Rotor to Distributor gap...

[JV Puleo]

I know there is a gap, but I am finding it difficult to get any idea as to what the gap is. I realize its an odd question since its not adjustable in the normal course of events but in this case I'm making a combination generator/distributor unit to use on my 1910 Mitchell in place of the magneto. I'm making the entire distributor and, when I make the rotor, need to know how much gap to leave.

[Guest stude8]

I assume you mean the gap between the rotor tip that moves past the fixed sparkplug wire posts of the distributor cap. I would say .030 to .050 would be fine to allow the voltage to jump from the rotor tip to plug wire post and still give enough room to avoid mechanical collision of the tip to the post when distributor shaft bushing wears in time and rotor takes a wider swing at the post.

The point gap is another item that has a closer tolerance and most distributor breaker plates are designed so that the gap of the fixed point contactor is adjustable to within .015 or .020 of the moving point contact that rides on the shaft point cam.

Attached is an image of a Delco Remy 636-X distributor for a 1928 Studbaker Big Six engine. The gap adjustment slot is not visible from the angle of the photo unfortunately.

Stude8

[JV Puleo]

Thanks. That is what I need to know. I'm actually using a set of Corvair points in this distributor. The distributor shaft runs on a ball bearing so I'm not worried about it wearing out in my lifetime. I'll probably aim at the small end of the potential size since the engine will only have about 5 to 1 compression.

Do you know if the condenser has to be grounded? Thats another question I can't seem to find an answer for. In my distributor the base plate is made of canvas phenolic which doesn't conduct electricity so if I attach the condenser to it, it won't be grounded... but I can't find anything that says it has to be.

Edited November 26, 2011 by JV Puleo (see edit history)

[Rusty_OToole]

Condensers do not need to be grounded.

[McDar]

Interesting question. Tomorrow I'll cut the top off a distributor cap and measure the rotor to plug wire contact distance. Will do the same to an electronic cap to see if the gap has styed pretty much the same through the years. Since it on a curved surface I will use a wire gauge instead of a flat feeler gauge so keep that in mind.

[Guest Snopack]

The case of a condensor has to be grounded, in your distributor that means a seperate ground wire.

John

[Rusty_OToole]

Seems to me you could take any distributor cap and matching rotor, measure the distance from the center of the rotor to the tip and the distance across the cap from one contact to the opposite one and have your answer.

And, condensers still do not need to be grounded.

[JV Puleo]

I did think of that, but the only cap and rotor I have loose, from an old car, is from a 4 cylinder with a wipe contact rotor. Also, because the measurement is in thousandths, I'm not confident of getting the measurement from the center of the rotor to the tip accurate enough. This is all good though, as its allowed me to redesign the rotating base plate inside the distributor. I'm not including a centrifugal advance mechanism so it is a simple distributor, though nothing is simple when you have to make all the parts. The idea is to come up with a unit that bolts on in place of the magneto and looks like the distributor/generators that were sold as accessory items around 1910-1912. I have it in-line running directly off the back of the generator so that the advance lever will work "up and down" exactly as the magneto advance did.

[McDar]

Your question intrigued me to the point I had to find out. What I found was VERY surprising to me. Pictured is a cap with an access hole to reach in with a wire type feeler gauge. It's hard to make out so I painted the gauge white--it's "L" shaped and you have to look close to discern the rotor tip and the contact in the cap. The gap is a measured 0.105 but I'd bet it was 0.100 before erosion from use. I stacked flat feeler gauges to obtain 0.100 and it fit also but not a nicely as the round wire gauge since ones dealing with a curved surface. I would have guessed the gap would be about the same a a sparkplug but not so. I have several 235 Chevy point type distributors and caps but haven't dug up a rotor yet. When I find one I'll measure it since they have the old Delco point type and the gap may be different.

[NTX5467]

The gap between the rotor and the distributor cap plug wire terminals could well be determined by the mean output voltage of the ignition coil being used. In this scenario, it could be highly variable for an earlier vintage vehicle.

Other than cutting up distributor caps, you could measure from the center of the carbon contact in the middle of the cap, outward, to the sparking surface of the spark plug wire terminals inside of the cap. Then, take a matching rotor for the particular cap and do the same. The difference should be the spark gap for the cap/rotor assy.

In the later 1960s, the gap was widened a little for better RFI control. Many Delco rotors of this nature had an "E" stamped into the rotor's contact lug, near the outer end of it. Many hot rodders sought out used rotors without the "E", thinking they'd be better. The slightly wider gap will make the coil build more voltage before it can fire, which might also result in the need to slightly advance the distributor a few degrees to compensate for the later spark.

Although the .100" gap might seem wide, compared to spark plug gaps, it might well need to be that wide to get the coil to build sufficient voltage to efficiently fire the spark plug. If such a gap is the "industry standard" (of sorts), then that would explain why ignition coils can be seemingly be swapped around among different brands of vehicles as it seems they can be.

Just some thoughts,

NTX5467

[Joseph P. Indusi]

Both leads of the condenser, or more properly the capacitor must be connected across the two elements of the ignition points. This is so the condenser can absorb the high current flow across the points when they suddenly open. With the points now opened, the condenser and the coil primary form a LC circuit that can induce a voltage in the coil primary of over 200 volts, much more than the 12 volt battery voltage. If one side of the points is normally grounded, as the stationary points are in a GM car, then the case of the condenser must also be grounded. In any case, the condenser leads must be connected across the two elements of the point set.

