Technical Distributor Rotor Tip to Terminal Clearance?

Is there a standard gap measurement inside the distributor, between the rotor tip and the cap terminals?

Everybody seems to spend a lot of time obsessing over spark plug gap but it looks to me like different rotors are all over the place (in these pics (not mine) all of them are listed as "correct" for the application.) I'm measuring about .090" or better using my calibrated eyeball in my Ford distributor. Seems to me maybe it's a later, emissions era rotor? Did gaps widen out, is this a good thing, ? etc.

 

NOOOOO.
Back in the "old" days, the rotor tip actually touched the terminal tips if the distributor cap was just a LITTLE crooked on the housing.
The widening of the rotor to cap came along with some of the other silly smog regulations in the mid 70's. I've heard several different reasons, I don't know what the engineers were "actually" thinking. Unless you were an actual engineer or designer, it's "all" just speculation as to the reason the gaps got larger.
Then came the first HEI distributors from GM. The power output actually burned thru the stainless tab on the rotor causing shorts that allowed no spark to the terminals.
GM cut the power back in the coils to let the rotor tab live.

You say you have (I see your rotors..!) a .09" is incredible to understand.
The gap in my Chevy cap and the MSD rotor on a Studebaker home made crank triggered assembly is "about" .03", as viewed thru a big hole in the "setup" cap. The MSD cap vs. the MSD rotor appears about the same.

Me, I'd set it up with the smallest gap possible. There's less possibility of the spark trying to jump to another terminal. There's less chance of general arching, and throwing tracks out to the inside of the cap (that black line sometimes seen). There's better chance for the full power of the spark to actually reach the cap terminal. You have a better chance of the ignition timing to be closer to actual, rather than the extra time it takes for the spark to find the place where it's supposed to be going, rather than a "hunting" operation to find the right terminal and not jump the gap to the arching track in the inside of the cap, that will eventually become an actual place for the spark to go after building up enough burnt plasma arc material.

Mike

 

Did you take a look @ a Match set and see if there was a Difference. Same Manufacture Cap and Rotor mite be a Little Better. Just A Guess. Never really paid it any attention, Thanks for giving me something else to look at.

 

Well I wasn't the first person to wonder about this. That's what's great about searching discussion forum archives.

Here: http://www.slantsix.org/forum/viewtopic.php?t=60552

Somebody did some measurement, and found QC all over the place:

Biggest Gap Combo: New Echlin MO 40 cap with new MO 13 Rotor with gaps of .089 to .114" (varying with plug terminal)......19,000 to 22,000 volts on coil wire.

Smallest Gap Combo: New Different Echlin MO 40 cap with new Mo 3000 Rotor with gaps of .018 to .022" .........6,000 to 9,000 volts on coil wire.
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Summary: http://www.plymouthcarclub.com/wp-c...-of-Distributor-Cap-to-Rotor-Gap-Testing.docx

Now maybe forcing the coil to spool up to a higher voltage is a good thing. But maybe not, seems like coil would run hotter than it might otherwise.

 

Plasma trail. Causes corrosion too. Hm....

The reason I became curious was looking at excessive firing line voltages over my "new" ignition scope. Once wires and plugs and fuel mixtures are eliminated as a cause then one thing that can cause problems is a cocked distributor cap. Or old worn out tooling, that produces out of round terminal ring and/or defective distributor caps. Sloppy parts, defective rotors etc. Every gap should at least be consistent, whatever it is, I'd think. Or maybe .035" for both plug gap and rotor gap. I don't know, that's why I'm asking. Thanks for your advice.

 

Very interesting. I would like to read what Jim Linder (@GMC BUBBA) has to say about this topic.

 

I ordered a couple more different rotors of different makes to see if they make any difference on the scope trace voltages, I think they are longer and the gap should tighten up.

It "seems" like a consistent and close gap between the rotor tip and terminal would be optimum, the only place a wide gap would be wanted is at the plug itself. But, I dunno. That's why I asked.

Now it is true that anywhere there is a physical gap in the secondary ignition circuit, it will cause the coil voltage to spike on that particular cylinder, whether it's inside the distributor cap, a loose plug wire or a break internally in the wire itself, excessively wide plug gap &c. Most people are familiar with the old trick to getting a fouled plug to fire, say on a lawnmower. Pull the plug wire so it's about a 1/2" or so.

 

Found this in a 1954 patent application by Mallory:

"In a distributor cap having a jump contact it is essential that the gap between each of the distributor segments or contacts and the rotary distributor arm should be maintained within a close tolerance, for example, .001 inch to .003 inch."

https://patents.google.com/patent/US2677855

This would require a high level of precision in the components. .003"! Pretty fur piece from .1" eh??!

I think the argument is that a close tight clearance will result in the parts lasting a lot longer. The wide gaps were a result, it is argued, of 1970s lean burning air fuel ratio strategies. And that's right about the time everything started burning up, too. The aftermarket ignition parts may not be very good to begin with. Mallory at that time was one of the best, I'd think.

 

Pretty tough to maintain that with any tolerance stacking at all. Gives us something to strive for, though

 

I recently read (but don't remember where, may be Model A banger stuff?) it should be approx the same as the plug gap. That would make sense on the .025-.035 that we all use on points ignitions. I can't see .003 as enough for the parts to not crash when anything goes wrong. Rotor phasing is another thing that can make the gap be huge, too. Radial gap. especially with Vac and mechanical advance.

 

I've read the same thing. No, not going to try and achieve anything down to a hairs width, but maybe .020 or .030 inch, we'll see if these older rotors are a little longer than the newer blue type. Should be here tomorrow maybe.

 

Something else to ponder. A scope can only display the widest gap in the circuit...........??????????????????

