That would be great news if true.

Providing a detailed background and write-up on the three measurements to take (and how) and coil voltage check at this location (part 1 just posted):

http://www.plymouthcarclub.com/2018/01/ ... ity-check/

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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/

Left hand giveth (less eccentricity), right hand taketh away (aluminum and black plastic instead of brass and tan alkyd).

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

Sorry to dig up an old thread but I’ve noticed that this is the correct thread to discuss the issues. I’m having a hard time finding a long tip or wider tip rotor button. I’ve been using the regular one so far and it’s decent but probably not as good as it needs to be running HEI. What would everyone suggest? I’ve also blown 2 ignition modules from water intrusion (last week was one due to a large water puddle). I have it mounted on the fender well and the splash guards are in place. Any suggestions on what to do about this as well?
Edit: Almost forgot I’m in a 78 dodge d100 that has been upgraded to HEI.

The extended rotor use to carry the NAPA number MO 3000. Use care when using this rotor to insure the rotor is not hitting a couple of the distributor cap terminals.

Remember what the real issue with modern caps that is causing your coil wire voltage to spike & shorten life of the HEI components.
1) Terminals were pulled away from the standard rotor back in the 70's = more rotor to cap terminal gaps (makes up for sloppy machining at that time)
2) Sloppier machining now of caps results in the center of the rotor not lining up with the center of the sloppy machined cap terminals
Consequently if you put a tester together and measured the gaps between the rotor tip and the distributor caps you would find the gaps vary by almost 1/8"
For example: #1 gap might be .100" and #6 gap be .015" with a standard rotor.
The gap of .100" means there way more sparking in the cap than at the spark plug and coil wire voltage tripling!!
You can imagine what happens if you extend the rotor; #1 distributor terminal for our case could have a direct hit with the rotor and strip your distributor plastic gear.
Been there, done that.....and it initiated my studies on the subject.

Rotor tip to Distributor gaps between .015 to .025 are your best limits to maximize life of your HEI components and distributor cap center electrode.

I can email you pictures of the gap tester,
I suggest you test every rotor & distributor cap for gaps before use. Frequently I grind a couple of the cap terminals when using the extended rotor.

I now only purchase new old stock caps to get away from the poorly machined caps sold today that are non fixable in many cases they are so bad.

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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/

This very thread is all about how to put together exactly such an item. Read through it—the whole thing, start to end—and then you'll know what to do.

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

Good reading, thanks for bring it back up.

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Hyper_pak

I never understood why they called them "rotor buttons"
The actual button is captured as part of the caps construction.
Ai remember reading on this thread quite a while back, and around that time I found a handful of the "premium" old school "blue streak" caps that are probably 35 years old or more. I forget the number, and I ain't home to go look for a couple days. It's the one with the "x" after the part number. I also got a few gp Sorensen, Sorensen before the gp came out and I think the other one may be filko. All look better than current stock common brands I've seen.
I also got a can (remember the old squarish cardboard cans with metal ends) of new but old Delco Remy rotors for a /6. I don't feel like cutting up my stash for testing (then I won't have any to use when my current ones in use, wear out) but I have an "x"cap on mine with one of those Delco rotors and it starts easy and I don't have any misfires happening....
And for the record I have always preferred brass contacts to aluminum

Ok I went back to page 1 and looked as a mini refresh
The blue streak caps I have are the CH410X...

"I don't feel like cutting up my stash for testing"

Testing can be done without the need to alter or damage a cap you want to use in the future.

Rather the testing is done by mounting it to a re-bushed testing distributor. The cap then rests on it's internal plastic shoulders as it would on your driver distributor. The gaps are then measured using also the rotor you intend to match with that cap. The cap and rotor are then a matched set to be mounted to your driver distributor (after your own re-machining of poorly gapped cap terminals).

The "best caps" are those that have wide terminals which address another operation issue; advance gaps. These "best caps' are likely 6 decades old & nearly true on dimensions giving about a .020" gap between terminal & rotor tip.

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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/

Because different variants of English have different names for things. Americans say "Bumper guard"; Brits and Aussies say "Overriders". Americans say "Elevator"; Brits and Aussies say "Lift". Americans and Aussies say "Parking lights"; Brits say "Sidelights". Americans say "Carb"; Aussies say "Carby". Americans say "a-LOOM-in-um"; Brits and Aussies say "Al-yuh-MIN-ee-yum".

They're going to keep right on calling it the rotor button, because that's what they call it. ¯\_(ツ)_/¯

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

Nicely made, but available only with aluminum contacts. Hence the recipe in this thread for achieving the same thing with a wide rotor tip.

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