Joe, BCA 33493

[JV Puleo]

I'm not going to be using an antique coil or points... or the rest of the charging system, most of which will be hidden except the generator which will be as "disguised" as I can make it. But, you bring up an interesting point... since I took the points out of a Corvair distributor... and the generator is also from a Corvair (because it runs counter clockwise) I'll get a Corvair cap and distributor and measure those... it just so happens that I have a friend with a building full of Corvair parts so those aren't hard to come by. I'm still dealing with a much lower compression ratio than any modern car so I wonder how important it is to maximize the coil output. Magnetos and early distributors and timers had wipe contacts.

[NTX5467]

Up into the 1960s, magnetos were the distributor of choice for many high-end racing engines. Their output was higher than any point system of that era PLUS there was no need for an energy-zapping/horsepower-robbing charging system. Simplicity and high voltage worked well together, back then. I'm sure there were some design parameters for mags that would affect their ultimate spark plug firing voltage, but even for the lower compressioin ratio motors, you might be surprised how much zap they had, even if it was overkill for a 5 to 1 CR.

Just some thoughts,

NTX5467

[JV Puleo]

I'm quite familiar with magnetos. It was the original system for my car and I know that when they are good, they are very good. I'm really doing this to fit the generator, not because I think that a modern point system is better. Twice I've been stuck on the road on a brass car when it got dark and had to come home on the kerosene lights. That was 30 years ago and were it not that I was in my own town, I wouldn't have chanced it. Later I fitted a generator to that car, though all it did was keep a battery up, but it was useful when I came back from Long Island - the end of a 300 mile trip on a 1910 car (this was in about 1975). It got dark two hours before I could get home so having an electric headlight was a huge plus. My feeling is that if you're going to drive these really old cars, there has to be some effort to make them as safe (for yourself and others) as is practical. And, I'd bet lots of 1910,11,12 cars had electric lights. They were readily available accessories. We don't see them because they aren't as "sexy" as the gas lamps and restorers usually have no intention of ever using either. The Mitchell didn't even come with headlights... you had to buy your own. We don't see brass painted black, but that was common... Gray & Davis offered their lamps "blackened" for .50 cents extra.

[NTX5467]

I concur with the orientation of updating various vehicle systems for more modern driving situations.

There used to be a story floating around in the Buick Club about a member who drove his very early model Buick to national meets . . . on the Interstate. He had it fitted with a generator, more modern lights, AND a CB radio. And he did like to talk on the radio has he cruised down the Interstate highways! (As I recall) He was talking with a trucker and getting compliments on how good his radio sounded. The trucker wanted to know what kind of vehicle he was in, after identifying himself. The response was something like . . . "I'm in that 191_ Buick you dang near ran over two miles back."

Keep us posted on your progress, please.

Happy Holidays!

NTX5467

[Manuel]

There has been some posts from the Model A Ford boys about this. It seems the new repro caps and rotors do not want to work with each other :-)

Manuel in Oz

[Rusty_OToole]

Have you considered putting an alternator under the car and driving it off the driveshaft? This has been done before, on cars with open driveshafts. It is possible to put a pulley on the back end of the shaft and mount a small alternator onto the rear axle or under the floor. Import cars these days have small, powerful alternators that are self contained and need no regulator.

[JV Puleo]

I did think of that but the Mitchell has torque tube drive. I might have done something with the connection between the engine and transmission but the car utilizes an unusual system where the transmission is fixed to the torque tube and pivots on trunnions at the front end. Also, I am determined not to make any permanent changes. Should someone in the future want to go back to the magneto, all they would have to do is lift my generator unit off and bolt the magneto down.

Edited November 29, 2011 by JV Puleo (see edit history)

[Guest De Soto Frank]

Seems to me you could take any distributor cap and matching rotor, measure the distance from the center of the rotor to the tip and the distance across the cap from one contact to the opposite one and have your answer.

And, condensers still do not need to be grounded.

I thought all capacitors had two terminals ? ( Not counting multi-section capacitors as found in power-supply filters)

In a conventional distributor, the "second" terminal is actually through the condenser "can", which is mounted to a grounded part of the distirbutor ?

[NTX5467]

Import cars these days have small, powerful alternators that are self contained and need no regulator.

One reason they might not "need" a regulator is possibly (the same as many new USA-brand vehicles) that the alternator is now turned "on" and "off" by the Body Control Module, to maintain adequate juice in the system. In reality, this is now how things work on almost every vehicle. Is there something I've missed on import brand vehicles?

Just some thoughts,

NTX5467

[Coley]

The condensor DOES indeed, need to be grounded. The clamp holding them in usually does this. If they are not grounded the points burn quickly

Edited December 1, 2011 by Coley
speling (see edit history)

[Guest 47jag]

The non invasive way to check rotor arm gap is to use a 'scope and ground one plug lead with the engine running. There should be a maximum voltage spike of 6-8 Kv.

Art

[JV Puleo]

The problem arises from the fact that I'm making both the rotor and the distributor cap... so there is no gap to measure and it will likely be 2 years before the engine will be reassembled. Even then, it will probably be another 2 years before the car is reassembled enough to run it. I am planning on building a test rig for the generator/distributor unit... but if all works out properly it will then be boxed up and put aside until the engine is back together. This is the most complicated thing I've ever tried to design and make from scratch so there is hardly a part I haven't done twice... sometimes even more times!