 

OK the rotors came in today and the older style definitely have tighter clearance with the terminals. Will check on the scope later and see if there's any difference on the firing line.



edit: Bought an old school Motorcraft tune-up kit and while the points & rotor are in good shape the condenser surprisingly failed in testing at around 100 volts DC, got a passle of very old condensers laying around (from makes like Standard and Echlin and Motorcraft) that test perfect for leakage all the way to 500 volts. Wouldn't have expected that.

 

Thanks for all your effort.
Have you had a chance to check the difference with your scope?

 

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I have, it was just a quick & dirty to quickly see what's what, if anything. It looked though to my untrained eye to be an improvement, the firing lines are lower and the height is now about the same across every cylinder. I will try to take some more pics. Also want to try a different coil wire just for comparison, have one spiral wound with 500 ohms, versus the current carbon core, which is several thousand ohms.

 

Here's my real world experience. I finished a total restoration of my '67 Corvette L79 (327/350) about ten years ago. This included a complete engine rebuild, with all new components. After I got it broken in, I gave into the tendency that we all have to run it up to the red line (6000 rpm). To my dismay, it started to misfire just before the yellow region (5800 rpm). I was wondering what I was going to do about this when I read an article in the NCRS Restorer magazine about this very subject. It seems that in about 1973 GM and all of the other parts manufacturers decreased the length of the tip on the rotor by about .060". This was a cheap way to decrease emissions. For my car at least, it also had the effect of causing the high RPM misfire (which matters not a whit to the emissions police). There is a company out there that sells rotors manufactured to the exact original specifications. I bought one and when I installed it, I could easily see the difference with the naked eye. I installed the new rotor, and the engine went right to 6000 without a miss. I'm sure that it would have gone further, but I have a lot of money in this rig.

Knowing the way the aftermarket industry operates, I would expect any rotor produced after 1973 was built with this additional gap, no matter what it was originally designed to fit. Had I not had the easy way out of buying an original spec rotor, I would have probably brazed some extra material on the rotor tip and filed it back to spec. I think solder might have worked too.

 

Sort of wonder if this wide gap deal has also happened in the high performance system such as MSD.

 

How do you accurately measure the gap with the cap off?

 

Well I've learned there's quite a few things to look at with ignition that's for sure. Good quality ignition parts make a huge difference. All the ignition coils pretty much look the same so we have to trust they are selling good stuff, distributor caps made with old tooling might be out of round or fit sloppy. And the rotors. Distributor itself is prone to wear and excessive clearance as it is a mechanical item. An ignition scope is really interesting to use and should improve your tuning right away. I can see why every shop had one. Course all this doesn't mean anything really, I haven't driven it yet with the latest batch of new parts - that's the only thing that matters. The plugs look outstanding.

 

Probably the easiest way is to measure the distance from two opposing terminals inside the cap and divide by half. Then measure the rotor length from tip of electrode to the spring contact point. The difference between these two figures is the gap.

 

I used some kids birthday candles and melted the wax on the distributor terminals. If the rotor tip took all the wax off, i tried again with less wax. It's kind of a time consuming task.

 

Took it for a spin got it good and warmed up and it idles real smooth and "solid", runs good. The traces on the scope, I think they look more consistent across every cylinder but the firing lines all seem pretty high. But I really don't know what I'm looking at because I don't have a baseline.

Have no idea on the finer points of how to read the waveforms as much as I study, because of lack of experience. It's my understanding the use of resistor wires and resistor plugs changes the voltages displayed.

I would say a reasonably close clearance on the rotor does seem to pull harder under load. Don't get too froggy, or stuff starts banging into each other. A little attention to the plug wires & ignition parts pays off, make sure the terminals are clean and shiny and fit tight and are fully inserted all the way into the cap receptacles. Good solid grounds too, all that stuff, really makes a difference. The scope does let you see the results of your handiwork.

 

The gap there will normally not cause problems. Yes an excessive gap can cause high RPM misfires, but it would have to be a huge gap. Bushing wear in the distributor will cause more problems than the gap will ever cause.

 

The closer it is the less arcing occurs. Arcing at higher rpm will cause the arc to drag to the next contact. Arcing at higher voltage also causes corona ( not the beer) which is also not wanted because of the tracking on the internal cap surfaces. Caps should have a breathing system when used in racing or high performance applications.

 

One thing I wondered about, most spark plugs available today are resistor type. Resistor plug wires. Resistor coil wire. And I guess, even resistor distributor rotors, depending on the rotor gap. Is it really necessary to load everything down every step along the way? I mean, if RFI is a consideration, wouldn't resistor plugs alone do the trick?

Don't currently have resistor plugs installed, though using carbon core resistor plug wires. Swapped out the resistor coil wire today for grins, from 9k ohm wire to 500 ohm wire and then checked the spark from the coil on one of those adjustable gap testers, looked a little fatter and hotter? I dunno, everything has to work together, it's probably worth experimenting at least. As hot a spark as possible short of any arcing or crossfire inside the distributor.

 

I don't know what the exact measurement should be but I had a outboard motor stop dead in the water 8 miles from shore and refuse to run. After much tinkering a new rotor solved the problem.

 

Yeesh. Didja have a new rotor witcha, or did you have to row row row yer boat gently cross the lake??

 

A gap in the distributor is easyer to jump that the same gap at the plug because of the compresson pressure.

 

Right, but I don't understand what the engineers were trying to accomplish. Plug gaps, sure. I guess. It's easier for voltage to jump a gap in open air, but it still increases the overall load in the secondary circuit. Rotor gaps (along with plug gaps) increased when HEI was developed, and lean fuel air ratios, emissions testing &c. And then they started burning up rotors, too. GM had factory .060" plug gaps for a year or two.

And see weasel wording here: