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Antique Gas Engine Discussion

FM Mag


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  #1  
Old 08-21-2005, 05:19:04 PM
Ralph Leonard Ralph Leonard is offline
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Question FM Mag

Found this FM X1A21 at a yard sale. No impulse, and has hot spark twice each revolution. What do you think it goes on?

Could it be adapted to a 3HP Z? If I could find the right gear count I could fabricate a bracket. Have any of you tried this?
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  #2  
Old 08-21-2005, 07:49:42 PM
KidDynamo KidDynamo is offline
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Default Re: FM Mag

My old interchange book pre-dates the FMX type magnetos, covering only the FM-J, M, MK, and O types but the manufacturer's numbers have always run true.

This means that the 21 number corresponds to the Homestead Valve Company and my catalogue suggests this mag's Type J and O predecessors fit somethings called the Hy-Pressure Jenny. They don't show a coupling type or a lag angle, suggesting that they were gear driven without an impulse. It could be from a 2 stroke engine or maybe it was used for a furnace ignitor like on a steam cleaner.

The mag should be a clockwise configuration which is what the Fairbanks 3 hp would have used, but the 2 sparks per turn might throw a permanent curveball into the works, but maybe not. I guess the thing would be sparking on the exhaust stroke T.D.C. as well as on the compression stroke, where it belongs. This might not cause a problem or ??

The Fairbanks Z with the "correct" replacement type mag would have used an adapter plate # B2561A although I don't know it's thickness, it would appear that it would allow the mag to mount on the same holes as a Fairbanks type "R" magneto and give the same shaft height. The resultant height appears to be about 40mm, total, as I measure one of my "R" magnetos.

I'm guessing that your mag has a height from base to centerline of the armature shaft of 45mm which is too high already to use the original style gear and an adapter plate would make this higher still.

I believe that the Fairbanks conversion mag's, such as a FM-J had a 35 mm height to shaft center and the adapter plate raised it another 5mm to achieve the 40 mm height similar to the "R" magneto.

The 3 horse "Z" requires a 15 degree lag angle impulse arrangement and I believe the same gear for the 3hp "R" equiped engine would fit right on your magneto and be at the right elevation and location provide the base plate adapter could be correctly fashioned. However, this may be problematic if your magneto starts at a shaft center height of 45mm, without the adapter.

I have never done this and know this stuff from "book learning" only but believe I've got most of it correct. I would encourage you not to leave out the 15 degree impulse if you can make the conversion, as this would make starting a dangerous proposition.

I'd be curious to hear what the measurement is, in mm, from the base of your mag to the armature shaft center. (Most any old 1 foot rule will have a mm scale on it.) Looking at your photo leads me to the suspicions I have about your mag's "height", and it looks built-in. Other varieties of this mag look similar but there is actually a 10mm adapterplate fastened on. In this other case, this 10mm plate could be removed and a "new" 5mm "Fairbanks" adapter fashioned.

If your mag is only 35 mm high, and if the double sparking doesn't bollux up things, you may be able to convert it with a Fairbanks impulse type gear from a 3 hp Z. If you have the incorrect height, then finding a gear with the correct number of teeth and correct pitch and still has the 15 degree impulse capability gets immeasureably more difficult.

If the height is too great, perhaps you could mill off 5mm from the base, but it might be easier to persue a different route.

In any case, if the mag is hot, you've likely got a bunch of good parts like a cap, condenser, points, fasteners, and more!! They may come in handy even if the magneto, as a whole, is too much trouble to adapt. In any case, don't let anyone discourage your original thinking. There's more way's to skin a cat and besides, I might have my "facts" all haywire. Good luck, I'll stay tuned in.......
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Old 08-21-2005, 09:19:55 PM
Ralph Leonard Ralph Leonard is offline
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Default Re: FM Mag

Kid, thanks for the quick responce and especially for more info than I expected. I believe some small engines operate their points off the crankshaft. Extra sparking may not be a problem.

We have been off again, on again about Portland. Since I posted the Q the "boss" has changed her mind and we are going. Maybe we can bounce some ideas after the show. HOT DANG, GONNA GET OUT OF THIS HEAT!!!!
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Old 08-21-2005, 10:26:37 PM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: FM Mag

This mag was turned by an electric motor, to provide spark for the ignition on an old steam generater-cleaner. it had a 3" diameter friction drive pully that ran off the back of the V belt that ran the hi-pressure waterpump, and fuel pump for the burner. To get this mag to work on your engine, 3 things can be done: 1) provide a gear twice the diameter of the existing cam gear (impractical). 2) provide a gear the same size as the cam gear. You may want to grind off one of the timing cams within the mag, so that it provides 1 spark per revolution. 3) after removing said cam, provide a gear 1/2 size of the original cam gear (you will get 2 sparks per engine revolution, but will save space, using the small mag gear). You will have to figure timing - spark advance setting.
Andrew
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Old 08-22-2005, 12:21:00 AM
JoeFisher JoeFisher is offline
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Talking Re: FM Mag

Two sparks per cam rotation equals one spark per crankshaft revolution which is no problem as most modern car engines, without a distributor, spark every crankshaft revolution. this is known as a waste spark. Hot in central Texas, Joe Fisher
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Old 08-22-2005, 01:06:45 AM
KidDynamo KidDynamo is offline
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Default Re: FM Mag

Yeah, I don't think the wasted spark will cause a problem. My Onan has a dual secondary coil and two cylinders that are on t.d.c. at the same time and waste half of the sparks.

I think the mag height will be the undoing of this senario, unless he's got a clever solution for the gear and impulse.

We shall see. I'm curious about the mag's dimensions.
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Old 08-22-2005, 08:08:21 AM
Ralph Leonard Ralph Leonard is offline
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Default Re: FM Mag

Here is the shaft view. The shaft center is 35 MM high. Notice it is also tapped for side mount. It will need a bevel tooth gear.

I don't think I can add impulse. What I had in mind is set timing about 4 or 5 degrees for slow running. I have other engines with fixed timing and dealing with them has not been a problem.

The mag has good strong lumps and will produce a 1/8 inch spark when I turn the nut by hand. That don't mean I can get it started though.

Is it worth a try? I've only got $10 in the mag so far. Hell might freeze over before I come by a steam jenny with a bad mag. LOL
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Old 08-22-2005, 03:31:57 PM
KidDynamo KidDynamo is offline
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Default Re: FM Mag

If your shaft center is 35 mm high and you can find a gear, "you're in like Flynn". If you would like, you could email me your mailing address and I could mail you a photo copy of the exact specification for the adapter plate. Although I believe it is based on a 5mm thickness, you could either find a piece of 5mm thick aluminum plate or try using 1/4", which is pretty close.

I wouldn't rule out the impulse gear. I guess it is a combination affair, and is an original Fairbanks part, although I don't know if it came in a bevel gear variety. This business of straight versus bevel gears on the Fairbanks engines is a minor subject of some speculation on this site and elsewhere.

Perhaps another person can inform us as to whether an impulse style gear for the 3 horse was made in a bevel gear ????? Are the 3 hp and the 6 hp gears the same??

Also Ralph, what year engine are you working with here, in other words, what style of magneto would have originally come on this particular engine?? Fairbanks "R" ? American Bosch AB-33/34 ??
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Old 08-22-2005, 05:39:31 PM
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David M. Lyon David M. Lyon is offline
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Default Re: FM Mag

I have a friend in our local club that has a three horse, open crank, FM Type "R" mag with bevel gears with an impulse set up...SO they were avaliable, but his is the ONLY one I've ever seen...Could be that some one hand made the set up years ago...Wouldn't be that hard to do if you had a pattern to go by...Also met a guy at the Portland show a few years ago that was selling three horse gears with impulse that he had made himself...Said he only made one of them at a time, and when he sold that one, he made another to sell...I have since lost his card, and I didn't find him last time I went. ..I think the one he had was straight cut, but I bet he could make a bevel gear just as easy!...Seems like he wanted $100 bucks for the comlete gear, spring, washer, and catch....YES!!! the three horse and the six horse open crank engines use the same gear. ... Looking at the photo of YOUR mag, Looks like it is drilled & Tapped for the impulse catch, So I don't see Why it shouldn't work.

David M.
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